re. 1988 Audi 90 - Air-Fuel Mixture Problem

[Ben Swann]

Bruce,

Try disconnecting the O2 sensor while the problem is happening and see if 
the problem goes away.  You likely need a new O2 sensor and I have seen them 
cause the very problem you describe when they fail.

Don't replace the differential pressure regulator - at most a gentle 
cleaning would suffice, but unless you have been running gummy fuel, this is 
not likely a problem.

Ben

[Date: Tue, 4 Oct 2005 21:24:46 -0500
From: <schief53562 at peoplepc.com>
Subject: 1988 Audi 90 - Air-Fuel Mixture Problem
To: <quattro at audifans.com>
Message-ID: <000801c5c953$f4ebf4a0$b16c9741 at DDSNPH41>
Content-Type: text/plain; charset="iso-8859-1"


I am new to audifans and am not a motor guru but will do my best to describe 
my problem.

Our 1988 Audi 90 is a 5 cylinder 5-speed with 198,000 miles.  It has been my 
son's college car.  Early this summer it started running rough.  I was busy 
working on my house this year and we had an extra vehicle for him to drive, 
so once he got home from college, the Audi stayed garaged most of the 
summer.

About a month ago, I started driving it to work occassionally to see what 
the car was doing.  Like I say, I'm not a motor head, but it sure seems like 
a air/fuel mixture issue.  In general, the car chokes and stumbles at times, 
depending upon how long it's been running, whether you're idling down, etc. 
Will explain.

I drove it home from work tonight and it was the typical story.  The car had 
been sitting at work over the weekend and it started right up (one turn of 
the engine).  The first three blocks, the car ran fine.  I made my left turn 
(from a dead stop - car was still running ok) and then in the next 1/4 mile 
as I was driving about 35 - 45 mph, it started choking and stumbling when I 
would start idling down (when it would come down to about 2500 rpm).  For 
the next two miles it ran rough.  During this two mile stretch I had a 
couple of stops and as stated earlier, it was a challenge to get going from 
the dead stops.  I had to ever so gently give it more gas until it reached 
an rpm (around 2500 again) to where it felt and sounded like it had crossed 
the barrier and was at least putting out enough power without choking that I 
would be able to start out.  This gentle ramp up stage of the rpms takes 
about 5 - 10 seconds.  If you give it too much gas too quickly, it chokes to 
the point
 where it almost kills and you get a strong gas smell.

At about the 2 - 3 mile mark the car started running fine and continued that 
way for the next 10 miles or so.  Then it started running rough again.  When 
you're doing highway driving and it's running rough, what it will do is 
start choking a little if you idle down a bit.  I often keep it in 4th gear 
on the highway just to keep the rpms to around 3,000.  As I bring it down to 
2500 rpm it will start choking.  Choking when idling down is a common theme 
when it's running rough.  The other thing I noticed tonight is that there is 
some correlation to the braking and the engine running rough.  When the 
engine was running rough, I noticed that when I stepped on the brake as I 
approached a stop, the car would idle up from 1,000 rpm to 2,000 rpm.  When 
I let off the brake, the car would idle back down to 1,000 rpm.  Sure 
enough, during this phase when the braking affected the idle speed, getting 
started from a dead stop was a challenge.  In contrast, during that 10-mile 
stretch where
 I said the car was running fine I had a couple of stops.  At those stops, 
braking would not affect the idle speed.  It remained at 1,000 rpm.  Getting 
started from these stops was a cakewalk.  The car ran as it should with 
plenty of uninterruptible power to get started smoothly.

I've had the car into a couple of different shops in the last two weeks to 
figure this out.  The first shop replaced the air boot and oil breather hose 
and then on a second visit, did a tune-up.  Obviously, these items didn't 
fix the problem.  The second place performed the following.  These are their 
words from the invoice:
"Owner states car runs rough.  Sometimes when down shifting or coasting, 
sometimes when cruising at lower rpms.  Multiple possible causes.  Test 
drove.  Confirmed owner concern.  Inspected.  Possible power output stage 
malfunction.  Installed power output stage.  Some improvement.  Car still 
stumbles.  Replaced idle valve.  No improvement.  Disconnected fuel pressure 
regulator.  Engine ran ok but with disconnected regulator, starting car 
difficult at best.  Next step would be to replace regulator."

So . . . the last recommendation is to replace the fuel pressure regulator. 
I declined for now because they wanted $500 for the part.  Furthermore, I'm 
still wondering about the braking affecting the idle speed.  Is this a 
vacuum problem?  Supposedly the first shop checked all the hoses, but who 
knows?

Thanks for your help!

    Bruce]