!!! Idle issue! w/ complete test results. Solve the Mystery !!!
 

Here's the situation... Over the past year or so my car has had a rough idle and has been pulling a mere 14-15 in hg. Since this time the car has undergone numerous changes including a Vitesse MAF, Paragon 8mm wire set, ngk bpr7es plugs, new idle stabilizer valve, new coolant temp. sensor for the dme, plx r500 wideband, etc. The only actual attempt at solving the concern was the purchase of the idle stabilizer valve. All other purchases were unrelated in their intentions, however I feel it significant to post as it ensures other components are not the cause of this mystery.

Recently I purchased a used wiring harness that was in significantly better shape than mine. I checked every wire and found only a few minor problems, so in the end after repairs every wire had 0.2 ohms or less and all connectors are tight. One of the many reasons for my purchase of this harness was the wiring to the idle stabilizer had been poor with spots that were missing insulation: did not fix concern.

Throughout this year I have thried endlessly to adjust the idle through the idle screw, the tps - both using the factory method of shorting the plug and just adjusting it without. WHEN THE IDLE IS SET OFF THE IDLE CONTACT ON THE TPS (AT THE SAME RPM) THE ENGINE PULLS 17-18 IN HG AND RUNS SMOOTH. So you would think it is a bad tps. Using the factory tests and testing each wire for resitance, shorts, etc. led me to no conclusions. The tps potentionmeter measures perfectly with no breaks, proper resistance where it should be, idle to wot. The dme also cuts fuel on decel when the idle is set correctly, however when off contact of course it does not. THE CAR WILL IDLE WITH 16-17 A/F RATIOS WHEN THE TPS IS SET CORRECTLY. AT THE SAME RPM ADJUSTED OFF CONTACT IT IDLES @ 14-15 A/F RATIO.

I've had another suggestion from a reputable source that a weak ignition coil can cause only a concern at idle.

Lastly, if there is a way to test the DME I am unsure, so any help would be appreciated. Clearly it is the final culprit.


thanks :surr:

It may be worth getting a new dizzy cap and rotor while you're at it. Are your plugs gapped to spec?

plugs gapped to spec.

I think we need more data. How does the car perform otherwise -- under different loads and different RPMs?

Does this happen 100% of the time, or only when the car is warmed up?

Have you pressure tested the intake system to eliminate vacuum leaks or bad check valves?

Have you actually swapped the TPS out with a known good one?

Have you done a diagnostic with a Motronic tester?

Car performs very well under all loads. Idle is the only concern. This does happen 100% of the time and is not temperature dependant.

The intake has been tested and is currently leak free.

I have not swapped out the TPS with a known good one because I have no source to try. I am making this post in an attempt to find the solution without making any wrong guesses.

As far as a Motronic tester I am also unsure where to look. I am an Audi tech and am aware of their similar electrical systems in the 80s, however is there a generic Motronic tester that works on Audis and Porsches, or is this 944 specific?

Thanks.

Perhaps you can answer the following questions, it'll help eliminate possible problems.

- Are you using the stock cam?
- Did you check the voltage between Red & Black at the MAF connector with Ignition on
- Check for any voltage between MAF connector BLACK and Battery Ground terminal, both IGN ON & OFF.
- Rotate the MAF some, does it make a difference?

Valve timing?

Thanks for the response John. The car has a stock cam and I will check voltage where you stated and attempt to rotate the sensor. I will respond with results tonight.

Also, now that I think about it I recall us discussing the idea that my injectors may be the issue. Do you think this is still a possiblity based on my current findings? From what I understood the same Siemens 55# injectors from one company to the next have occasionally proven to not flow consistently with the sets sold by others.

Voltage between red and black was 11.78 V (battery was at 11.98 - car has been sitting all winter and only run for a few minutes so far this year)

Voltage between black and battery ground was 0.021 V

Rotating the maf showed no results

how are your vacuum lines? my car had a rough idle (wish i knew my vacuum readings as i have a gauge, just cant recall and i took the engine apart)... but anyways, i found a few cracked vacuum lines that were in dire need of replacement. a couple were completely broken, no wonder the car had a hard time starting at times!

You might want to take off your intake manifold (yes, i know, its a beast), but check all your vacuum lines to make sure they are serviceable.

if they are hard, they are most likely bad.

Give it a shot!

Vacuum lines all replaced within the past year. Initially I did have a few vacuum leaks so I gave up on this being another issue and fixed the vac. leaks first. Problem still persists. I am going to smoke test the car on Saturday to ensure I am leak free but I rarely require a smoke machine and I regularly diagnose vac. leaks on customer cars at work. Assume for now I am leak free.

Let me ask the first question that needs to be asked.
What is the idle speed?
And you need a portable tach to be accurate or a digital datalogger because the stock vdo unit is not acceptable for speed setting no matter what the geniuses say.

After this, assuming all the other checks you mentioned, you will need the leakdown to find out which valve has the potato chip under it. :)

Idle speed does not seem to affect the concern. The car has a lightened crank and flywheel so I typically try between 900 and 1k rpms and see where it runs best. For example, if I set it at 1k rpm with the tps adjusted properly the car runs poor (13-14 inHg and rough), however if I set it at the same 1k rpm and adjust the tps off idle contact it runs great (17-19 inHg and much smoother). As far as utilizing a poortable tach I have had to opportunity to use one a few times in the past and it appeared my factory gauge was very close. I do understand your concern with using the right equipment when making fine adjustments, however in this it seems to happen wherever I set the idle. I can set it much lower or much higher and have the same issues (more rpms seem to mask to problem slightly but it is clear the concern is still present).

Any chance you could go into further detail. You think I have a valve not seating correctly?

I am assuming that by setting the tps off idle, but at the same rpm, the engine will have more load and possibly assist in masking the concern as well? If not I am unsure as to what you are suggesting.

By the way, I want to thank all of you that are taking the time to assist me in this matter. It seems the majority of my posts go unnoticed and this time I really need help! Thanks!