Car running like sh$t...no codes, no leaks but runs well with MAF unplugged.

[Blue AWD]

So I was doing a pull in 2nd and right as boost was ramping up, all power cuts. Having blown a lot of TBB off, I assumed boost leak. I have done MULTIPLE boost leak tests and nothing. Car will idle fine but once I apply any throttle in just neutral it studders and wants to stall. Unplug the MAF and will rev freely like normal. I haven't been able to drive the car yet. No CEL and have some misfire codes.

Things I've tried:

New MAF sensor
New divertor valves

Now I know unplugged MAF means leak somewhere but there is nothing. I haven't replaced any check valves or anything.

Any thoughts? I'm worried the motor is popped or something but it doesn't add up.

[vavJETTAw36]

Log your misfires. I think it's measuring block 15 and 16. Report back. Could be one or more bad coils and or icms


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[e30mclow]

He is on 2.0T Coils, but yes, it will still show misfires.

You'd expect that if it did this while on it in a 2nd gear pull, that you'd at least get a code relating to boost deviation or something.

[Zba]

Check you MAF wiring.

[Blue AWD]

I would suspect a code if MAF wiring was off?

[Zba]

Computers can be finicky. Have you scanned with a legit scanner such as VCDS or are you assuming no codes because there is no mil?

[Blue AWD]

I have Vcds amongst other things. It all stemmed from one pull with no leaks so it's suspect.

[LINDW4LL]

This sounds exactly like a typical charge piping leak where a hose pops off during a pull. The same exact thing happened to my friend this afternoon.

How confident are you in your leak testing procedure? (Have to ask )

[cjt671]

Plugs good?

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[LINDW4LL]

Plugs good?

If it were plugs, then unplugging the MAF wouldn't change anything.

[nonstopgo68]

unplugging the maf takes it out of closed loop

[Blue AWD]

Pressure test is completely leak free. I don't think plugs would be issue here because of the way it runs with MAF unplugged.

[vavJETTAw36]

What do your fuel trims show? Log 32 and 33


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[hotshot32890]

just sell it to me cheap....im in boston and we have talked a handful of times.

[jpurre]

Did you hear any kind of "pop" when it cut power. You would have heard a pop if a intercooler hose came off under pressure. But you said you pressure tested, so... What about the brake booster vacuum valve directly behind the intake manifold? I had that fail once and it exhibited the same signs as you.

[Blue AWD]

Im not really sure if it made a pop or not but after blowing the TBB boot off several times it felt exactly like this. Absolutely no leaks to speak off. I haven't looked into the brake booster valve. I assume I can just remove it and plug the pipe to just see if it idles better. I'll give it a look otherwise I'm running out of ideas.

Did you hear any kind of "pop" when it cut power. You would have heard a pop if a intercooler hose came off under pressure. But you said you pressure tested, so... What about the brake booster vacuum valve directly behind the intake manifold? I had that fail once and it exhibited the same signs as you.

- - - Updated - - -

They seem to be within reason -3 and 4% on idle. Partial hasn't adapted as I haven't driven the car yet since this issue. Only idle and revs.

What do your fuel trims show? Log 32 and 33


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[vavJETTAw36]

Checking your idle trim at current state is useless unless you reset you clear your codes and check them quickly. They will literally adapt the car to compensate for any vac leaks. Your partial is what is going to tell us a clue into your overall health.


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[Blue AWD]

I have cleared it and checked immediately. I will try and take it out shortly but the car literally can not drive with MAF plugged in. I am going to do a compression check this week although it seems unlikely that the motor is shot since it reacts well to MAF unplugged.

I appreciate all the help and whatnot but I am 100% sure there are no boost leaks that I can find through a pressure test. I have been through every possible scenario with B5s in 15 years of ownership as well as 10 motor pulls and turbo swaps but this one has me stumped.

Only possible scenario I can think of is that the motor popped or turbo are blown (both unlikely I know but I'm out of ideas)

I checked shaft play and everything when I had the motor apart and they haven't seen a mile since, could a turbo be blown with no shaft play and good condition blades? I saw .08 of boost yesterday just revving so I assume this isn't an issue.

[Zba]

Try disconnecting other components until it clears up. I am still a firm believer that you have a wiring issue. Not necessarily a short to ground or B+ but high resistance or a couple of wires shorted together. If unplugging the MAF really sends the ECU into open loop then you have quite a few inputs to check. Personally I would start with the MAF wiring although you seem hesitant to check wiring since you brushed off my earlier post...

[Timtheguru]

I just changed my FPR, been about 15-16 years since the new one that came with the car was installed. Every driving condition is smoother now, and the FPR was only $30 on Rockauto. If you're running 5.0 bar then try another 5.0 bar in there. :) It's worth changing at this point, for this age of car.

[Blue AWD]

MAF and o2 wiring all good.

All check valves good.

Another pressure test up to 20psi. Good.

No codes except for MAF when I unplug it.

Will it do any good to log it with MAF unplugged for o2s?

[verstappen]

How's the idle? How old are your Coolant temp sensors? I had the same problem, only ran good without maf, changed maf, all boost hoses, etc. new spark plugs and 2.0t coils. nothing, no codes... One CTS was bad and I was dumping fuel when boost was coming on. bogging down, I also saw the throttle plate was closing down at that point, the car was falling on its face. New cts's and boom, problem solved.

[Blue AWD]

All are new besides the one on the passenger side of motor. Took it for a short spin today with MAF unplugged and it was running pretty badly, barely made it up a small hill. Trying to move it with MAF plugged in and it won't even move. No codes still. Compression test tomorrow.

[e30mclow]

When did you last fill up with gas?
I chased a similar demon last year, turned out to be bogus gas. Plugging things in & out made differences, but it was gasoline in the end.

[Timtheguru]

Like I said, swap your Fuel Pressure Regulator. It's either air or fuel, and you seem to have covered air..

[Blue AWD]

Swapped fpr. No change.

[Blue AWD]

Did a compression test on the motor and everything came back very solid. Changed spark plugs, no change. Still no codes. Clogged fuel filter? Dieing fuel pump? Exhausted over here...

[e30mclow]

You poor bastard.

Definitely not shitty fuel?
I had fuel pressure, spark and air - it ran so bad it popped an intercooler hose when it started to cough. I only had about 300 yards warning and it needed a tow.
Weeks digging into it, and it was finally draining the fuel tank, running fresh fuel through the lines and out the fuel rails that solved it.

I had tried:
N75
MAF
MAP
Plugs
Compression
FPR and EVERYTHING ELSE

[Blue AWD]

I will try fuel. It literally happened out of nowhere.

[e30mclow]

Sounds silly, but you can smell if the gas is off.
Pop off the "in" line by the air box, and turn the key to prime the pump. That'll get the potentially problematic fuel into a container so you can at least take a look at it.

Mine didn't have much potency to it, which made me suspicious. Eventually I drained the tank which couldn't have been easier.
It took a bit of messing around to run good fuel through it before it came around, but it then started up fine and never gave a similar problem again.

[Zba]

Yes, what the Irishman said. If the fuel is contaminated you will it see it as separation.