Rough idle only while cold

[dsmithwc04]

So I've searched around and found many threads on rough idle.

I took out the ICV, which was a PITA, and tested it and it tested fine.

I don't think it could be the AFM or the TPS since it idles fine once warm.

So what exactly can cause a low (600-700rpm) idle that stays pretty consistant (doesnt hunt around) and feels like I'm sitting in a message chair due to the shaking.

If I give it gas to about 1150-1200 rpm the shaking is completely gone. This is also the rpm range where the car idles at when warm.

I don't have any "flat spots" during acceleration as well. I don't think it is a vacuum leak as it would also have problems when warm correct? I'm stumped.

[Gawernator]

DME temperature sensor

[Ky914Newbie]

Originally Posted by gawernator

dme temperature sensor

+1

[dsmithwc04]

That is what all the other threads said but the PO just replaced it... Maybe he didn't replace it with a new one?

Well, I guess its easier to just check it instead of going through everything else only to come back to this later. Will keep you posted, thanks!

[porsche924-S]

Hello All,
I am going to replace DME Temp Sensor for my 1988 924S, there are 2 part numbers but I am not so sure which one is a right one, can someone point me a right part, thanks
944606125 OR 0280130026

[Dave Colangelo]

Motor Mounts could be the culprit, I have this issue in the winter. If its very cold out and I leave my car sitting for more than 24 hours, when I get in and start it up it shakes quite a bit. I think that the motor mounts get hard when they are cold and once the engine hear warms them up they often out and the shaking stops. The reason why it goes away when you rev the engine is because it does not actually go away it just increases frequency. The shaking becomes so fast that your body does not notice it like it does at the lower frequencies.

Regards
Dave

[rgp944]

Cold idle enrichment valve under intake manifold. Bosch 0 280 140 501 for 944 turbo.

[Gawernator]

I'd also definitely take a look at your motor mounts

[dsmithwc04]

Nice call, I will check the motor mounts. The DME sensor doesn't get here until next week from Pelican so something else to check in the meantime.

rgp944: if your speaking about the Idle Control Motor/Valve then yes, I've already checked it as stated in the first post. And my car is an NA not a turbo Still fun though!

[Ky914Newbie]

It also sounds like it idles fast when "normal". The idle should be around the 950 range (+ or - 50). Might want to check for vacuum leaks also.

[dsmithwc04]

Well, just replaced the DME temp sensor and it still behaves badly. I'm not sure if the shaking changed at all but either way it's not a good thing.

Also, the Tach decided to stop working all of a sudden?!?! Guess it could be either signal wire, or tach died... Would have been nice if I could actually fix a problem before new ones pop their little heads.

Motor mounts huh, maybe I could run a torch near their location and warm them up. Start the car and find out if it's them or not.

[dsmithwc04]

BTW, if not motor mounts what else?

[Gawernator]

Would not hurt to test the operation of the TPS. Just replaced mine to fix an idle surge (hopefully) and the car definitely drives noticeable better. Especially acceleration. Could also be because I rebuilt the throttle body. Clean all your grounds especially under the dash. If that doesn't fix your tach follow my write up for cleaning the instrument cluster. If not then you might want to send it to Hollywood VDO or get another.

[dsmithwc04]

Well, as stated in my other thread I just found out that my '86 has an '87 engine in it. Was going to adjust the belt tension and found it had an auto tensioner in it. It was stamped M44 /07 on the top of the block enar the bellhousing.

The TPS is probably fine since I don't get hunting RPM's and it idles fine when warm which the TPS would cause adverse affects while both warm and cold. Started car while in warm garage and still ran really rough.

[dsmithwc04]

BTW: where is the specific ground for the tach? Maybe it was bumps while installing new radio??

[Gawernator]

Instrument cluster grounds are right under the dash, driver side, there's two side by side. I highly doubt your engine swap is affecting anything electrical, my 86 has an 84 swap no issues

[ernie9944]

Originally Posted by dsmithwc04

BTW, if not motor mounts what else?

Check if this hose # 7 on the diagram (circle in Red) is well connected to your air boot,if not it will cause rough idling.I've seen lot of time when people take things apart they completly forget about this hose,make sure the clamp is really snug on it.

[Gawernator]

Hey Ernie, that part #7 is a plastic or metal tube actually but same concept!

[Ederd]

Whats the average life span of motor mounts on a non tracked car? I have PO receipts of my motor mounts being replaced mid 2009. I definitely get the massage chair feel at 950 RPM and goes away when hit 1100 RPM.

[Gawernator]

Well Genuine Porsche have a two year warranty, and even on my new mounts and all the fixes I've done there's still a slight vibration at 950, you can feel it in the chair but just barely.