mc_opp

I have bought a 993 S (1998) with 30k miles a year ago. It came with the following idle problem that I have not been able to solve:

The problem is that the car starts shaking (roughly every 10 seconds) when the car is in idle mode and the engine is warm (RPM at 800). (the mechanic said something like misfiring a cylinder). When the engine has just started (and the RPM is higher) everything works perfectly. Likewise, if one puts the AC on full power (higher RPM) it works perfectly fine. Also, once you are driving, it is fine as well. Thus, the problem is only there if the engine is warm & in idle gear & AC is off.

I went to 4 shops (Dawydiak, Stable, AVR, SCARGO) in SF and already spent a fortune. None of them could solve the problem. We have a regular OEM dual-mass clutch and no chip tuning. The issue is that there are no error codes. These are the things that have been tried:

- O2 sensor checked
- Vacuum leak checked (engine was smoked out)
- New MAF
- New idle stabilizer valve
- New air filter
- Ignition: New caps, rotors and plugs. Wires have been checked
- Check for grounding engine harness
- Hall sensor checked
- Temperature sensor checked (no error code pulled)
- Check fuel pump and fuel filter

Not checked yet (engine has not been removed yet)
- Issues with dual mass flywheel
- ECU issues
- Wearing valve guides
- Immobilizer system electrical problem

Now, I have heard from several people that my car should be fine and a quick fix would be to simply raise the idle speed by modifying the ECU to always put the car in AC mode. Others have strongly advised against this... I am running out of suggestions. Is it possible that the ECU does not store any codes? Could the ECU be the problem?

I am just a little reluctant to get my engine put apart before ruling out every other possibility.

If you have a valuable suggestions that turns out to work, I will certainly reward you with a $100 check.

I just want to get this problem solved

ReinerFink

Broken Distributor belt?

-reiner

ReinerFink

Pull off the caps and make sure one of the distributors isin't "loose". I suspect that whomever changed the caps and rotors would have noticed this, but you can never be sure:

http://p-car.com/diy/dualbelt/

If I am right, you just keep your reward. :-)

-reiner

brucec59

I'll venture a guess, and it's purely a guess: Clogged cat.

diet97993

Sounds like a vacuum leak. I had a similar situation and replaced a rubber "T" connector that was cracked (weathered) on the drivers side (left) engine compartment in the back next to the Varioram. Just a suggestion.

mc_opp

Thanks a lot. (Danke, Danke (some guys seem to be German)). Concerning the last comment (vakuum leak). Wouldn't this show up if you smoke out the engine? My mechanic said he could rule it out because of this. But did you have the same issue (that it only occurred when the engine was warm and AC off)?

mc_opp

I will check the distributor belt.

diet97993

Only did it when the engine was warm. No problems when cold and mixture was enriched which compensated for teh vaccum leak. Not saying it will fix it...... but worth looking into, your call. It is a rubber manifold that joins 3 plastic Vacuum lines together and is somewhat stressed and has tendencies to kink also. If the smoke test is accomplished using a positive pressure it may be visible. Good luck!

Stealth 993

Check the dist belt & make sure you can't wiggle the rotors. It might also be a bad FW sensor.
It's a long shot, but might as well check the coil packs, one may be weak, or getting a weak single.

Have you pulled out the plugs & checked them?

Could be a clogged cat, but performance would be weak.

mc_opp

Thanks, I appreciate the comments. plugs have been pulled out and checked, but distributor belt is on to-do list. (coil packs will be checked as well).

Just another question. Is it possible to swap out the ECU from another 993 to see whether it is the ECU that is causing the issue?

Amfab

DME relay first

PNogC2S

I know this suggestion will sound absurd/crazy etc.
-- Make sure you have "Porsche" fan & alternator belts. Not "OEM", but ones that actually say Porsche on them.
I had an intermittent idle surge (albeit a subtle one) and was generating misfire codes (I know yours isn't generating codes).
I was told by some of the same places here in SF area that you mention that it might be a DMF issue or worn valve guides, faulty injectors, etc. Then Burce Anderson (PCA) suggested changing the Alt & Fan belts for factory belts (mine were supposedly "OEM" but not actually "Porsche" belts). I was skeptical but made the change (the belts only cost about $60) and it took care of the problem! No more issues in over a year!
I can think of no logical reason for the belts having this effect, and no has been able to pose a reasonable answer...but it worked!!!
Sure beats some of the other options you are looking at, so I'd suggest you give it a try unless your car has fairly new Porsche belts.
Good luck!

mc_opp

Andrew, thanks. What do you mean by the DME relay. Should I just swap it out?
Best,

Amfab

Yes just swap it out. I had a similar problem. Stalling when the car would get warm, then randomly it would be ok, but then it got consistantly worse.

I checked vacuum connections, cleaned the ISV, cleaned the MF sensor, changed the air filter, then finally, just 'cuz I had one, I swapped the DME and it cleared up.

It then kinda seemed to get a little rough here and there again, but then I cleaned the ISV and it seems to have cleared up. It's been four months now and no stalling.

Now and then the idle seems to go a hair low. I have a feeling I was dealing with a combination of and ISV problem and a DME that was on its way out.

Anyway. Its a cheap thing to try, especially with what the expenses and troubles you have you have been through

chris walrod

I would go with bad DMF. Sadly the only way to inspect it is to pull either the trans out or drop the whole shebang and have a look.

When the A/C compressor is engaged, it will act as additional flywheel weight which may be the reason the problem isnt much of a problem with the A/C selected.