very rough-running 1987 Audi 4000CS Quattro

I have a 1987 Audi 4000CS Quattro with 145000 miles on it. The car currently runs very rough, with the following symptoms:
1) Idle warbles between 800 and 2000 rpm
2) car shudders and lurches, as if it isn't getting enough fuel, when coasting downhill or accelerating uphill. It is roughest when I step on the accelerator or suddenly release the accelerator with the car still in gear.
I had the O2 sensor and the idle valve replaced, as well as the cold start valve. The only known issue with the car is a hole in the muffler - my guess is that somehow the hole in the muffler is distorting whatever sensors are in the exhaust system?
I am considering replacing the muffler, but wanted to have a better sense that it is causing this problem.
Thanks for any advice, Audi fans!
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Idle and rough running sounds like some combination of: vacuum leaks:
Vacuum leaks: check by removing the oil filler cap wen running. It should cause engine to die. Remove dipstick when idling. It should cause engine to run bad and nearly die. If either of these don't cause bad running, check all vacuum lines and repair as necessary.
dirty fuel filter: replace if dirty or old
dirty Idle Stabilization Valve: remove and clean with brake cleaner spray and dry well. Check the closed throttle switch in the throttle body to be sure it is telling the ECU to ISV.
May want to try injector cleaner too.
Tony '91 100Q 5spd
Chris Donahue wrote:

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Like TonyJ says, check the 'closed throttle switch' -- it's below the throttle butterfly casting (opposite the switch that detects full-trottle which is on top). My 2.2L 5-cyl did the same lurching and idle warbling, until someone else suggested I check this switch. I just soaked it with WD40 and voila!
Jaz

{ Please excuse the 'burp' when replying } "O""O""O""O""O"
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While you're at it, pay particular attention to the big rubber 'vacuum junction' the ISV plugs into. It has about 5 different sized appendages going off in weird directions from it. It tends to get hard (from engine heat, IMO) on the bottom and cracks there. It's not very expensive or difficult to replace and is quite possibly your problem. As a very temporary measure, you can slap some RTV goop on the crack and wrap it with about 20 feet of duct tape (The Handyman's Friend) and let it cure overnight before driving it. Don't expect that to last even a week. Just long enough for your special-ordered new one to come in.
OTOH, all of us have avoided mentioning the $$$ po$$ibility: the fuel differential pressure regulator. It's the little grey-green electronic thing on the front of the fuel distributor with two screws and a connector going to it. It's labeled "Bosch", which explains its $260+ price tag. At least you can install it in under 5 minutes ... -- C.R. Krieger (Been there; done that - on the road)
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Seems to me that I remember someone checking this by unplugging it. When unplugged the engine ran, not well, but corrected the symptoms. This is just a SWAG and others might have better information.
Tony '91 100Q 5spd
C.R. Krieger wrote:

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Chris/Tony, Do normally aspirated (aka non-turbocharged) engines stall when the oil cap is removed? My '80 5kS (ok, it's not the same engine, etc.) runs fine when you remove the cap - see the camshaft merrily spinning in there - flinging oil out and everything. (then again, my '80 doesn't have a O2 sensor, so....) You're right on with the vacuum leaks - check the big rubber meter to throttlebody corrugated boot - they often have cracks on the underside. Don't use brake cleaner on the ISV, though....yikes!!! Brake cleaner is way too agressive and will destroy the rubber seal in there. Use throttlebody cleaner - even carb cleaner will damage the rubber. Spray the stuff in and use a 9 volt battery - yes, 9v, not 12 - to operate the valve open and closed a few times. The hole in the muffler will also decrease the backpressure in the system, try patching it with whatever muffler bandage you can get/make and take the car for a drive to see what has changed/improved. Cheers! Steve Sears 1987 Audi 5kTQ 1980 Audi 5k 1962 and '64 Auto Union DKW Junior deLuxes (SPAM Blocker NOTE: Remove SHOES to reply)

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Steve, thanks for correcting the ISV cleaner recommendation I made.
The oil filler cap does kill mine when I remove it. My '91 is of course the NA NF engine and does have an O2 sensor. I wouldn't think that the O2 sensor would be involved in that check because it should only be affecting the timing and not the vacuum system.
Tony '91 100Q 5spd
Steve Sears wrote:

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Thanks all for your suggestions. I'll try some of them out this weekend, and if I get stuck, will bring a printout down to my mechanic (Tony Dvorak at Euro Quattro in Mountain View, CA, in case any of you are from the area) next week.
My last symptom which I forgot to mention is that the car runs much better at highway speeds than at slower speeds. If it is a vaccuum problem, is it possible that high speeds makes the vaccuum issue less noticable?
Thanks again,
Chris

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That would be my guess.
Tony '91 100Q 5spd
Chris Donahue wrote:

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I got this problem fixed at Advanced Audi VW in San Francisco (415-901-7270) - the problem turned out to be a bad vacuum advance, and a broken ground for the fuel injector (broken at the manifold). With these replaced, the car ran perfectly. Thanks all for the suggestions - hope this dialog proves useful to someone in the future.
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