CEL if AC is turned off?
 

I recently purchased a 1996 993 which is, for the most part, running wonderfully. There is just one problem:

-With the AC off, idle seems to be about 50rpms or so low (~800-850rpms) and I get an occasional slight "puh" sound in the exhaust. After a couple minutes of accrued idle time during normal driving, the CEL will come on (misfire codes for all cylinders).

-With the AC turned on, idle speed increases slightly (~50rpms) and the engine runs perfectly smooth. CEL light, if on, will go off after a couple driving cycles and stay off so long as I'm running the AC.

I had the car serviced (oil/filter change, air filter, new plugs, new distributor rotors, cleaned the IAC valve). It also passed smog with flying colors (with the AC on). There are no SAI issues and it easily passes all diagnostic testing so long as the AC is left on. Plugs and coils (primary and secondary) test out fine as well. The car is using Porsche belts.

It seems to me that I could solve this issue if I could simply raise the slightly low idle RPM's by about 50-75rpm, but there doesn't seem to be a way to do that (that my searching has found thus far).

Anyone have any suggestions?

Can't help with the idling symptoms you're experiencing, but the best thing you could do is buy an OBDII scanner from your local Pepboys (or whatever is close to you) which will tell you what code your CEL is throwing. I bought mine for $60 (give or take), and it's been an integral part of my roadside kit. It will save you any silly CEL diagnostic charges at shops and you'll know what you're up against right away.

Post the code!

:thumbup:

The codes I get are basically misfires on all cylinders, but I'll see if I can get the actual error code(s) and post them up.

So I finally bought myself a reader for the 993. Here are the codes I get when I turn off the AC:

P0300
P0301
P0302
P0303
P0304
P0305

I know it doesn't seem to say much other than it's misfiring on all cylinders (except cylinder #6).

Over the last couple months, I've been driving the car without any problems...no CEL, smooth idle, etc--but I left the A/C running the whole time for that result. This morning I turned off the A/C, and the codes came up. Idle gets slightly lower/rougher, with an occasional very soft "puh" sound coming from the passenger side exhaust...which is likely an audible misfire..and can only be heard clearly when you are outside of the car and bend over the exhaust tip to listen for it.

Turn the A/C back on, the idle clears up, the "puh" goes away...and the CEL will turn off on its own after a few drive cycles.

My guess at this point is that it's a belt issue. I figure the additional load of the A/C running on the belt adds just enough tension to correct the problem...what do you think?

Interested in some expert replies on this. My 97 runs noticeably smoother with A/C on as well. I've also found that it has also triggered misfire codes and a SLIGHTLY rough idle without A/C engaged on a few occasions. 97 currently with 87000ish miles.

Misfire CELs and lame idle
 

I've had both issues, misfire on cylinder 6 (odd that it's just the opposite of the OPs), and lame, misfire'ish idle.

The misfire CEL was remedied by cleaning the numerous grounding points on the car. I posted about this maybe 6 months ago.

The poor idling goes away when the AC is switched on -or- when I switch on the headlights. I conclude that it is not the increased RPM (although it certainly doesn't hurt), but the increased alternator load on the engine that results in the improved idle.

Does switching on your headlights also improve your idle? What I found is that it will take about 20 seconds after switching them on to make a difference. Let us know if you find the same or not.

do you have oem belts?

I was one of the lucky ones that random misfire cleared up with install of new OEM belts. My old ones were not horribly worn, but not oem. Several others have had this problem. I would start there since that is probably the cheapest starting point. Then I would probably head to BesideTheBox's solution.

BesideThe Box, did you have the upgraded battery ground cable already? I need to add one of those to my shopping list. I'm still on the original. No misfires, but seems that would be a good prevent. for me...

Your plug wires are definitely in the correct spot on both distributors?

Belts, as mentioned, is another check.

Do you have an Amp clamp? You could easily check alternator output that way and verify if the DME request for an idle up with a/c on makes the difference.

Thank you for the suggestions so far (lights, ground checks, alternator amp check). I'll look into those.

The belts do appear to be OEM (as labeled), but through my searches I've discovered that sometimes labels can lie...I bought the car with these belts installed, so I should probably have them changed out to be sure (cheap and easy troubleshooting step).

The plug wires are correctly routed/plugged into both distributors. I did a check of those, too, and they seem fine (including disconnecting each distributor bank to see if the other was still firing properly).

Thank you kindly for the suggestions so far, folks.

Lucky, indeed. New OEM belts made zero difference for me, but I think only the smallest of the 3 was non-OEM originally.

Thanks for the reminder. I do have the original. This is so inexpensive that I'm just going to add it to my list.

Might be useful to clean your ISV (idle stabilization valve, search for it, you'll find out how).

Others have mentioned some great items to check, like grounds and belts.

But it could be your dual-mass flywheel is on its way out, and may need replaced. It's quite a bit more $'s, so hopefully it's not that.

Start with the cheap/easy stuff.

I don't think it's the DMF, because if it were the case I'd expect misfires to be occurring regardless of the AC being on or off.