frob

Hi folks, new member here, desperately seeking help!
I have a 99 A4 1.8T Quattro (~ 170k Km) , which about a month ago started doing this while on a long car trip, seemed to happen only occasionally but now consistently whenever the car is started cold-
no problem starting ( instantaneous) but idle speed is unsteady, and when taking off, a few seconds later the engine really bogs down, hesitates, gives an occasional small backfire, and just seems to loose practically all its power, barely being able to accelerate at all. flooring the pedal only makes it worse. then, after a minute or 3 of this, the car surges forward and everything seems to go back to normal, for the remainder of the trip.
The CEL has been on since i got the car, i took it then to an independent Audi-VW mechanic and they said the turbo wasn't working ( no boost) due to a broken wastegate, they found bits of it in the front of the cat. Other than having a little less engine power, the car has been working fine and was drivable. I was waiting to find a good deal on a used turbo to change it, until this last problem appeared.

The mechanic spend an hour checking things last week and came up empty - he's baffled. Coils are good, changed plugs, no apparent vacuum leaks.
Said maybe the cat, but every experienced mechanic / muffler shop says when the cat is plugged, it stays plugged, so it wouldn't act intermittently like this is.

Scan codes read:
O2 sensor - B1S1
Charge pressure deviation ( i.e. turbo not working, knew about that)
B1S2 Limit reached
LTFT B1 Lean
LTFT B1 Rich

Any ideas ? Could this all be caused by a bad O2 sensor, or is that just a symptom of a dead cat?

marcj

fuel pump? could explain the sudden loss of power and lean condition.

frob

That occured to me too. If it were that though, i would expect to have trouble starting it too, no?
I bought a new fuel filter and will try to change that tomorrow.
If i can find appropriate fittings i'll check the fuel pressure. according to my new Haynes repair manual it should be ~ 55psi. I suppose i should invest in the proper Audi shop manual at this point, don't know if they're even available or how expensive that would be.

Anyone know if a dying fuel pump would normally be identified with a DTC, or does the ECU have no way of knowing ?

25orsix24

FROB: I suggest you FIRST fix what is broken: the waste gate! Assuming that that (waste-gate) is not affecting the normal operation of your car is not the correct process for trouble-shooting.

frob

I'm up for that- thanks for the suggestions.
I just removed and inspected the cat, inside its as good as new.
This also allowed me to inspect the turbo exhaust outlet and see that the problem there is really just the waste gate "f'lapper valve" button that had fallen off, and landed in the cat where it made only a small dent into one side of the catalst, hardly even noticeable.

The flapper part is long gone, but it looks like the turbo is otherwise serviceable.
Is there any way to get just the replacement waste gate lever arm/flapper assembly so i can repair it in situ? that would be both a lot cheaper and easier than changing the whole turbo.

The fuel filter they sold me was the wrong type, so now i have to go exchange that for one made for the bajo fittings.
If i can get my hands on a front O2 sensor at the same timei'll change that while i've got the airbox off, and pray for a miracle...

Thanks and keep please the ideas coming !

25orsix24

FROB: I will suggest that at 170,000 Km you seriously look for a new turbo.
Turbos do not last forever, and while you should not look for fixing something that is not broken, in this case the thing is at "end-of-life".
Look for something a little bigger than OEM for that added kick!
Later on you can replace the DV, injectors etc. and get a nice "chip" and the car will make you fall in love with it all over...
Just a suggestion!

frob

I would love to do that, but the financial reality right now is that i cant afford to. i just need to fix whats really broken with absolute minimum cash outlay until the situation changes. When it does, I'll probably trade it in for a new(er) model, rather than put a lot of money into performance enhancements into a car that is nearly end of life, which could just push it over the edge anyway without a major engine rebuild.

I had a phone discussion with the senior mechannic at the local dealer and he suggested the most common cause of this kind of symptom, which doesn't kick any diadnostic codes, is a bad mass airflow sensor, and possibly the coolant temp sensor. any idea how to test these two to see if they're really bad?

Update:

Well, its not an intermittent problem anymore, it seems to be permanent now, i.e. the car is pretty much dead. i used electrical contact cleaner on both the O2 sensor and the MAF, and used shop air to blow them dry, ( no touching the MAF) so i'm guessing in doing so i inadvertently finished off the one of those two that was dying. i'm hoping its the O2, i rather not have to change the MAF, anyway it looks mint.

25orsix24

FROB,
Firstly: I am not sure what shape your car is in but a 10 year old AUDI, normally has a lot of life left in it! IT IS WORTH INVESTING INTO IT!

Secondly: The golden rule of trouble-shooting says that when in doubt you replace the cheapest, easier to access parts first.
But keep in mind Murphy's Law which says that the $100 part will blow to protect the 5 cent fuse.

frob

Yup, agree there.

Having already spend over $2G on repairs over the last year, and now just found/bought a used turbo and new stainless flexpipe and exhaust manifold, i'm pretty committed to restoring her to top shape, with maybe a few more HP to boot.

Since the front end needs to come off to do the timing belt/ water pump, i might as well go ahead and put in a $50 front mount intercooler, which might help keep her running cooler ( of course i wont complain about a few extra horses either).

frob

Well i'll be a Bavarian monkeys's uncle.

Got the new O2 sensor in and ..... drumroll.... tada! she's purring like a hungry Doberman on a T-bone. (I would asaid like a kitten, but the rusted exhaust coupler after the flex pipe now leaks so she purrs a lot louder now...)

The CEL even went out on its own!! i'm really pleased about that, its the first time i've ever NOT seen it on my dash.
Guess that was it all along. I'm still glad i got the cat off to confirm its still in great shape inspect the Turbo. I found some info on these sensors, and found the heater is fine, in the old one it showed 3 ohms, so i'm guessing the actual sensor element got contaminated or otherwise compromised due to bad gas on that road trip.