Major Overboosting Issues

[gilber33]

Well, I just picked up an 05 A4 Avant with 144k miles a few weeks ago. I babied it until I was able to give it a complete tune up. It drove ok and felt like it was boosting ok. I just finished working on it. The work consisted of all new coolant hoses, forge tip, all new vac lines, new pcv, new ko3 turbo, turbosmart recirc diverter valve, spark plugs, and maybe a couple other things. Also installed a boost gauge.

All back together and it's running great. The old tip had a couple tears and the spark plugs were worn to a .04+ gap, so it's not surprising. My idle is great at -20 in/hg and -22 on decel. But my boost seems to be erratic. It was easily boosting to 20psi in higher gears. And this was at partial throttle. I haven't gotten on it yet because I don't know if it will stop at 20 or keep going. I've done a couple steady pulls in lower gears and the boost creeped up to 15 psi and let off the throttle. This is driving the same way it did before in regards to boost characteristics, it feels the exact same way. So I do not believe it's something I did.

Wondering what I could check? I do not have a cel or anything. Not sure if a bad N75 valve would trigger that or not. Does anyone know what stock wastegate pressure is? I was thinking of running the vac line from the compressor housing to the wastegate to rule out the N75.

Any input would be appreciated.

Gabe

[old guy]

Stock wastegate should open around 6 psi. Check your hoses from the high pressure port on the turbo to the N75 and from the N75 to the wastegate. You may have a crack or split in one of those hoses. This would prevent the N75 from passing the boost pressure to the wastegate and consequently you will be in an overboost condition.

[gilber33]

Brand new silicone vac lines on that. I will check them again, but it shouldn't be that.

I guess what I could be feeling is the wastegate not opening and the turbo is continually boosting.

[old guy]

Brand new silicone vac lines on that. I will check them again, but it shouldn't be that.

I guess what I could be feeling is the wastegate not opening and the turbo is continually boosting.

Unplug the electrical harness to the N75. That will prevent the ECM from activating the N75. It should also limit you to the wastegate spring setting. If you unplug the N75 (leave the vacuum lines in place) and still get high boost you probably have a sticking N75.

[gilber33]

Thanks. I will check that first.

[Audibot]

This wasn't that car up near Buffalo, was it? If so, I was seriously going to go after it until it just vanished

[gilber33]

I'm in Wisconsin.

[eljay]

I'm in Wisconsin.

With that boost level, you were probably in Buffalo just a few minutes ago!

Let us know how goes the N75 test. I think I'll be checking mine soon as well per OG's instructions.
I think you canalso test N75 when it's fully warmed up by checking the resistance across its terminals. A previous advice I got is that it should be around 25-35 Ohms.

[Furly]

Plot twist:
You purchased a car with a tune and you're hitting 20psi because it's running properly now.

[old guy]

Plot twist:
You purchased a car with a tune and you're hitting 20psi because it's running properly now.

LOL....Certainly a good possibility. What color are the fuel injectors?

[gilber33]

Plot twist:
You purchased a car with a tune and you're hitting 20psi because it's running properly now.

That thought crossed my mind. I found a few things that I believe is not factory.

Amber has been removed from the headlights, it has a poly snub mount, and this shift knob:



Leads me to believe some type of enthusiast owned it.

Is there any way to see if it has a tune on it with vcds?

[old guy]

Is there any way to see if it has a tune on it with vcds?

Yep. Check your requested/actual boost.

Here is a great chart for checking measuring blocks: Clicky click®

[gilber33]

Yep. Check your requested/actual boost.

Here is a great chart for checking measuring blocks: Clicky click®

Thank you! I will be ordering vcds very soon so I will look into it. It's hard to drive the car and feel like I have to be very careful.

[eljay]

That thought crossed my mind. I found a few things that I believe is not factory.

Amber has been removed from the headlights, it has a poly snub mount, and this shift knob:



Leads me to believe some type of enthusiast owned it.

Is there any way to see if it has a tune on it with vcds?

Certainly looks like an enthusiast owned the car seeing that you also have a B7 center console with cupholders and RNS-E headunit.

[Charles.waite]

Plot twist:
You purchased a car with a tune and you're hitting 20psi because it's running properly now.

I was going to say this...

My b6 boosted to 22psi all day (well spiked to 22 and tapered down to 16-17ish) on APR Stage 1+. If you were hitting those numbers on a stock ECU you would be in Limp mode for sure.

My best guess is its tuned.

[gilber33]

Certainly looks like an enthusiast owned the car seeing that you also have a B7 center console with cupholders and RNS-E headunit.

I didn't know that was a b7 center console. But now that I looked at some pictures, I see that. Wow, I'm really glad that was swapped. Not having those cup holders would be super annoying.

I was going to say this...

My b6 boosted to 22psi all day (well spiked to 22 and tapered down to 16-17ish) on APR Stage 1+. If you were hitting those numbers on a stock ECU you would be in Limp mode for sure.

My best guess is its tuned.

Interesting. I want to confirm in vcds now.

[MacFady]

As OG mentioned, what color are your fuel injectors? You should at least be able to tell if it's a result of tuning by looking at the injectors, no need for vcds, you wouldn't be at those numbers with a tune on stock injectors IMO.

[gilber33]

Best pic I could get with the injectors installed. They look to be light blue. I can pull the rail off after work and get part numbers off of them.

[old guy]

Light blue would be the TT225 386cc injectors. You are tuned with a big injector file ;)

[eljay]

Best pic I could get with the injectors installed. They look to be light blue. I can pull the rail off after work and get part numbers off of them.

They look greenish to me, but that's likely the lighting.
If they are light blue, they could be the Bosch 386cc. If they are green, then they are 440cc. Click here for possible options.

Sounds like you are discovering Christmas presents all around the car. I would check under the spare tire to see if there's a pot of gold there.

[gilber33]

Light blue would be the TT225 386cc injectors. You are tuned with a big injector file ;)

I'll pull the fuel rail this afternoon. It looks like there are part numbers on the side of the injectors just to confirm.

But this is so far a pleasant surprise.

[gilber33]

They look greenish to me, but that's likely the lighting.
If they are light blue, they could be the Bosch 386cc. If they are green, then they are 440cc. Click here for possible options.

Sounds like you are discovering Christmas presents all around the car. I would check under the spare tire to see if there's a pot of gold there.

They are definitely not green. I've had the 440s in several builds over the years and these are not them. It's really bad lighting.

[old guy]

Clicky click®

[gilber33]

Confirmed. They're the 386cc injectors.




I have a set of low mileage used ones I bought in anticipation of getting a tune that I'm going to put in since I dont know the age of these ones. At least I don't have to buy a tune!

[DownhillA4]

I have to ask why or how you bought this without the person not telling you or you knowing all this is aftermarket, haha. Welcome, in for more photos. I have a feeling you got a steal on a gem that you don't even know about.

B7 Console with RSN-E upgrade means previous owner had taste, I like.

[Charles.waite]

Probably bought it from a dealer.

[gilber33]

I have to ask why or how you bought this without the person not telling you or you knowing all this is aftermarket, haha. Welcome, in for more photos. I have a feeling you got a steal on a gem that you don't even know about.

B7 Console with RSN-E upgrade means previous owner had taste, I like.

I've owned 15 Volvos over the past 5 years. Decided to mix it up and bought this kind of spur of the moment. I don't know what interior upgrades are the hip thing to do or what to look for.

Probably bought it from a dealer.

Correct, from a dealer. I believe I'm the second owner. I think the 1st was the enthusiast, and the second was the deferred maintenance type.

[Bmase]

Drive it like you stole it don't be scared to go into the 20lbs of boost. The 1.8t loves that much boost from the baby turbo it has. Maybe check the turbo to and see if you have a Ko4 as apposed to the ko3

[eljay]

Unplug the electrical harness to the N75. That will prevent the ECM from activating the N75. It should also limit you to the wastegate spring setting. If you unplug the N75 (leave the vacuum lines in place) and still get high boost you probably have a sticking N75.

Sounds like OP discovered many pleasant surprise on his car, but the title of the thread is relevant to what is happening to me, so if I may pick up on this...

My 1.8T with GTRS Eliminator kit has been surging and recently I've noticed what I'd consider major overboost. Podi gauge showed a noticeable spike over 30. The boost has not been very smooth for a while, peaking around 24psi, which I think is inline with the mods I have. And I've only seen it spike to 30 only once before almost a year ago and after I replaced the DV, some vacuum lines etc. I thought I was fine. I have tested my N75 resistance and it is within range, so I thought it was all fine. However, today, I decided to perform the above test and I unplugged the N75 connector. Well, the car is just running incredibly smoothly (besides a small misfire on idle that's been there before). The boost builds smoothly, peaks at 14psi, which I believe is exactly the wastegate rating for the GTRS turbo. It's so good, that I almost want to say to hell with power, I'll keep this smooth running car as is. Of course, that would not be addressing the problem.
So, does this mean I have a faulty N75 and I should replace it? Or what other diagnosis/test should I do?

p.s. No codes showing.

[agentsmith988]

Sounds like OP discovered many pleasant surprise on his car, but the title of the thread is relevant to what is happening to me, so if I may pick up on this...

My 1.8T with GTRS Eliminator kit has been surging and recently I've noticed what I'd consider major overboost. Podi gauge showed a noticeable spike over 30. The boost has not been very smooth for a while, peaking around 24psi, which I think is inline with the mods I have. And I've only seen it spike to 30 only once before almost a year ago and after I replaced the DV, some vacuum lines etc. I thought I was fine. I have tested my N75 resistance and it is within range, so I thought it was all fine. However, today, I decided to perform the above test and I unplugged the N75 connector. Well, the car is just running incredibly smoothly (besides a small misfire on idle that's been there before). The boost builds smoothly, peaks at 14psi, which I believe is exactly the wastegate rating for the GTRS turbo. It's so good, that I almost want to say to hell with power, I'll keep this smooth running car as is. Of course, that would not be addressing the problem.
So, does this mean I have a faulty N75 and I should replace it? Or what other diagnosis/test should I do?

p.s. No codes showing.

Have you tried checking resistance when the engine is still hot? My N75 tested good at ambient temps, but was out of spec when hot (>35 ohms).

[eljay]

Have you tried checking resistance when the engine is still hot? My N75 tested good at ambient temps, but was out of spec when hot (>35 ohms).

Yes, I did. My notes from last year show 29 ohms cold and 31 ohms warm after a good run. I will retest it today.

[eljay]

just tested it right after a morning drive and it's a steady 32.6 ohms when hot.

A new valve is $100, so I don't want to buy one unless I know I need it.
I haven't tried logging the car yet. I think I can test the N75 duty cycle with VCDS, correct?
However, I'm hesistant doing a logging run at WOT until redline with the boost surging this high. I don't want to damage anything.

[old guy]

It may simply be that your N75 is sticking a little. Try removing it and cleaning it out. You can use WD-40 or similar. Apply a 12v signal to cycle it on and off while cleaning the solenoid. You might get lucky.

[eljay]

It may simply be that your N75 is sticking a little. Try removing it and cleaning it out. You can use WD-40 or similar. Apply a 12v signal to cycle it on and off while cleaning the solenoid. You might get lucky.

Thanks OG!
If that fails, do the symptoms point to a bad N75 or can something else be the culprit?

The car has a Maestro tune and one of these days I will have to read up on all that tuning stuff and see what it is I actually have there.

[old guy]

That would be my guess based upon the information provided. Apparently this is a new development and previously your turbo boosted OK. You have determined that your wastegate is functioning properly. You aren’t getting any codes to indicate an electrical issue with the N75. Your N75 coil resistance is in spec.

The most logical explanation would be a sticking N75 solenoid. Another possibility is that your MAP sensor is acting screwy. You could get some insight into that by logging your actual versus requested and look for extreme variations.

[agentsmith988]

just tested it right after a morning drive and it's a steady 32.6 ohms when hot.

A new valve is $100, so I don't want to buy one unless I know I need it.

I just purchased a revision E N75 from ECS last week. They're on sale right now for $49.95 with free shipping. A bit less painful than the $140 the my local stealership wanted for one...

[alimo20]

wow OP i can't believe your luck. that is amazing! Christmas in early august for sure

[eljay]

I just purchased a revision E N75 from ECS last week. They're on sale right now for $49.95 with free shipping. A bit less painful than the $140 the my local stealership wanted for one...

Sadly, I'm in Canada.

[eljay]

That would be my guess based upon the information provided. Apparently this is a new development and previously your turbo boosted OK. You have determined that your wastegate is functioning properly. You aren’t getting any codes to indicate an electrical issue with the N75. Your N75 coil resistance is in spec.

The most logical explanation would be a sticking N75 solenoid. Another possibility is that your MAP sensor is acting screwy. You could get some insight into that by logging your actual versus requested and look for extreme variations.

Just looping back here for anyone peaking into this thread because of its title.

I shelled out the CAD$105 for a new OEM N75 valve, installed it with new clamps and boom... smooth smooth boost. Wow. It has not been this smooth since I've owned the car and because I've never owned a big turbo 1.8T before I didn't know what should it feel like.
Very happy now!

After installing it and returning from a test drive with a smile I've handed the keys to my wife and she went to take care of some errands. 10 mins later I get a text message that whatever I did, she absolutely loves it and it's a completely different car. I just need to keep repeating to her: no boosting and redline until the oil is warm!
Who knew N75 valves make wives happy.