tobone

Hello,

I´m new here, and I have a 94 Porsche 993 C2. I bought it 2 months ago, just done 3 mods (cat bypass, air box cover and install RS carbon fiber duct). When starting at cold, the first 1-2 minutes everything seems ok, but then it start to misfire until the engine stop. After that, it´s very difficult to start the engine again. When It starts, it seems to run ok after a few miles. Sometimes I also can not start the engine at normal temperature!

It´s seems that distributor caps and the ignition leds are new (perhaps someone was already looking to fix this problem). I already change the spark plugs, fuel filter, air filter, DME relay, clean ISV.

I waiting for the OBD plug to scan for some errors, but it can not solve the problem.

any help on these?

best regards

pp000830

If one runs the OBD Codes the problem may become self evident.

Otherwise;

I have not experienced this issue but it may be a mixture issue where the fuel air ratio when cold is a little rich to help with starting and possibly later when the computer senses a warm engine it cuts back the fuel in the fuel to air ratio to a point where there are running problems.

This would suggest that the mixture at running temperature is a little too lean (too much air). I would look for an intake or vacuum leak after the air flow sensor as the computer can't compensate for these uncontrolled sources of air to the intake if they are too large.

Keep in mind that sometimes there is a computer identified (OBD)running problem on a 20 +/- year old car where a sensor communicates erroneous information to the computer that then tries to compensate for it. In these cases the problem really lies with a bad sensor or sensor connection.

I assume you kept the before the CAT oxygen sensor and the temperature sensor in the airbox intact.

Churchill

Sounds a lot like a bad cylinder head temp sensor. Cheap part, pain in the *** to replace. Lots of info in the archives.

pp000830

There must be a test procedure to test this sensor at its pigtail connection point with a Volt / Ohm meter. Might save having to dig deep if the signal from the sensor is within specification.

Or better yet analyze the running temperature data with Durametric and see if it records reasonable warm up temperatures.

nine9six

Without being as knowledgeable or magically intuitive as others, I have a couple of comments/questions... otherwise it's more of a guessing game, predicated upon the limited amount of information provided,

The first course of action would be to run the OBD codes on the vehicle...

Was a PPI conducted on the vehicle prior to purchase? If so, results?

Was the car running fine before the mods? No misfiring or starting issues?

If the answer is yes, I would suspect the mods in terms of an O2 sensor issue. Does the cat bypass have an O2 bung and was the O2 sensor tip handled/touched by bare hands or dirty gloves prior to installation into the bypass?

More info needed...

993Maineiac

I may be wrong but I believe the early 993s did not have an OBD port. Codes were flashing lights on the panel.

pp000830

They all have an OBD port. Pre-96 cars had a round DIN connector and there is an adapter to convert it to the US mandated plug style where it works with Durametric and others.

P-daddy

FIFY

tobone

Well I run the OBD and no codes...

And yes, when I bought the car the problem was already there, but not so frequently.

What voltage values should the MAF read?

prschmn

The symptoms aren't typical of a bad temp sensor but that's
easily checked with an ohmeter. Around 3K cold and 200 hot.
Wondering if the car seems rich when it's acting up?
Checking fuel pressure running and resting might be a good idea.
Diaphragm in the regulator can go bad and let raw fuel into the intake tract via vacuum lines.
and as was said-checking for vacuum leaks would be a good plan too.

Endoman

Crank sensor failed on mine, would cold start then die after 15mins. Would not restart until cooled for 2 hrs or so then same again. Similar on 944, bad hot start occasional misfire. Totally wierd as it happened a few weeks after the 993 one failed.

Jay J

Sub'd

danco_

I had a very similar problem as you are describing and was able to fix all symptoms as you with the change to a new cylinder head temp sensor.


Don't think, just do. You will be pleasantly surprised.

Edit: Here is a thread I started a few months ago. The CHT resolved all my issues.
https://rennlist.com/forums/993-foru...g-at-idle.html

HDA

ha! I had a misfire code pop up once a few miles after I installed the fabspeed airbox thingy.

Put the oem airbox cover back, reset the light with a OBD Scanner and it hasn't had a problem since.
I guess she didn't like to be modified...

Agree with the above post, was it running OK before the mods?

shadow993

I agree it doesn’t sound like head temp sensor. Mine would start fine, and it was the restart that was the problem.

Check to make sure both distributors are working. Mine had a weak spring on the cap that would get stuck. Ran fine until under load.

Clean MAFS, clean O2’s with MAFs cleaner. No luck...then

Replace head temp sensor. Sacrifice the socket for the install with a cut. I fought mine for 30m by hand, right on with cut socket.