[TurboLover]

Hello everyone I recently bought a 91' 200 avant and since I bought it it has begun hitting over boost fuel cut. I have been going through the SJM diagnostic guide and I believe it is the Pressure Transducer in the ecu.

I teed a boost gauge in the line going to the ecu and it read up to 15 psi before fuel cut. my dash boost gauge is reading very low, when I put 15 psi of pressure into the ecu line it reads 1.2 bar and at 15 in hg it reads .2 bar.

However the dash has a few issues like wavering speedo and most of the trip computer outputs don't seem accurate so i dont know if i can trust the boost gauge.

I disconnected the plug on the boost solenoid and the car works fine only boosting up to 6 psi which makes me think it is not the wastegate. did the output test on boost solenoid and it clicks.

am I on the right track? the thing I haven't really figured out is if the pressure transducer is bad how does it know it is overboosting? I imagine it must be with the MAF sensor i just haven't read that anywhere.

anybody replaced the Pressure Transducer? is that a common failure? Is there a way to test it directly?

Thanks.

 

[A20V200]

Check for any vacuum leaks. All the little 1/4 inch lines. Check all your hose connections/clamps etc. Pull your codes under the dash and that will give you a better idea. SJM has the instructions.

 

[TurboLover]

pulled the codes got 2224 "overboost fuel cut" checked hose to ecu for leaks, hose to waste gate for leaks, checked waste gate diaphragm if it holds vacuum, cleaned and checked MAF sensor,

Pulled the top off the wastegate to see if it was sticking and i noticed it sticks if you put any kind of side load on it is that common?

I haven't pressure checked the whole system yet because i need to go find a air compressor and a cap to fit the intake. I also cannot wrap my head around how a leak after the turbo outlet could cause a overboost condition as stated on the SJM page?

The thing is if the waste gate is working fine when i disconnect the boost solenoid. That makes me think the computer is telling the boost solenoid to stay closed for to long... making me think the computer doesn't know how much boost it is running. but if the computer doesn't know how much boost it is running how is it cutting the fuel?? oooo

Thanks for the quick response!!

 

[yodasfro]

The fuel is cutting because you're hitting 15psi. In stock form it should only make 12psi of boost. Has your WG cap been modified IE has the plug been taken out and has the adjuster bolt been tightened down? I would also make sure the vac. lines to the ecu are good and tight. In the engine bay you'll see a green plastic line coming out of the fire wall that's is ecu vacuum line. Could also be that wastegate valve is sticking. You may be able to pull it apart to put some high temp anti seize on the valve stem. You said you checked the the WG by applying vac. via mity-vac to the nipple on the WG cap? If so did it hold steady or slowly bleed off down? If latter it could be a torn diaphragm.

 

[AudiSport4000]

I was gonna suggest a torn WG diaphram as well.

 

[TurboLover]

The fuel is cutting because you're hitting 15psi. In stock form it should only make 12psi of boost. Has your WG cap been modified IE has the plug been taken out and has the adjuster bolt been tightened down? I would also make sure the vac. lines to the ecu are good and tight. In the engine bay you'll see a green plastic line coming out of the fire wall that's is ecu vacuum line. Could also be that wastegate valve is sticking. You may be able to pull it apart to put some high temp anti seize on the valve stem. You said you checked the the WG by applying vac. via mity-vac to the nipple on the WG cap? If so did it hold steady or slowly bleed off down? If latter it could be a torn diaphragm.

everything is stock. Vacuum line to ecu are good will hold 15 in hg of vacuum and I pressurized it with 15psi.
The vacuum line to the ecu is what i tied into to actually check the pressure.

waste gate diaphragm is not torn holds vacuum perfectly. put anti seize on what?

 

[yodasfro]

On the wastegate valve since you said it was binding? Another thing to check is the MAF, try another MAF. http://www.sjmautotechnik.com/trouble_s ... tml#oboost

 

[TurboLover]

if the wastegate binding was the problem, when i disconnected the boost solenoid wouldn't the problem still be there?

I don't believe the wastegate is binding. but I do not know how they are supposed to feel.
however, when I took the cap off and moved it up and down it was free except when pushed to one side at all it would stick which seemed odd. but like I said i have never played with one before. It seems to me that it should never experience any kind of side loading that would cause that so it is not a issue.

cleaned and checked MAF sensor outputs it is working fine.

 

[A20V200]

The WG only goes up/down...not side to side...throw some lube on the shaft......not the problem. Make sure you erase ALL of your codes, pull fuse or disconnect your ecu for a couple of minutes and plug back in. Sometimes it will throw a code, you pull the code and are erased automatically, but not always, they will stick around even if you have cured the problem, making you scratch your head and cuss.

 

[84urq20v]

Have you found the issue yet?

My 3B is doing what sounds like the same thing (so far only on 3rd gear pulls onto highways) - except I have an RS2 turbo and EM, and proper Motronic software. I have pressure tested the system many times - although not in the last few months - no leaks - and cannot hear any leaks on boost. My WG is tightened down several turns.

My thought was that, with my mods, the boost builds fast enough to exceed the software parameters before the WGFV bleeds it off - thereby tripping the overboost code - which sends engine to a limp home mode where it cuts out around 10 psi boost.

But with you being stock - I doubt we'd have the same issue, but...

Brandon
'84 urquattro 20Vt

 

[TurboLover]

No I have not figured it out yet I really still think it is my pressure sensor inside the ecu because my stock boost display reads .3 bar when the engine is turned off. When I pressurize the ecu boost line with 15 psi the stock boost gauge reads 1.3 bar which is wrong.

this also screams a bad ground. However, I just pulled my dash apart and looked for any broken solders and was unable to find any.

Thing is I have been unable to find a way to test the pressure sensor inside the ecu. the only thing I have found is that according to http://www.sjmautotechnik.com/trouble_shooting/10vgauge.html the sensor ranges from 0.2 to 5.0 volts. I think I am going to test it by assuming that it is linear from 0-2bar in that range then using a volt meter to test outputs. What I need to find is what output on the ecu goes to the dash display. Does anyone have a diagram showing the ecu plugs outputs?

 

[TurboLover]

Have you found the issue yet?

My 3B is doing what sounds like the same thing (so far only on 3rd gear pulls onto highways) - except I have an RS2 turbo and EM, and proper Motronic software. I have pressure tested the system many times - although not in the last few months - no leaks - and cannot hear any leaks on boost. My WG is tightened down several turns.

My thought was that, with my mods, the boost builds fast enough to exceed the software parameters before the WGFV bleeds it off - thereby tripping the overboost code - which sends engine to a limp home mode where it cuts out around 10 psi boost.

But with you being stock - I doubt we'd have the same issue, but...

Brandon
'84 urquattro 20Vt

You said yours is tripping the overboost code correct? do you have a pressure gauge and are you seeing it go boost to high? I imagine your ecu is chipped? However, I bet it still uses the same method as mine to determine over boost just at a higher limit. Mine allows boost to build all the way to 15psi and then it holds it there for a second before it finally cuts fuel. So I don't think I can see your car building boost so fast that it is grossly exceeding the preset boost limit. However, if that was the case your waste gate frequency valve is only pressurizing a line that opens your waste gate so your waste gate would be the factor that is not allowing you boost to regulate in time.

In that case, I feel like it has to do with your waste gate being tightened which I imagine is kinda the same as installing a stiffer spring? Because, in this system your waste gate is not controlling your maximum boost it is your boost solenoid which the ecu allows to send pressure to your waste gate when you are approaching the set boost limit, then causing the wastegate to open which bypasses the exhaust. The stock waste gate spring in my car opens at 6 psi. So if your wategate is now no longer opening at the point your ecu expects it to because of the stiffer spring it could not be opening in time to stop boost from exceeding the limit.

Sounds like a awesome car pretty much what I have always wanted to do is put a 20v in a ur. I obviously am just starting to understand these cars so I might be way off. good luck

 

[TurboLover]

Just thought I would let everyone know it was the pressure sensor. I fixed it by getting it replaced and the ecu chipped by IA :-D

 

[84urq20v]

Glad to hear you figured it out.

Mine was just exceeding the 2.5 Bar software limit (~22psi). Now my question is why all of sudden...? Nothing has changed on my setup in a long time. So I'm working on it....

Anyway glad to hear you figured it out.

Brandon

 

[auditude]

Just thought I would let everyone know it was the pressure sensor. I fixed it by getting it replaced and the ecu chipped by IA :-D

So the sensor tested bad or something?

If you have a leak in the hose to the WGFV or FPR you will get overboost also.

 

[MrTurbo0290]

Have you found the issue yet?

My 3B is doing what sounds like the same thing! (so far only on 3rd gear pulls onto highways) - except I have an RS2 turbo and EM, and proper Motronic software. I have pressure tested the system many times - although not in the last few months - no leaks - and cannot hear any leaks on boost. My WG is tightened down several turns!