vans944

Hi, I have this car that is missing heavily on cyl 3 when cold, gradually picks it up as the engine warms up. Runs fine when warm, or when revved up cold. Compression is excellent on all cyls, leakdown test is fine. I have switched injectors between cylinders, no change. Have lifted rail up and pointed injectors into jars, fuel spray and volume the same on all cyls when cranking. Spark output to all cyls is good. Has new coil, wires, cap, rotor, fuel inj temp sensor, plugs, idle air motor, etc. Belt is on time. Spark timing is correct at each cylinder. Sparkplugs show more brown on porcelain of other cyls, while 3 tends to stay more white. Seems like it is lean on that cyl. Have been unable to find a vacuum leak. Have checked camshaft lobes and lifters, found nothing unusual there. This is just the 2.5L normally aspirated engine. I am running out of ideas, wonder if anyone else has run into something like this?? Thanks for any input.

ClassJ

Have you done a compression test by chance?

pjburges

Maybe some kind of intermittent head gasket issue?

The bad fuel injector may output reasonably at cranking speed and then not release enough fuel when running.

Do the easy stuff like swap plugs and wires and make sure the rotor and cap are good too. If the contacts on terminal 3 in the cap are bad it could cause erratic issues.

944s can run a little rough on cold startups...

Jay W

Did you swap spark plugs around to see if maybe you got a bad one when you bought the set? If that is ok then did you spray carb cleaner around the intake runner for cylinder 3 (CAREFULLY so you dont blow yourself up) while it is running to see if the engine speed picks up? Speed up gives away a vacuum leak between the intake runner and the head etc.

vans944

I continued searching for clues on the car. Determining that the DME drives the injectors in pairs, I was able to swap injector leads 3 and 4 and find that the miss stayed with cyl 3. That proved that any of the wiring or DME functions were not at fault. Redid a smoke test, and found a large leak out of the oil separator lower oil ring. Replaced both orings on the separator and that took care of the bad miss when cold. Thanks for the input from those who responded.