Chris in Detroit

My 993 pretty regularly lights up the Check Engine light at track events - which then goes out after a few hours. Obviously some code is tripping, then resetting.

Applying the "hammer" the diagnosis is seemingly random (various cylinders) and very occasional misfires.

Since the car runs absolutely fine - and no intake/engine/exhaust mods, am I reasonable in putting the above down to a combination of street gas / running v.hard and hot at the track leading to an occasional cylinder misfire.

Or is this something I should be more concerned about ?

Robin 993DX

I know you have been having this missfire issue for awhile now. And this might be a long shot, but this just came across my mind. Did you ever check the condition of the distributor drive belt for the 2nd distributor?

Like a said a long shot.

Robin

Curtis - '97 Targa

Chris,
I've been dealing with a similiar, although more frequent situation. I get random/multiple misfires about every 100 mile or so. Gas, load, or ambient conditions don't seem to make any difference. I started with the cheapest and easiest things first. Caps/rotors, then plugs. Now, all indications are that I have bad plug wires. I should get around to changing the wires in the next week. I'm trying to decide if I change the all of the wires or just the consistent misfires(2,3, and 6). I'll probably bite the bullet and do them all because it is such a pain in the rear.

I bought the OBD-2 tool from Alex Peper (See Robin's site for link) and have been able to rule out about anything else other than plug wires....

Good Luck!

Chris in Detroit

A long shot ........ but well aimed the tech who works on my car DID check the distributor drive belt and that is fine.

I recently replaced plugs, dsitributor, rotor - but it was "check-engine-ing" before that, so I don't think I have been ham-fisted.

I'm not that concerned - car runs fine and I wouldn't even know without the obsessively **** OBD-II system - but wanted some opinions on whether occasional misfires (a) seem normal enough given the conditions on track and (b) could be causing any damage.

TIA

johnsopa

Chris, I have the same problem. The check engine light comes on every month or so and the code is the same -- misfire. I don't recall which cylinders. Same as you, I notice no poor performance and the code goes away after a few days.

I would be interested in your eventual diagnosis.

Chris in Detroit

Well, this may be confined to the "mystery" pile - misfires are randomly and occasionally occuring on all cylinders BUT only under track conditions - so it would not seem to be e.g. bad plug wires / plugs on a couple of cylinders.

Most likely cause would seem to be (street) gas which isn't quite up to the task when the engine gets real hot / conditions pretty extreme i.e at track. Does this make sense ?

Marty 98'C2S

I had this happen on my 98'C2S. The check light was random and could not be replicated on demand. It turned out to be bad exhaust valves . Hopefully your issue is less severe.

Marty

Chris in Detroit

Marty

What engine check code were you getting - the misfire code ?

FWIW another idea my tech had is that the battery is beginning to expire - although it tested OK, his theory is that under heavy engine load (which can't really be simulated by load testing) is doesn't quite give enough juice - hence occasional misfire. Could be right - it's the original (now 5 year old) battery. What do you guys think of this theory ?

Chris in Detroit

Update:

My tech suggested it m-i-g-h-t be a weakening battery - I still had (note tense) the original battery.

So, off I went to a track weekend during which the battery proceeded to go into terminal decline - evidenced mostly bu sundry electrical problems, due I think to weakening voltage being insufficient to trip relays - examples:

- the rear spolier stayed up and didn't go down when I slowed;
- the passenger side electric window wouldn't work without the engine running
- the air blower motor, top drivers side of engine behind all the black plastic cowling stayed permanently on (and flattened the battery !)

Given these symptoms I was actually lucky enough to locate close by the right kind of Interstate battery - fitted and all problems went away - also no check engine light (yet ....)

So, I think maybe the random misfire was the battery crapping out - likely to start happeining now these cars are 5 years old is my guess.

Curtis - '97 Targa

Chris,
Hopefully, you've got it nailed. I wanted to share with everyone else, though, another piece of information that I've gathered in trying to fix mine.

The "Random/mulitple misfire" code happens whenever the OBD detects more than one misfire. For example, if you got a misfire on 2 and 3, the OBD would actually throw three codes: Misfire on 2, misfire on 3, and a random/multiple misfire. I kept a detailed log over the course of a couple of weeks and determined that they weren't random at all. In my case the misfire always included #3 and sometimes #2. Never any other cylinders. I'll replace the #2 and #3 wires this week and will hopefully put it to bed.

This is one of those bodies of knowledge that wish I never would've needed to get into...

Kevin Reilly

Bumping this one to the top. My '96 has been misfiring the last 6 weeks or so. One misfire and check engine light per week. Light goes out within a day. Almost always comes on when engine is warm. Misfires are on different cylinders.

Just had the 30K service recently, so plugs, rotor etc are all new. Battery seems fine. Car runs great.

Mechanic thinks it is due to bad gas at the local Chevron...he actually told me he "bet he knew where I got my gas", and he was right. Apparently the local Chevron station uses lousy gas, and some of his other clients have had similar problems.

So I'll start fueling up somewhere else and see if this fixes things. Man I'm getting tired of staring at that orange light!

Curtis - '97 Targa

Finally got mine fixed in the last weeks....

After I narrowed down the conditions that caused my misfire (sustained, steady operation above 3000 RPM) the Service Manager at my local dealer made a couple of calls. Sure enough, he found that there is a new control unit that specifically addresses the random misfire above 3K problem. We put the new cu in and it has worked great for a couple of weeks now.

E. J. - 993 Alumni

Curtis, I am glad you finally found the problem. I know you have been working on it for a long time.

E. J.

Robert Henriksen

Curtis, you've described exactly my problem. Sustained steady throttle at 3,300 - 3,400 rpm, I have the same MIL condition - random/multiple misfires. Have changed the same items (except control unit - you mean the DME?).

Please, please can you give me the name/number of your service manager? I'd be thrilled to be able to cruise at a decent rpm w/o tripping that #$@! MIL light...

thanks,

RNH

Steve Weiner-Rennsport Systems

Chris, et al:

Just a quick note about OBDI & II misfires,...

The ECU will log any detected misfires (incomplete combustion) and trip the MIL light whenever several of these occur in a row. This may be a false alarm as indicated by some folks with defective ECU's, or this may be the Real McCoy.

Now, repeated misfiring in OBD I & II cars will crank the catalytic converter temps through the roof and melt the core into a big ugly solid mess.
Plus, its glows a nice pretty dark red during this process and that, for damned sure, isn't good for the surrounding components, either.

I'd respectfully suggest that one is VERY certain that everything is in fine order: plugs, wires & connectors, caps, rotors and all wiring, before chalking this up to a bad battery or anything else.

Red Herrings' abound on these computer controlled cars,.......