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Help: Rough Idle and dip in RPMs off start

Posted: Sun Jul 16, 2023 2:01 pm
by lukegoode8
Hey guys, just had a issue flare up on my 944s. When I start it, it dips sharply in RPMs around and below ~600 before it evens out. It also could possibly be ticking louder than usual for this car with it possibly misfiring while it bounces around 900-1000 RPMs. First time it completely died off start and I had to try again. I just replaced the distributor cap and rotor along with the plugs, but touched nothing else. I don't think it has anything to do with that though. Belts all are tight and fine, didn't blow the engine, battery is fairly new. I'm not sure where else to dig. Would love to hear some thoughts.

Re: Help: Rough Idle and dip in RPMs off start

Posted: Sun Jul 16, 2023 3:37 pm
by Tom
Maybe post a video. What plugs did you use, and are you sure you got the firing order right when reinstalling the wires?

Assuming that all checks out, I'd suspect a vacuum leak. Any chance you knocked something loose when changing the ignition parts -- FPR vacuum line, etc.? If you spray starter fluid around the vacuum fittings, you'd notice a change in the motor if the starter fluid gets sucked in anywhere.

Re: Help: Rough Idle and dip in RPMs off start

Posted: Sun Jul 16, 2023 4:52 pm
by lukegoode8
I used the Bosch OEM equivalents and gapped them correctly and the wires are correct. I don't think I knocked anything loose.

Re: Help: Rough Idle and dip in RPMs off start

Posted: Sun Jul 16, 2023 5:23 pm
by usury
It the spark plug wires aren't fully seated, either at the distributor or at the plug, it would probably run weird.

Re: Help: Rough Idle and dip in RPMs off start

Posted: Sun Jul 16, 2023 5:37 pm
by lukegoode8
All of them seen to be seated okay, I'm fighting the idea of the ignition coil being the culprit. Could that cause all that?

Re: Help: Rough Idle and dip in RPMs off start

Posted: Thu Jul 27, 2023 6:27 pm
by usury
Did you find a solution? I'm curious what it was

Re: Help: Rough Idle and dip in RPMs off start

Posted: Thu Jul 27, 2023 6:47 pm
by lukegoode8
usury wrote: Thu Jul 27, 2023 6:27 pm Did you find a solution? I'm curious what it was
Nope, seemed to work itself out somehow. It isn't a perfect start up every time but always gets going so I'm not gonna touch it.

Re: Help: Rough Idle and dip in RPMs off start

Posted: Wed Nov 01, 2023 9:09 pm
by notny41
i had a strange problem where my car would hiccup and almost stall every so often at idle. i tried many many different things. finally got it fixed - my throttle pos sensor was bad.

Re: Help: Rough Idle and dip in RPMs off start

Posted: Wed Nov 01, 2023 9:11 pm
by lukegoode8
notny41 wrote: Wed Nov 01, 2023 9:09 pm i had a strange problem where my car would hiccup and almost stall every so often at idle. i tried many many different things. finally got it fixed - my throttle pos sensor was bad.
That's funny I was just thinking about replacing mine because of the same thing. That makes me more confident in it then.

Re: Help: Rough Idle and dip in RPMs off start

Posted: Thu Nov 02, 2023 1:20 am
by notny41
unfortunately they aren’t exactly cheap. i found a new one on ebay for my turbo at a decent price and ever since I put it on, it’s been great.

mine used to also start difficult (mostly after it was warm). i replaced the fuel pump check valve too and all has been good since.