mattmondo

Hi All,


Audi is finally starting great 100% of the time! Now the other pain....


Idle is always slowly revving from 1200 to 1400, then back down to 1200 to 1400, drops down.... well you get the idea. Not a show stopper, but annoying as hell. When standing near the open hood by the fuel distributor, when I hear the plate drop and the idle drop down.

Replaced ISV with a known good unit (also hear it pulsing) but no change.

Any ideas where to start?

Satch0922

I was going to say the Idle Stabilization Valve but you have that covered. Only other thing is to chase a vacuum leak.

socalclimber

Did it suddenly start to do this, or has it been progressive? Any history going on? Miles on current oxygen sensor?

Have you ever replaced oxygen sensor? Sometimes mixture control (oxygen sensor) feedback loop can cause irregular idle as it's trying to compensate.

Look for vacuum leaks, old vac. hoses, injector o-ring seals.

mattmondo

Brent,

No, never changed the o2 sensor. Would the computer throw a code if it is having issues reading the signal?


It did it for a while a few months ago, then stopped. Now for the last 2 weeks it has been doing it. Also, get thew occasional 2800 rpm idle and checked the throttle cable for getting stuck, but nothing.

I could start looking for old vacuum lines and change one-by-one, not to create a new problem by doing everything at once!

New injectors 6 months ago....

socalclimber

Searching for "idle hunting" turned up these:

Bad coolant temp sensor
https://www.audiworld.com/forums/sho...t=idle+hunting
seems like a good place to start, low cost comparatively

Basic idle speed adjustment with ISV unplugged
https://www.audiworld.com/forums/sho...t=idle+hunting
On some later cars, idle screw is sealed, and not adjustable

Vacuum leak at ISV hose
https://www.audiworld.com/forums/sho...t=idle+hunting

These are good places to start. Check for vacuum leaks, replace any suspect hoses or lines. If you have new injectors, they should have new o-ring seals (this can be a source of vacuum leaks).

Do you have a repair manual for your car? Bentley is best, in it there are specific tests for coolant temp sensor, reading KE mixture control, oxygen sensor, and reading basic fault codes.

Start with the basics, as above, and move on from there.

mattmondo

Good idea. Is the coolant temp the MFTS? I replaced mine, but not sure if it is a POS (China made, not German.)

No Bentley here! I'll have to get one soon!

I have to get the big vacuum hose from the crankcase to the valve cover. Mine is a 'custom made' hose from the previous owner....

socalclimber

The bentley manual is the best out there and very comprehensive. I have never worked with MFTS, but I found great info on an ECU temp sensor that you may have, see here:

ECU temp sensor info on 5000
http://www.sjmautotechnik.com/troubl.../10vcoolt.html

Don't know if you have 10v or 20v engine, or these apply to your car or not, 5000 turbo seems to be different from 5000CS, you have to investigate what you have.

See here as well, he talks about MFTS and related topics
http://www.sjmautotechnik.com/troubl...g/cooling.html

This site is pretty cool, just found it by searching for "audi 5000 cs mfts."

Temp sensor can be tested with normal volt-ohm meter with Ohms scale, there's a graph on this site, and probably one in Bentley manual as well.

The vac lines attached directly to intake manifold will be the most important, generally these are smaller than the crankcase breather hose.

I would say your best money spent is getting the Bentley manual for xmas if still in print, or see if your library has it!

audi_marv

IIRC...Temp Sensor for the idle is the single wire connector on top of the coolant hose at the cylinder head. Costs about 8 bucks.

Turn screw on top front of throttle body clockwise to lower idle. You may want to remove the screw, and spray throttle body cleaner inside to remove built up sludge. Screw should back out 1-3 turns from closed on turbo's.

May just need to give your ISV a good cleaning with tb cleaner.

To check for a vacuum leak....remove oil cap with engine running. Car should stall and die. If not, you have an air leak somewhere.

Check ebayz for a good deal on a Bentley, or even a Haynes manual. Both have the specs for testing the temp sensors.

Air Plate at fuel dizzy may be sticking. If you have removeable cap on top of the rubber air boot, take off cap and spray throttle body cleaner inside the air boot to give plate a good cleaning. May want to replace your air filter after doing this, as it will catch most of the dirty residue.

Throttle Plate may also be sticking. Spray tb cleaner on a rag and wipe both sides of throttle plate (should not spray directly into throttle body if a Turbo). Spray the outer linkage also.

02 sensor will throw a code if bad.

As for the SJM site....read it, live it, and then read it again

mattmondo

So far....

Oil cap pulled and car stalls.


I forgot about the 02 issues. My friend had an old cavalier that had a bad o2 sensor. Car started fine, but when it warmed up, the idle would jump between RPM's. A new sensor fixed the idle

When I started the 5000CS this AM, it idled fine, but started jumping when warmed up.

I trust pulling the code is the same. When warm and idle jumping, insert the fuse for 4 seconds, remove and watch the flashes....

Gas MPG is garbage. 18 MPG now! Was 24

socalclimber

I think I would focus on coolant temp sensor and O2 sensor.

Try unplugging O2 sensor and see what idle does when warm.

O2 sensors can start being sluggish as they get old. Poor mileage also supports a bad O2 sensor or possibly temp sensor.

Bentley is calling, my man, Bentley! Need to do the tests for O2 and temp sensors.