Intermittent underboost issues

[PRY4SNO]

I have a persistent MIL light that pops up and goes away after a while, can't remember the exact code but it's for failure to meet requested boost.

Asked the mechanic to log measuring block 115 and he said it's meeting boost required during drive up to 120 km/h at 2080 millibar.

From another thread that held my other clue:

As far as the code goes for requested boost not meeting actual boost, this is typically a hardware issue and not a software.

Potential reasons for this would be...

1.) Boost leak...could be anywhere, most likley it exists in a check valve coming off of the intake manifold or a blown diverter valve (710N). Injector seals are known to leak as well if the injectors have been removed and replaced.

2.) Bad MAP sensor, there is no actual leak, however the software thinks there is because the sensor is not reading correctly.

To rule out the MAP sensor, you will need a VAG-COM and log measuring block 115. Compare the actual boost shown to a boost gauge you have in the car, if you do not have one installed, you can borrow one and tap into the boost reference signal on the manifold temporarily.

A pressure tester will help isolate leaks.

My mechanic wasn't concerned enough about the MAP sensor, or leaks in the plumbing to start testing them, insomuch as pointing toward potential software issues, and he mentioned an airflow meter that could cause sporadic boos issues. Said it was a $300 maybe...

Any suggestions?

TIA

[PRY4SNO]

ttt

Any suggestions?

Pretty please???

[FourRings80]

Faulty N75 ,N249 or G31 sensor? 5.1 bar, that's like 100psi of boost.

[PRY4SNO]

Did you do a boost leak test. Faulty N75 maybe? 5.1 bar, that's like 100psi of boost.

Sorry, thought the ***EDIT*** was clear... I'll edit it better this time. Boost requested vs. actual was 2080 millibar, twice in a row up to 120km/h / 75mph.

No, I haven't had time to do a leak/smoke test (barely managed to eek out my oil change... 1,650km over interval, *erg*) and my mechanic seems so incredibly sure it's not the N75 or MAP sensors that I haven't investigated further as of yet due to lack of time/cash. Kind of want to get it right the first time if you know what I mean.

He keeps pointing to an airflow meter and/or the software. He's becoming a Unitronic dealer so at that time he can help (thus saving me the 3 hour round trip) with the software but said the airflow meter was a $300 maybe, at best.

[PRY4SNO]

Faulty N75 ,N249 or G31 sensor?

Okay, here are a couple things I can sink my teeth into with the search button!!!

I was thinking N75 myself but, really, what the hell do I know... there's so much to the turbo system on this car that it could be anything and I'm unfamiliar with it at this point. FWIW the boost hoses on the bottom of the car don't feel as though they're losing any pressure under load.

[FourRings80]

sorry about the edit part, my bad.

Did a search on rosstech boost related CEL, and 95% of them relate to N249, N75 and G31 :
http://wiki.ross-tech.com/index.php?...it=20&offset=0

good luck and I hope it's nothing big

[ProJect_*A4*]

Not sure if this is it, but other day i got mail from audi stating that they are extending the Emissions Control Systems Warranty for the engine coolant temperature sensor (ECT)

`reason being that the ECT (and important emisson control device) might malfunction and will cause the malfunction indicator lamp (MIL) to go off.

not sure if this helps but its something i would look into, I JUST recieved the letter.

then again i can be totally wrong.

GOOD LUCK! =]

[PRY4SNO]

Right on guys, thanks! I'll be looking into it!

[mike-2ptzero]

Was it a Deviation code? It isn't just the peak boost reading, you have to look at the whole boost log during the run because if there is a good size gap between requested and actual boost the car can throw a boost deviation code. Usually this happens if it is more then 300mbar which is about 4psi.

[PRY4SNO]

Was it a Deviation code? It isn't just the peak boost reading, you have to look at the whole boost log during the run because if there is a good size gap between requested and actual boost the car can throw a boost deviation code. Usually this happens if it is more then 300mbar which is about 4psi.

OK, I think I get the gist of what you're saying: make sure the boost is on target throughout the entire range. Not just lining up at peak boost? Correct?

When I VAG the car, how / in what channel do I observe said Deviation code (maybe a link to an existing writeup)? What specific code(s) should I be on the lookout for? I'll be getting set up to do some heavy logging in a few weeks, but I'd like to have the info handy if an opportunity arises beforehand.

I'll be installing my boost gauge once I get set up at work, so this coming week in the evening(s). Then I can physically observe it throughout the range. With stage 1 weak sauce, I'm looking to peak at 21psi and hold 18psi, no?

Thanks Mike!