1988 Audi 90 - Air-Fuel Mixture Problem

[Tony Hoffman]

Yes, it sounds like the car runs rich. The fact that the brakes bring the engine speed up confirms this. The brakes use vacuum, and it acts a lot like a vacuum leak in that case. The first thing I'd check is the engine coolant temp sender. I beleve it is the one at teh front of the engine on the bottom of the upper radiator hose neck. It has four wires going to it, Check the plug for good contact, and there is a way to check the resistance of the sender itself with boiling water. I'm sure someone can send you the values from teh manual if you need. That coolant temp sender basically tells the engine control computer how warm the engine is. It will make the engine run richer if cold, then leaner as it warms up. Next thing to check would be the connection on the differential pressure regulator. It is the grey plastic thing on the front of the fuel distributor. The fuel distributor is the alluminum piece on the passenger side with the braided lines going to it. 
 
Hope this helps,
Tony Hoffman

schief53562 at peoplepc.com wrote:

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
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