Cold start cylinder 3 Misfire issue ( long running issue, even longer post )

[Dangerb]

Hey guys,

sorry for the long post...

I've had an intermittent coldstart issue that began a few winters ago and has progressively gotten worse this past winter. I've read a bunch of threads where people experienced similar symptoms as me and swapping heads for the most part seemed to fix the problem but that hasn't been the case for me. I've narrowed down the possible causes but feel i may be missing something, and i'm sure this may be the case for others too? I have access to Vagcom, so any recommended test are greatly appreciated.

I've tried to list any relevant history/information i can think of that may relate to the issue since it started and repairs i did to try and narrow down the issue:

2003 A4 1.8t Tiptronic


Intermittent Hard start issue started 3 winters ago ( winter 2016/2017 ) after i went stage 2 Unitronic with TT injectors, new plugs and stock coilpacks. Car started consuming oil 5 months before performance chip. Car would struggle to start periodically and would skip cold start warm up and idle at 800rpms, , but never threw CEL and condition went away with the warmer weather. As weather got warmer, car would perform cold start warm up procedure normally even if it was warm overnight? Replaced Camshaft and crankshaft position sensors, both O2 sensors and coolant temp sensors.

Installed ECS fuel gauge and fuel filter spring/summer 2017 to narrow down possible fuel pressure issue. Not an issue. Installed 2.0 coilpack conversion kit Dec 2017, no change..but car ran better. Same Coldstart issue continues throughout winter 2017/2018.

Spring 2018 car started smoking on decel and during long hill climbs ( full smokeshow ) so i bought an ecs catchcan. Can catches lots of oil...Summer 2018 Installed new stock fit turbo as the OE one had 250,000kms and a lot of shaft play. Smoking issue was lessened, but still present after turbo swap, but car felt better. Oil consumption about same, but catch can now stays dry/ minimal vapor. Planned on replacing valve seals. Replaced fuel injector harnesses at same time as well replaced all evap/breather hoses with 034 silicone kit and billet PCV and new crankcase vent valve. 034 block breather hose didn't fit soo great at the billet end, but no oil buildup is noticeable around the connection.

This winter hardstart issue returned. Swapped in a good Bw K04 + new CTS HFC over christmas and stayed with stage 2 software for 3 weeks. Switched to stage 2+ software, periodic hardstart continued ( so i guess not a software issue ).
Found a leak around the intake manifold gasket for cylinder 3, so i swapped in a fully cleaned and powdercoated Intake Manilfold with new gaskets and billet fuel injector seats. No more leaks but problem persists after a few days. Also cleaned a spare Throttle body i had painted to match my intake manifold, performed adaptation with Vagcom, problem still persists.

February i started getting misfires with flashing CEL/codes for cylinder 3 when the car would sit overnight. Problem continued to get worse and would misfire with CEL after work and started to happen on cylinder 4 as well. Cylinder 3 appeared to be wet, couldn't smell coolant though. Stuck a wooden dowel down the plug hole and all i could scrape off was oil. Ran pressure test, all cylinders within acceptable specs with 2+3 being a few PSI lower than 1 +4 but still good. Systematically swapped Coilpacks, plugs, and fuel injectors back and forth, problem did not follow movement of components.

Car was loosing coolant so i thought maybe time for an AWM head swap with new valve seals and kill 2 birds at once. Found out coolant loss was from a warped coolant sensor clip on the rear coolant flange. Replaced clip. no more coolant loss. Same time i replaced my shitty coilpack connectors.

Car started misfiring while driving, so i swapped in some NGK copper core plugs as 3+4 were fouling.

Rebuilt a good, straight AWM head while dealing with nonsense above and installed with all new gaskets/bolts/hardware. All Problems go away for a week, car now surges at coldstart again in morning during warm up cycle. No CEL light on, but soft code for cylinder 3 misfire persists. Oil Consumption issue is gone, Oil catch can still dry/ minimal vapor.

Checked old AMB cylinder head, no visible evidence of cracking or warping. Head gasket looked ok too.


short list on Repairs i've done personally since the coldstart issue started that i thought may be related (new to old)

New spark Plugs (May 2019 )
AWM head swap with all new gaskets/seals/hardware/tensioner pads ( apr 2019 )
New Coil pack connectors ( feb 2019 )
Intake manifold swap with new billet injector cups ( dec 2018 )
New injector connectors ( Aug 2018 )
NEW oem MAF ( aug 2018)
034 silicon breather/PCV hoses, billet PCV ( Aug 2018 )
2.0 coilpack conversion ( dec 2017 )
ECS fuel pressure gauge ( aug 2017 )
Replaced Fuel filter ( aug 2017 )
Camshaft and crankshaft poistion sensors, both 02 sensors, coolant temp sensors ( winter 2016/2017 )

thats everything i can think of at the moment

Today:

Watching my boost gauge at cold start ( overnight ), it immediately goes down to -5hg for about halfway through the cold start warm up process, then climbs up to -11, and then settles out to -15/16 once the idle goes back down to normal. Maybe Vac leak at coldstart? Seems a tad low at regular idle even for a tiptronic. Car will hit -20 at cruise with foot off gas.

As it stands right now i'm planning on going back to square one and start checking for some vac leaks. I have a wobbly/poor fitting billet end on the crank case breather hose from 034 that is getting some attention today and I've also ordered a bunch of new vacuum valves from US plastics, so those are on the to do list.

I'm wondering if anyone may have some insight to whether or not a vacuum leak could cause a cylinder specific misfire? Multiple cylinders i could believe but just cylinder 3? Bad PCV? Maybe The Silicon line to the Intake manifold near cylinder 3 from the PCV is sucking in too much air at start up? All i know is once it warms up all seems fine.

Could this be a 30397 driver issue with the ECU that k0mpresd has repaired for other members?

Any ideas or thoughts would be greatly appreciated!

[Gin+]

That's a long list. Injector maybe? Strange it would be isolated to cyl 3?

[Dangerb]

That's a long list. Injector maybe? Strange it would be isolated to cyl 3?

I swapped injectors 3 & 1 a couple months back with no changes. I could always swap em back though. I had the injectors professionally cleaned and flow tested about 2 years ago when i installed the fuel pressure gauge and fuel filter

[old guy]

The low idle on a cold start is normal when the temperature drops below the mid 30's. Clicky click®. The reduced vacuum is also normal. When you first start the engine the intake cam adjuster goes to the full 22° of advance and the ignition timing goes to around 5° ATDC. This pushes some of the gas ignition further downstream and gets the cat up to temperature faster. As the cat temperature increases the timing goes to BTDC and cam advance goes back to 0° advance. At that point you will see an increase in the manifold vacuum.

[Gin+]

Is cyl 3 still wet in the morning?

[Dangerb]

The low idle on a cold start is normal when the temperature drops below the mid 30's. Clicky click®. The reduced vacuum is also normal. When you first start the engine the intake cam adjuster goes to the full 22° of advance and the ignition timing goes to around 5° ATDC. This pushes some of the gas ignition further downstream and gets the cat up to temperature faster. As the cat temperature increases the timing goes to BTDC and cam advance goes back to 0° advance. At that point you will see an increase in the manifold vacuum.

Thank you for the link! That’s cool Info about the dew point. Does the system measure temp/relative humidity and constantly adapt to an ever changing dew point, or does it run off a fixed temp or some other reading?

I kind of figured the vacuum readings/ behaviour is normal for start up. I can watch the vacuum climb as soon as the SIA pump shuts off. I guess i’m more curious what vacuum readings other tiptronic users are seeing though/ what constitutes a healthy vacuum for tiptronic. Mine seems to be around 5hg lower than what a healthy manual would be at.

[Dangerb]

Is cyl 3 still wet in the morning?

Warmish morning so I checked, Cylinder 3 is bone dry. I’m going to swap Injectors back after work.

[Dangerb]

Car still misfires at cold start first thing in morning and after work, with at least one of those starts throwing the CEL now. Got my usplastics order and replaced all valves last week, and i swapped injectors around with no changes. While fuel rail was out i double checked intake manifold bolts. everything is nice and tight. Going to swap spark plugs betweeen cyl 1 + 3 tonight to see if anything changes.

[wafi ahmed]

I had exactly Same problem. In winter I replaced the timing belt. And this weird cyl3 misfire started.

The misfire is so small that I can’t even feel, but the cel comes out and show po303.

This problem gone away as it got warmer. I don’t have any cel or hard starting problem.

You should check your fuel filter line. My one was little moved and that was the reason for hard starting...


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[Dangerb]

I had exactly Same problem. In winter I replaced the timing belt. And this weird cyl3 misfire started.

The misfire is so small that I can’t even feel, but the cel comes out and show po303.

This problem gone away as it got warmer. I don’t have any cel or hard starting problem.

You should check your fuel filter line. My one was little moved and that was the reason for hard starting...


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I haven't had any hard starting issues since the head swap. The car fires up instantly, but the rpm starts surging immediately as cylinder 3 misfires. After the idle settles back down to normal, everything is fine.

I've changed out my fuel filter twice in the past 4 years, but its easy enough to take a quick look at it. My fuel pressure primes right to 58lbs with key on and will stay there at idle so i'm not sure if its a supply/pressure issue but i'm open to anything.

Let me know what fuel line/location you had moved and i'll give it a shot.

thanks again

[EuroxS4]

Cracked head maybe or a faulty coil maybe. Get the car hot drive it park it come back to it after 2 hours. if it does it maybe you have a head thats partially cracked.

[Dangerb]

I Just swapped in a good rebuilt AWM head a month ago, and its a brand new coil pack as of two weeks ago. I Haven't had any hard starts since the head swap, now just getting misfires at cold start.

[wafi ahmed]

Did you tried a fuel system cleaner?
I remember that, I put two bottles of fuel system cleaner in 1 month.



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[jarrodblake]

+1 on cracked head, also maybe the head gasket is going or warped head. I had the same problem for two years. I installed a brand new german audi head fixed the problem. That said I also did the vacuum pcv system simplification at the same time. Maybe do a cold compression test and leak down test, then redo it when the system it up to temp if the first test does not show an obvious problem.

[Dangerb]

+1 on cracked head, also maybe the head gasket is going or warped head. I had the same problem for two years. I installed a brand new german audi head fixed the problem. That said I also did the vacuum pcv system simplification at the same time. Maybe do a cold compression test and leak down test, then redo it when the system it up to temp if the first test does not show an obvious problem.

My AMB head had the same symptoms on cylinder 3 before I swapped in a new rebuilt complete AWM head and the symptoms persisted.

After checking the original AMB head, there’s no obvious signs of cracking or warping.

While anything is possible, I have a hard time believing both heads were magically warped/cracked in the exact same way. Head swap was done April 27, 2019

Before I swapped heads I did multiple compression tests while hot over a couple weeks and every time Cylinder 3 & 4 were the strongest.

I can do another compression test cold/hot and a leakdown test though?

[Let it snow]

I had a similar problem. Every Autumn at around 40 degrees the car would misfire on startup on cylinder #3 on cold start. I thought it was SAI related or a cracked head. I checked air and coolant sensor at startup and they always read ambient. Much later on my coolant sensor did fail. I replaced it and my odd cold start problem ended. I know you replaced the coolant sensor at the back of the head it may be worth trying again.

[Dangerb]

I had a similar problem. Every Autumn at around 40 degrees the car would misfire on startup on cylinder #3 on cold start. I thought it was SAI related or a cracked head. I checked air and coolant sensor at startup and they always read ambient. Much later on my coolant sensor did fail. I replaced it and my odd cold start problem ended. I know you replaced the coolant sensor at the back of the head it may be worth trying again.

I'm game. I Just ordered two, so i'll pop one in today after work before i fire up the car

[Dangerb]

I'm game. I Just ordered two, so i'll pop one in today after work before i fire up the car

Update: New Coolant temp sensor was No Dice. cold start after work was perfect, but cold start this morning was same old misfires

[Let it snow]

Update: New Coolant temp sensor was No Dice. cold start after work was perfect, but cold start this morning was same old misfires

Sorry that didn't work. It seems the problem is time and or temperature related. What is the time difference between morning and afterwork starts? Big difference in ambient temperatures? Engine temperature at ambient both times? Can you check your intake air sensor reading before startup?
As for your head leaking I posted a thread a while back called (Coolant pressure test low tech) sorry not a link.

[Dangerb]

Sorry that didn't work. It seems the problem is time and or temperature related. What is the time difference between morning and afterwork starts? Big difference in ambient temperatures? Engine temperature at ambient both times? Can you check your intake air sensor reading before startup?
As for your head leaking I posted a thread a while back called (Coolant pressure test low tech) sorry not a link.

The time difference between cold starts is relatively the same, between 9-9.5 hours. After work today ambient was 13, IAT fluctuated on the border of 13/14, and Coolant Temp read 15 before startup. Once i fired up the car it had a normal, healthy start with no perceivable misfires or rpm fluctuations. Once the idle settled down to 800rpm i drove around the block to pull into the garage at work and use the hoist. Once parked in the shop at idle, cylinder 3 started misfiring constantly with a flashing CEL.

Started watching the misfire counter for all cylinders and #3 just climbed until i gave it gas. Any increase in RPM halts the misfires in that cylinder until rpm drops back to idle. out of curiosity i checked ignition timing retardation and cylinders 1-3 showed 0.0kw, with cylinder 4 showing fluctuations between 0.0-3.0kw. Also watched ignition knock control voltage, and cylinders 1, 3 & 4 alternated back and forth from 0.234 and .351 V with cylinder 2 showing exactly double those figures.

I'll check again first thing tomorrow morning

[Gin+]

I think you need to do a cold compression test.

[Kevin C]

I think you need to do a cold compression test.

Good call and a vacuum test of the intake when its running, look for an intake manifold air leak by cylinder #3 ( again). You will see a distinct pulsing on the gauge. The old axiom of fuel, compression and spark... One is not doing whats its supposed to. Any way to get a scope on the plugs to see if you have an ECU problem?

[Dangerb]

Good call and a vacuum test of the intake when its running, look for an intake manifold air leak by cylinder #3 ( again). You will see a distinct pulsing on the gauge. The old axiom of fuel, compression and spark... One is not doing whats its supposed to. Any way to get a scope on the plugs to see if you have an ECU problem?

I might have found the problem:

I monitored the car on cold start today, with usual misfires on cyl 3 for about a minute, then they stopped halfway through higher idle. i was sitting in the car while it idled for another 4-5 minutes and everything looked great...then Cyl 3 started to slowly misfire again, but not enough to throw a soft code. Up until now I had been leaning towards an ECU or 30397 driver issue, but i did a hot compression test which changed my direction.

Borrowed a compression tester from a co-worker and gave my car a good, spirited drive until i got home. Pulled coils, plugs and disconnected fuel injectors.

............Todays Hot Test results: Cyl 1-168lbs, 2-167lbs, 3- 125lbs, 4-172lbs.
Pre-Head Swap Hot Test Results: Cyl 1-157lbs, 2-152lbs, 3- 160lbs, 4-165lbs

I did a second compression test and they all read about the same, so then i poured a tablespoon of oil into cylinder 3 and compression went up to 155lbs. Better, but still lower than where it was reading before the head swap. Out of curiosity, i poured a tablespoon of Liquimoly Motor Oil Saver into the cylinder and checked compression: it now read 191lbs.

Sooo...just for fun, i put the motor oil saver in the rest of the cylinders and went like this: Cyl 1- 191 lbs, 2-195lbs, 3-191 lbs, 4-195lbs.

Cleaned up any oil from the spark plug wells and then put everything back to normal. Besides the obvious smokeshow, the runs great now. Planning on doing a piston soak sometime this week for 24-48hrs. Either way, i'll do a cold compression test tomorrow morning and post the results.

[wafi ahmed]

I might have found the problem:

I monitored the car on cold start today, with usual misfires on cyl 3 for about a minute, then they stopped halfway through higher idle. i was sitting in the car while it idled for another 4-5 minutes and everything looked great...then Cyl 3 started to slowly misfire again, but not enough to throw a soft code. Up until now I had been leaning towards an ECU or 30397 driver issue, but i did a hot compression test which changed my direction.

Borrowed a compression tester from a co-worker and gave my car a good, spirited drive until i got home. Pulled coils, plugs and disconnected fuel injectors.

............Todays Hot Test results: Cyl 1-168lbs, 2-167lbs, 3- 125lbs, 4-172lbs.
Pre-Head Swap Hot Test Results: Cyl 1-157lbs, 2-152lbs, 3- 160lbs, 4-165lbs

I did a second compression test and they all read about the same, so then i poured a tablespoon of oil into cylinder 3 and compression went up to 155lbs. Better, but still lower than where it was reading before the head swap. Out of curiosity, i poured a tablespoon of Liquimoly Motor Oil Saver into the cylinder and checked compression: it now read 191lbs.

Sooo...just for fun, i put the motor oil saver in the rest of the cylinders and went like this: Cyl 1- 191 lbs, 2-195lbs, 3-191 lbs, 4-195lbs.

Cleaned up any oil from the spark plug wells and then put everything back to normal. Besides the obvious smokeshow, the runs great now. Planning on doing a piston soak sometime this week for 24-48hrs. Either way, i'll do a cold compression test tomorrow morning and post the results.

Piston ring is leaking or something? You mean you put oil in the cylinder?


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[Dangerb]

Piston ring is leaking or something? You mean you put oil in the cylinder?


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Yes, i put oil in the cylinder for the 3rd, and motor oil saver in for the 4th compression tests. I'm hoping just the rings are stuck on that cylinder.

possible theory : The valve seals had been leaking oil in that cylinder for quite a while. Potentially the leaking oil was helping increase compression in that cylinder and showed higher numbers before i did the head swap. Now with no leaking oil in the cylinder the compression is reading true?

When we did the head swap, cylinder 1 was at set TDC, so we got a really good look at the cylinder walls for 2&3 and they looked great.

It's supposed to rain every day this week, so I pulled my car inside so i can do a cold compression test today after work. I also have access to a leak down tester that i could use next week sometime.

I'm still leaning towards doing a piston soak then an oil change. I just need to do more research on what product to use.

[Dangerb]

Cold compression test today showed basically the same numbers as the hot compression test did 2 days ago.
Cyl: 1-164lb, 2-162lb, 3-124lb, 4-163lb
Car sat for 10 hours before the compression test, and I’ve driven over 600km since the hot compression test. Misfired right at cold start like normal, then went away like clockwork.

[Kevin C]

How does your plug look after it sits cold, any signs of oil fouling? Also, since these are ULEV engines I can imagine that it doesn't take much to get into a misfire when cold. You could give it a shot of oil in the low cylinder so on the next cold start you get better compression. Also, low compression and any extra leanness will make things worse. That includes a poor injector spray pattern or flow and or anything else in the system getting the ECU to adjust the fuel trim to lean.

The compression sounds to be the root problem, but any other factors could make it worse. For a test I would bet that if you put in a slightly larger injector on that cylinder the cold misfire would go away. It's not a fix, just a way to see if its a corner case of tolerances. It should be possible to put one 06A 906 031 BA on cylinder three only, its 12% larger ( I think its a drop in , you need to verify). The extra fuel should help with the miss fire. The goal is to make sure it's not a missing spark issue, its not a fix to make it drivable.

Another option would be to see if a shop can get an ignition scope reading.

[Dangerb]

Progress Update: +30lbs compression increase after doing a piston soak overnight.

Process went like this: got my hands on a can of GM/ACDelco top engine cleaner ($60 CAD) and decided to do a cylinder 3 hot piston soak. Thursday evening I Removed the belly pan, Drove the car for 20 minutes to get hot then removed coils, iridium plugs and disconnected injectors. Hand Cranked cylinder 1 to TDC then drained the oil and set it aside to re-use. Left a catch tray under the car with the drain plug removed. Poured a couple ounces of TEC into cylinder 3 and let it sit over night from 8pm to 8am.
Friday morning I Pumped remaining fluid out of the cylinder and cranked over the engine a few times with a rag over the cylinder. Tons of little specks of Carbon fly out. After a few rounds of this I grabbed a long nose compressed air gun and blew out the Remaining crap from the cylinder into a rag. Piston looks cleeeean.
Cleaned up the spark plug well, poured some fresh oil through the engine to rinse out any leftover TEC in the oil pan then Replaced drain plug oil. Replaced oil, put in some copper core plugs (in the event the soak causes fouling) and plugged in coil packs and injectors. Start up car, tiny amount of smoke for first few seconds then took my car for a 20 minute drive.

Started car up today this morning, slight misfires (possibly a fouled plug)Did a compression test this afternoon ( Saturday ) and compression is now showing 155lbs in cylinder 3. Swapped back my BKR7EIX plugs and will monitor the car for the new few days.

Planning to Check compression again in a few days or next weekend. If it’s still sitting up in the 150lb+ range I’ll do a second round of piston soak and post the results.

[Furly]

is the head really "brand new"? or is it just new to you?
if there is no code stamped on the front where it should say AMB/AEB etc.. its a chinese replica head. if you got a used head, step #1 is to have an engine machine shop leak test it to make sure its good, then resurface the sealing surface. inspect valve guides and replace valve seals while you're there. make sure you thoroughly cleaned the block's surface as well. and inspected for flatness.

[Dangerb]

is the head really "brand new"? or is it just new to you?
if there is no code stamped on the front where it should say AMB/AEB etc.. its a chinese replica head. if you got a used head, step #1 is to have an engine machine shop leak test it to make sure its good, then resurface the sealing surface. inspect valve guides and replace valve seals while you're there. make sure you thoroughly cleaned the block's surface as well. and inspected for flatness.

The head is new to me. It's a low mileage AWM head that i had inspected, then i stripped it completely down and rebuilt with new seals. Valve guides were good. Myself and The tech who helped me swap the head cleaned the block surface. The only thing we didn't check was the blocks flatness

[Dangerb]

Update:
The weather has been dipping below the freezing point at night fairly often now, and every time my car has gone into the normal low idle on cold start, the car doesn't at misfire at all. Otherwise the car still misfires for a few seconds during high idle coldstarts if it has sat for 8+ hours.

Sidenote: I picked up a spare engine with forged IE rods ( among other goodies ) back in August and stripped it down completely for a full rebuild. The bottom end is getting bored, decked and assembled at the machine shop this week. I replaced the valve seals on my stock AMB head and will be swapping it back into the car and will have the AWM head that i'm currently using pressure tested and decked to install it on my newly built and painted block.

I've wanted to install forged rods for a couple years now, and learning that i have low compression on one cylinder has given me the motivation/excuse to rebuild an engine. I'm still not convinced a fresh block and head will fix the coldstart misfire issue though. I still want to figure out what the root problem actually is so i can be sure to fix it properly

[Let it snow]

Maybe your SAI system is causing an issue. It is active during cool weather with a cold car (high idle) situation .

[wafi ahmed]

Hey my car recently started to do cold start misfire again, it never misfired at summer, I don’t know what’s it about, MPG went down insanely


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[Dangerb]

Maybe your SAI system is causing an issue. It is active during cool weather with a cold car (high idle) situation .

I haven’t tested my SIA pump in Vagcom, but it seems like it’s working fine compared to previous ones I’ve had go south on me. Is there any possible way the SIA pump/ Combi valve could be causing a cylinder specific misfire though?

Either way I found an older troubleshooting post regarding the SIA from OldGuy that I’m going to try out tomorrow since it’s super easy. I’ll see if disconnecting the hose between the Combi and the SIA pump has any effect at cold starts.

[Dangerb]

Disconnecting the SIA line made no difference over the past week. I hunted around for the easiest spot to access and ended up disconnecting the coupling at the bottom of the hardpipe that bolts to the turbo.

Today I installed an inline coolant heater, so I’m curious to see if the coolant being at a higher temp will affect the cold start misfire issue. I’m thinking the car may start just as though it’s already running and was up to temp?

[rifleman223]

Yeah my car started to do this cylinder 3 misfire. I've already replaced the plugs and new coils. New injectors already last year. I'll keep checking. I'll try the injector cleaner.

"What we obtain too cheap, we esteem too lightly." -- Thomas Paine

[wafi ahmed]

Yeah I think my cold start misfire issue was from cracked headIMG_2241.JPG


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[Dangerb]

Every time that I’ve plugged my car in since I installed my Calix coolant heater, I haven’t had a single misfire at coldstart. I’ve had my blockheater timer set to cycle for 4 hours before I leave for work in the morning and as soon as I key on, my thermostat bumps right up between 60-90 celcius. Car fires instantly and idles at 800ish rpm and runs live I’ve been driving it all day.

After work I fire it up after sitting for 9 hours ( without plugging it in ) and I get misfires and high idle startup like before.

[wafi ahmed]

Every time that I’ve plugged my car in since I installed my Calix coolant heater, I haven’t had a single misfire at coldstart. I’ve had my blockheater timer set to cycle for 4 hours before I leave for work in the morning and as soon as I key on, my thermostat bumps right up between 60-90 celcius. Car fires instantly and idles at 800ish rpm and runs live I’ve been driving it all day.

After work I fire it up after sitting for 9 hours ( without plugging it in ) and I get misfires and high idle startup like before.

So it means the misfire only happens when coolant is cold, problem is why?


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[Kevin C]

Head cracks happen when the metal temps get high enough that the expansion permantly deforms the chamber. When the head cools back off the meat contracts and is put in tension. When this happen often enough or to a severe enough level a crack forms. The crack also seals itself as the head warms up. The other possibility is a bad temp sensor that is throwing of the fuel mix on cold start (or any other AF ratio issue) . If you had a cylinder with low compression it would be more prone to misfire from a fuel mix that's a bit off.