chengteik

SEE MY POST BELOW (#10) FOR AN UPDATE OF PROBLEM

Just got my car back from a complete top and bottom end engine rebuild. Added Schrick Porsche Mortorsports cams which I believe are the same cams used in the 993RS.

If you want more details, see:
https://rennlist.com/forums/964-forum/183728-engine-rebuild-modifications-964-w-6spd.html

I now have this PROBLEM. When I'm in 1st or 2nd gear at about 1500rmps, when I depress the clutch (accelerator not depressed), the engine revs drop to 500-600rpms and sometimes even lower causing the engine to stall. This also happens 50% of the time when I blip the accelerator pedal (not in gear).

The car idles fine at 800rpms. Upon a cold start, the revs hunt a little but stabilize after a minute.

I've read other posts about dirty idle microswitch, ISV and airflow sensor but don't think this applies because I have good idle stability otherwise.

Any ideas what this could be? Someone suggested maybe airleaks? I don't really understand that comment.... Thanks in advance guys.

Computamedic

This is beginning to sound vaguely familiar!! Your original post mentioned a lightweight flywheel. Was it fitted?? Maybe that could be the problem.

A recent (long) discussion on exactly that problem at https://rennlist.com/forums/964-forum/189532-idle-problem-after-installing-12lb-lighten-flywheel.html

Regards

Dave

AJKOK1

I agree with Dave's comments ref the LWF, however if you have not had one fitted, then the items you list are the most likely candidates.

Ref "Someone suggested maybe airleaks? " It has been raised a few times that if the Airbox cover is not securely fitted ( i.e has a gap at the rear) that this can cause a similar effect.

In my experience the most likely cause is after a service, Oil has been replaced, amount used was a touch generous and this has resulted in contamination of the ISV / airflow sensor.

I have to say though, your symptoms do seem to be spot on with those noted by many owners after a LWF has been fitted.

Al.

chengteik

No lightweight flywheel. I'm using the 993 dual mass clutch and flywheel (togehter with my 6spd 993 transmission). Will also check airbox and oil overflow.

chengteik

Hmmmm.... It's not the airbox or engine oil overflow..... Anyother ideas/suggestions? Thanks.

Youcef

I had a broken wire to the hall sensor that had caused the revs to drop when you touched the accelerator pedal.

Dave R.

Youcef,

Hall sensor at the flywheel or Hall sensor at the primary distributor?

chengteik

Arrrr.... the engine stalling is really embarasing and irritating me.... Any other reccomendations would be GREATLY appreciated.

JasonAndreas

Have you scanned the car with a Bosch hammer? Like Youcef I had a broken connector on the dual distributor hall sensor that showed up as an intermittent fault and that caused a bunch of unnecessary stalling when I had a DMF. You also might want to try what others have suggested and clean your idle speed valve (it only takes 15-20 minutes) even though your standing idle is steady.

chengteik

PROBLEM NOT FIXED. Here's more background so hopefully someone is able to help. Please.

1990 964 C4.

My engine has just been completely rebuilt. All new parts except crankshaft, engine casing, valves, pistons and cylinders. All parts stock except for Schrick Porsche Motorsports spec cams. Same spec as 993RS.

Before the rebuild, the car was idling fine.

Now when in neutral and I blip the accelerator to 2000rpms and take foot off accelerator, the revs drop to about 550rpms then jumps up to about 1200rpms and finally settles at 800rpms.

If I'm in 1st or 2nd gear and rolling at 1800rmps, foot not on accelerator, and I depress the clutch, the engine reves drop really low (about 500rpm) and the engine dies 50% or more of the time when I do this. If it does not die then it returns to 800rpms. It seems like the ISV has got a delayed retarded reaction to the revs dropping. This is what the lightweight flywheel guys face but I am using the standard dual mass clutch and flywheel.

Checked ISV (Idle Speed Stabilizer valve). It was very clean but we cleaned it again anyway. Airflow sensor is also clean. Idle microswitch is also working fine. Hall sensor is working as well.

We used the hammer on the car and the only faults that showed were Air-conditioner related. BTW, my AC cannot be turned off completely. Even at fan speed zero, there is very light air blowing.

My mechanic said that the AC was not signaling the DME when it was on and because the AC is alwasy slightly on, this might be the problem??

Do I replace the AC controlls?? I think this is really expensive and might not solve the problem. Will getting the chip reprogrammed with a 50-80rpm higher idle solve the problem??

Any other suggestions?? Thanks in advance guys.

Monique

If the RS cams are fitted, then the idle RPM should be adjusted to 960.

This is from the 993 RS technical manual.

Check your cam specs: if indeed an RS cam, intake lift is 12.5 mm and exhaust is 11.1 mms. Valve timing must be set using tools 9551 & 9552.

Good luck.

Luis de Prat

Miles on these parts? They can be clean, but worn out, too.

The ISV on my S2 had similar symptoms very sporadically. Replacing it cured the problem and idle is rock steady all the time now. It's a 964 part on this car too, btw.

The Throttle Position Switch (not sure if you guys have the same part on 964s) was "clicking" like it's supposed to, but replacing it solved an annoying hesitation problem I was having at WOT.

Sometimes electrical parts simply wear out. A lot cheaper to just replace them than taking apart the engine, IMHO.

chengteik

CAMS are 288/280/114 (Duration CRA, Intake/Exhaust, Peak Timing). Cam lift 8.8, Valve lift 13.2. Does this still need 960rmp idle? Does increasing the idle prevent the dip? I assume it will still dip after increasing the idle but dip to 600+ instead of 500+? Is this normal?

100,000 KM on the car.

Thnaks guys.

Monique

Don't have the specs for duration but yours seem quite perf oriented.

Your lift is high; check the values I posted before.

IMO, 960 rpm idle is prob valid in your case.. any of the real experts around???

Cheers.

chengteik

Thanks Jean-Pierre. Is the only way to adjust the idle to 960rpms to reprogram the chip? Will also talk to my mechanic about Valve timing set using tools 9551 & 9552.