I have been dealing with a bit of rough idle that I will like to get smoothed out. So far I have done:

1) New Rotors
2) New Dist. Caps
3) New Spark Plugs
4) New complete spark plug wire set
5) Cleaned ISV
6) Used a couple of bottles of Techron on the las 2 tanks.

Symptoms:
If I run the A/C, then the idle goes to around 750rpm's and there is a slight stumble from time to time but not as noticeable. If I turn the A/C off, idles smooth when cold, but when warm, the RPM's drop below 750 RPM's and idles rough. So I keep it mostly with the A/C on.

Troubleshooting:
To check to see if I had a bad distributor belt, I unplugged the coil cable that goes to the main (high) distributor and left it with just the secondary coil and secondary distributor, and there was no change whatsoever, it started and idled exactly the same with or without this cable connected. Which tells me that the secondary distributor (lower) and the secondary coil is working. But...why no change on RPM's, idle, etc? Maybe the primary coil is bad? Am I running only on the secondary distributor?

So, my next thing was to check to see if the primary coil/distributor was working? So, I unplugged both distibutor cables and plugged only the primary distributor to the secondary coil which I knew worked from my previous test. This would tell me if my primary distributor was working. The car started but idled very rough and back fired. So, the primary distributor connected to the secondary coil worked but VERY poorly.

So, does it sound like I have a bad coil, or one bad coil and one weak?

HELP! Confused...

 

I think you've got a vaccuum leak, air getting into the intake manifold not being managed by the mass air flow sensor. Look around for a broken or disconnected hose. Listen for a hissing sound. Spray carbuettor cleaner using the plastic tube extension around the intake manifold, around hoses, and tucked back in especially in hard to reach or hard to see areas around the engine. If the idle speed picks up, you've located the general area of the leak. Try to pin point the leak, find the parts you need and make the repair.

 

Thanks Z,

I will try that for sure. Someone else also mentioned the air leak possibility and the MAF senseor not knowing about this extra air going in.

The thing that I don't get is why the difference in idle behaviour when using one coil or the other in combination with either of the 2 distributors. Shouldn't the car behave the same say with coil#1 with dist #1, or coil #1 with dist #2?

 

May want to check that both distributors are turning and pointing at the same cylinder(s) as they spin. Take off both caps and compare the position of the rotor for each distributor as the motor cranks (DME relay disconnected). May be a belt with a couple broken ribs.

 

Heres an additional thread https://rennlist.com/forums/993-turbo-forum/97087-check-engine-code-p0441.html with some of the main checks for evap leaks of which mine related to low idle as well. My code and low idle for now seemed solved with step one, new gas cap.

 

Thanks Rich,

I will look into that as well. My problem is more of an erratic idle and once in a while, I get the CEL with P0300, P0301, etc... which are random misfires or misfires at certain cylinders.

This weekend I did additional tests.

If I unplug the main coil from the main distributor, it idles the same as it had both distributors hooked up...in other words, no change. However, this tells me that the secondary coil, distributor, rotor, distributor belt, cables and sparks on the secondary "chain" are OK.

If I unplug the secondary coil from the secondary distributor, then it idles very rough, almost like it is about to stall and it back fires.

If I plug both distributors back on, it idles just the same as if it had just the secondary only plugged in.

Since the car has new spark plugs, new cables, new caps and rotors. Then I thought it was the primary coil that was weak. So, I just plugged the primary distributor to the secondary coil and it did the same as with the primary coil...very rough and back fire idle. So I know is not the coils.

So, there is soimething on the ignition, but not know what else to check. Like I said, new everything with the exception of the coils which I swapped with no change.

 

an old thread but was wondering how your problem was resolved.
thanks

 

Wow...this is an ooooold thread!

Well, after replacing everything you see in this post and then some, the rough idle continues as of today. Maybe to a lesser degree, but is still there.

I have driven other 993's that are actually worst than mine, so I have learned to just live with it. It's the nature of the beast I guess.

Are you having similar symptoms?

 

yes....that seems to be the consensus....thanks for fast reply

 

I believe the ISV opening or closing "coil" or whatever can fail. Not sure I got that right - will have to do some reading this weekend since I don't know exactly how it works or how to test it.

 

I've done all the same fixes and used an unlit gas torch to search for vacuum leaks. No dice. My car has idled at a solid 500 rpms since the day I got her back after a 6 mos. accident repair, including LWF and RS kit over two years ago.

 

Rick,

Would it not be true that, unless the idle speed was adjusted (increased) at the time of installation of the LWF, the lesser mass of the LWF would generate less centrifical force on the engine and thus cause the engine idle to slow slightly? The slower the idle, the greater the possibility of "lumpiness" in the idle. 500 RPM seems a little low to me.

Just a theory.

TMc

 

That's one I haven't checked. Thanks.

 

i am waiting for rotors and caps and if that does not make it better i am going to try and adj. the isv to higher rpm.
anybody have any luck with changing the coils?
thanks

 

Mine was low before the LWF, and it's low after the LWF, too. Not 500, more like 700, but still shaky at idle without the AC on. With the AC on, it's fine.

My most-trusted mechanic told me that the only real "fix" is to reprogram the DME -- along with the extra benefits of more power, etc. But I'm not sure it's worth the $1400 or whatever.