B6 A4 Misfire Cylinder 2

**Audicouver** · 01-09-2025 06:40 PM

Hey guys, first post on the forum, glad to be here.

I bought a 2003 a4 1.8t a couple of months ago from a guy claiming to be a mechanic and have had a ton of issue with it, slowly getting them resolved but my current most pressing issue is I keep getting misfires on cylinder #2. When I first start the car cold it will misfire a couple of times sometimes. Driving in first and second gear is mostly good but when I'm in 3rd and above and pull power above 2500 rpm or am pulling power while going up a hill it misfires like crazy. Been trying to baby it around while I get this figured out but its not always easy where I live (good amount of hills in North Vancouver). This issue only started recently, before that the engine ran fine, I was able to drive up to the ski hill no problem. About 2 weeks ago I was heading up to the ski hill and it started misfiring like crazy in 3rd and above, I had to baby the car to make it as there are no turn arounds. It did have misfires 2 months ago but only slightly on startup, then my fuel pump died and I had no issues after it was replaced so I assumed my misfires were related to that and didnt have issues afterwards.

Whats been done to the engine by my local shop since I've owned it:

-valve cover seal
-all new ignition coils
-all new spark plugs
-new fuel pump
-2 injectors on cylinders 3 & 4 were swapped with spares that came with the car (amongst a box of other used goodies) they were not new but dont seem to be having an issue
-oil filter housing gasket. there is still an oil leak here, the shop that I've had a majority of this work done by thinks the housing itself is cracked but I dont think that is related to issue

The other night I plugged in VAGCOM and saw the following codes:

16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 001 - - MIL ON
16686 - Cylinder 2
P0302 - 001 - Misfire Detected - MIL ON
16839 - EVAP System
P0455 - 001 - Large Leak Detected - MIL ON
16826 - EVAP Emission Control Sys
P0442 - 001 - Small Leak - MIL ON
16727 - Camshaft Position Sensor (G40)
P0343 - 001 - Signal too High - Intermittent

I figured camshaft position sensor was a good culprit to my problem so I replaced it. I also removed all the injectors and tried to clean them out by actuating them with a power supply and spraying brake clean and sea foam through them. They all seemed to have the same spray volume and patterning. fired it back up and went for a drive while monitoring the misfires on vagcom, still misfiring heavily on cylinder 2. The camshaft position sensor code has gone away but they others remain. I'm not really sure where to go from here, could the EVAP Large Leak be a cause of this issue? and if so where do I look to find that leak?

Any help would be greatly appreciated.

**AvantNoir** · 01-09-2025 06:53 PM

Check wiring at the connectors

**Imtryingtofixit** · 01-09-2025 07:06 PM

If i were you. I'd try fixing that evap problem. Might not fix the cylinder 2 misfire. But my old 05 1.8 ran horribly until I fixed the evap.

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**Audicouver** · 01-09-2025 07:20 PM

OK I checked the connector wiring, they arent the prettiest but they arent terrible either. Cylinder 3 injector 1 wire the insulation was cracked so I threw some electrical tape around it. Some of the other injector wire have electrical tape around them so peeled them to check the wires are still connected and they were so I retaped them. The entire injector wire harness loom is just electrical tape around all the wires, no protective plastic sheathing.
Cylinder 2 coil wire the protective plastic sheathing has crumbled away a bit and there is a bit of green corrosion coming out of it, this could be my issue Im guessing?
How much does a new wiring harness usually run?
Any other possibilities?

Also I will try to track down my EVAP leak, need to learn the system to do this though.

**Audicouver** · 01-09-2025 07:26 PM

I just scanned the codes again and had the following 3. EVAP code is gone. Not concerned about the CAT at this point.

3 Faults Found:

16804 - Catalyst System; Bank 1
P0420 - 002 - Efficiency Below Threshold - MIL ON
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 001 - - Intermittent
16686 - Cylinder 2
P0302 - 001 - Misfire Detected - Intermittent

Readiness: 0000 0000

**Imtryingtofixit** · 01-09-2025 08:04 PM

Have you ruled out. Spark plug ignition coil and injector? If the wire is corroded, that could be the cause. Did you check if the pins were corroded?
Do you have fuel trim data?

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**Audicouver** · 01-09-2025 08:46 PM

Not completely I suppose. I assume that with the plugs and coils being new they would not be the issue but I can try swapping them with another cylinder 1 at a time to rule them out. Connector pins are not corroded.
How do I pull fuel trim data? not sure what parameters they are. I went through the monitoring parameters list when I was taking my misfire readings and dont recall a fuel trim parameter but would have to check again.

Also I think I found my evap leak issue. Check valve above the air box is blown out. Ill try to get another 1 and some hose clamps to replace it

**Imtryingtofixit** · 01-09-2025 08:51 PM

I've had new parts turn up bad. Move the injector sparkplug and coil to different cylinders. In case they are the issue, you can differentiate which one was the cause.

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**Audicouver** · 01-09-2025 09:08 PM

Ok I will try this when I have time again and see what I come up with.

**egovreau** · 01-09-2025 11:13 PM

One thing that hasn't been mentioned...the Coscast head used on the AMB engine tends to be notorious for cracking, especially around #2 cylinder.

Just to rule it out, I'd pull the plug and inspect the plug as well as look in the cylinder through the spark plug well.

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**Mike Tries** · 01-10-2025 04:16 AM

AvantNoir makes injector and ignition harnesses. His harness require a little work to install, but they're done the right way. I had a failed ignition harness, replaced it with a similar harness (didn't know about his at the time), and the problem went away.

Like others here, I also had a cracked cylinder head (that was early in my ownership of the car like 5 years ago). Look inside the spark plug holes for coolant on top of your piston. That's a good indicator. Also, if your spark plugs are hard to remove, that's another indicator (my guess is that the hole go out-of-round).

Start with the easy stuff first, though, before you go worrying about the head. Swap the coils and injectors like Imtryingtofixit suggested (lol I like that user name). Then get that trash wiring off of there and replace it. AvantNoir's harnesses go straight back to the ECU, are labeled, and you won't be connecting new wires to dead ones.

**walky_talky20** · 01-10-2025 05:04 AM

Looking down the plug holes with a flashlight, and compare the tops of the pistons (and the spark plugs that come out of each hole). If coolant is leaking into the cylinder, it will "steam clean" the top of the piston real nice-like. If that piston is way shinier and cleaner than the other 3, you know you've got water entering the chamber.

**Mike Tries** · 01-10-2025 06:08 AM

Good point! You'd also be consuming coolant. Check to see if your reservoir is full, or if it appears a suspiciously low.

**Kevin C** · 01-10-2025 07:10 AM

And as long as you have the plugs out, I would run a compression test. Most times the issue is the wiring harness.

**Audicouver** · 01-10-2025 08:16 AM

So I pulled the plugs last night a boroscoped all of the cylinders, all plugs and pistons looked the same, no steam cleaning or ring wash on any of them. #2 spark plug had a bit more resistance when removing but it wasn't very hard to remove it. Don't have a compression tester but I could maybe get one for when I try my plug swap to test compression.
I found Avantnoir's post about the injector wiring harness and rechecked my wiring this morning and I think that may be my issue, they are fairly crispy. Most have cracks in the insulation and tape wrapping them to hold the together. And the insulation is crumbling apart at the pins

**Imtryingtofixit** · 01-10-2025 09:40 AM

I belive you can rent one from auto zone or most local parts store

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**Mike Tries** · 01-10-2025 12:37 PM

Replacing the harness looks like an intimidating job, but it's really not bad. Just take your time and do it right the second time.

**Audicouver** · 01-10-2025 01:07 PM

Not to worried about the install, just wondering how I order 1 from Avantnoir?

**Mike Tries** · 01-10-2025 01:16 PM

PM him. He was the first one who responded to you above.

**AvantNoir** · 01-10-2025 02:15 PM

Originally Posted by Audicouver

OK I checked the connector wiring, they arent the prettiest but they arent terrible either. Cylinder 3 injector 1 wire the insulation was cracked so I threw some electrical tape around it. Some of the other injector wire have electrical tape around them so peeled them to check the wires are still connected and they were so I retaped them. The entire injector wire harness loom is just electrical tape around all the wires, no protective plastic sheathing.
Cylinder 2 coil wire the protective plastic sheathing has crumbled away a bit and there is a bit of green corrosion coming out of it, this could be my issue Im guessing?
How much does a new wiring harness usually run?
Any other possibilities?

Also I will try to track down my EVAP leak, need to learn the system to do this though.

Wiring most likely the source. Tape on it is a bandaid on a shotgun wound.

Replacement Fuel Injection harness is $135 direct from me. Or $145 on ebay.

**AvantNoir** · 01-10-2025 02:16 PM

And most likely, the coil pack wiring is shot.

**Audicouver** · 01-10-2025 04:24 PM

PMed you AvantNoir. Maybe Ill just get both off of you if you have them available

**Audicouver** · 01-13-2025 05:36 PM

So this morning I swapped spark plugs in cylinder 1 & 2 and monitored on my way to work. Misfire moved to cylinder 1, bingo! I realized they were NGK plugs so after work I got a set of 4 bosch plugs and changed them. Car wouldn't start... I swapped them all back and car still won't start. I'm guessing I somehow damaged the wiring to 1 of the connectors? Looking for insights.
I noticed when I was removing the bosch plugs they felt pretty hot. Hoping I didn't damage my ecu..

**AvantNoir** · 01-13-2025 08:11 PM

Yes, you probably damaged the wires on the coil pack harness. NGK plugs are good as long as they’re BKR7E gapped at no more than .028

**Audicouver** · 01-14-2025 09:26 AM

So I ran a code check in vcds this morning and have a P0688 ECM power relay load circuit : open circuit code. Maybe this is my problem, I blew a fuse or the relay is dead. Wondering which fuses, relays and power to check to diagnose this?

**AvantNoir** · 01-14-2025 09:31 AM

Originally Posted by Audicouver

So I ran a code check in vcds this morning and have a P0688 ECM power relay load circuit : open circuit code. Maybe this is my problem, I blew a fuse or the relay is dead. Wondering which fuses, relays and power to check to diagnose this?

http://wiki.ross-tech.com/wiki/index...2/P0688/001672
Usually wiring related.

And check your DM's

**Audicouver** · 01-14-2025 09:38 AM

Thanks AvantNoir

**Audicouver** · 01-15-2025 09:16 AM

So after reading a few posts about my no start issue I went and picked up a pack of 20A fuses and pulled fuse #29 and sure enough it was blown. Replaced it and the car started right up. Going to keep extra fuses handy and keep driving the car and not touch anything until I get my replacement harnesses installed for the coils and injectors. And yes I will install 1 harness and then see how the car runs before installing the other so if I have any issues with either install I can narrow down if I messed anything up. Will post an update once I've done this.

**Audicouver** · 02-12-2025 05:22 PM

Just an update, I've received my wiring harnesses, just waiting for some time to install them. Have also more recently had head issues that I've been dealing with as that is more important. A few weeks ago I cut all the boots off of the coil plugs and all of the insulation on the wires had disintegrated and thewires were corroded and shorting together. I carefully pulled them apart and isolated each 1 with electrical tape and since doing that I don't have any more misfires so that was definitely my issue. Will update again once I get the new harnesses installed.

**Audicouver** · 03-10-2025 11:47 AM

Update: I got both wiring harnesses installed and my misfire issues have disappeared. Thanks Avantnoir for all your help, great harnesses and instructions!