Techno Duck

Okay i used to think my intermitten rough idle happened on cold starts also, but frankly i have not had that happen now for as long as i can remember and im starting to doubt if it did ever happen on a cold start. I am having trouble researching reasons because all of these old posts seem to be related to a cold start rough idle.

I have however noticed i am getting it often when i park the car for say...15-20 minutes, come back out and start it. I do get that rough idle. It idles around 4-500rpms and wants to stall, but never has. Other than that, i give the car gas and it runs fine. It will sometimes go away by the first stop light, sometimes will not. I can be driving the car for a few miles on the expressway, get off at an onramp and i get the same rough idle. It dosent matter the situation, it seems to come and go as it pleases.

Vaccuum is good, gauge reads about 20-23 inches of hg.

Now im wondering, would a bad idle stabilizer cause this type of problem, or would i be getting the opposite, a high idle? What about that DME temp sensor? What is the part # for that sensor anyway, i cannot seem to find it?

This is bugging me because i would like to address this when i pull the intake manifold. I am probably going to end up buying a new ISV, including the two vacuum hoses attached to it. But i would HATE for that not to be the source of my problem. It leads me to think the ISV can be the problem, i have been sucking a huge amount of oil through the throttle body and j-boot from the air oil seperator (one of the things i am addressing).

So really, what does everyone else think? Any possible other thing i can look at or would cause a 'warm' start idle problem?

Here is a list of things recently changed on the car..

-vacuum lines (not hoses though)
-fuel pressure regulator
-fuel filter
-O2 sensor
-distributor cap and rotor
-fairly new ignition wires, about 20k on them
-Plugs have i figure less than 10k on them..ill get around to pulling them, inspecting and changing them also.

Im truely stumpted. Anyone with ideas...?

gregeast

I've been chasing a similar problem and my current theory is that it's the DME Temp Sensor. Having done all my vacuum hoses and replaced my ISV, all to no avail, I'm now operating under the theory that the DME temp sensor is not working properly, which means that the DME thinks the engine is still cold and therefore tries to use the cold idle maps. The O2 sensor disagrees (correctly) and then the DME tries to compensate based on the O2 sensor input but the range of adjustment available to it in the cold idle maps is not sufficient to smooth things out, so it just hunts around and never reaches a 'happy' place.

That's the theory at least, so I replaced the sensor (it's the one closest to the front of the engine with the blue connector on it) for ~$30. My turbo is supposed to be back from the shop on Thursday, so hopefully within a week or so I'll know if I'm right.

As for the ISV, you can bypass it to see if it's causing the problem. You can either jumper between pins B & C (please double check me on that, I think that's right) on the test port or you can pull the two hoses off and plug the port on the intake and the j-pipe, thus bypassing the valve.

Hope that helps, I'll keep you posted, please do the same.

Greg

Techno Duck

Hi Greg, i am reading threads about that sensor on both the list and the Pelican forums and finally found the part # for it, so for future reference to anyone else it is

944-606-125-00

Also, Pelican lists a gasket with it, did you need this also Greg? I see no reason a sensor like that needs a gasket. But maybe im wrong. The part # for it is...

900-123-101-30-OEM

So it looks like ill be doing the ISV anyway, including this little sensor.

Also Greg, Pelican seems to be out of stock for both the Bosch and aftermarket one, where did you buy it from? I really appreciate the help, im getting a better plan of attack every day now...including making this repair bill more than originally expected

kwdrus

Just to add a twist to this thread, I'm having a cold idle problem as well. When the engine is cold it will idle at about 400-600rpm, and judging from the fumes, it's running very rich. This has me thinking it's the DME sensor telling that the engine is actually warm and dumping more fuel. Once the engine heats up, usually a couple of minutes, it idles fine and no rich fumes. Now the flip side, once warmed up the engine has a intermittent high idle problem, usually 2100rpm. For that my guess is the ISV. To add to my frustration the cold start/rough idle is also intermittent. I've always operated under the assumption that once a sensor fails, it's done. Any ideas?

gregeast

Hi Jon,

I actually bought it locally at a Porsche specialist repair shop. They get their parts from World Pac so I'd think anyone locally that works with them can get it for you, if not I'd try 944online.com or Paragon-products.com

I don't remember a gasket but it did have a copper or maybe aluminum crush washer with it.

I bought a rebuilt ISV from 944Online for ~$80 and was pretty impressed with it (well, it was shiny and clean at least). I'd really recommend bypassing it first since it's not cheap to replace. Once the car is warm it shouldn't be engaged, although if the temp sensor is telling the DME it's still cold then it may be.

Kwdrus: Sounds like the ISV is being engaged when the car is warm, which is likely caused by this same sensor. Mine is intermittent as well.

I wish I could say for sure that this fixed my problem guys but I haven't been able to run the car since I replaced the sensor, I'm waiting on my turbo. I hope to have some feedback for you on Friday or this weekend.

Greg

Andy1212

I have the same problem did anyone figure this out?

minho78

I'm chasing the same problem. I have a rough idle when I start the car, then 1 minute later it's fine.
Any clues?

Will Feather

FPR is a possibility when starting after that car has been sitting, cold or not.

PeteL

The FPR is my best guess, given the symptoms.