88 924s shakes at idle
 

88 924s
car shakes at idle if it's set at about 1200 Rpm it is smooth and car runs fine other wise.
I have replaced all front timing belt components and YES the belt is adjusted and rollers positioned correctly.
Motor mounts are solid and replaced.
Subbed the Afm, compression is good.
Clutch replaced with none rubber style.
Need some feedback on this running out of options, I am wondering if the clutch and motor mounts maybe the cause.

shaking at idle is 99% of the time motor mounts, you said they are "solid and replaced" does that mean they are actually SOLID or just new and in good shape?

belt timing most likely wouldn't present at idle - they really start to get noticeable past 3000 rpm or so if it's set wrong.

when you set the engine to its spec'd idle speed of 900 +- 50 rpm how does it feel?

I have a similar issue; I just replaced the engine mounts on my '87 924S and noticed the same vibration at idle. My car has zero miles since the mounts were changed along with timing belts and other work because I am waiting on an antique plate from the state so I can drive it.

But, it SEEMS to be vibrating slightly less after running the engine at idle for 20 minutes or so with new gas (car had been sitting for about 4-5 years).

Anyway, is it possible that the mounts need a little time to "bed-in"?

The motor has been gone through and Mm are new along with a ton of other stuff. You are correct the balance shaft becomes in issue around 3k but that is not my issue. Wondering about the clutch and if I installed it correctly, car has been used by my son he mentioned the idle issue. It's odd at 1200 it's fine seems of clutch assembly is out of balance it would tend to be at all ranges. I just don't want to remove all the crap to check it and find out it's something else. The lower the Rpm it gets worse.

Did you use aftermarket motor mounts? I replaced mine a couple years ago with aftermarket and ended up with vibration and body noise at idle. Above 1100-1200 RPMs it went away.

But after changing to genuine Porsche MMs no vibration and all is well.

It's been so long but I believe they were after market solid.

Sounds like the dreaded early 944 shudder. (it affects all44s, but the early ones moreso). See link for a suggested fix.

Thats your problem, the only mounts that tame the "shake" are the OEM porsche ones. The aftermarket mounts don't have the liquid center and transmit the vibrations to the car. I had the exact same behavior with Lindsey racing semi solids on my S and also when i had a collapsed OEM mount on my S2. Get your hands on a set of OEM mounts if you want it to go away.

You may have something on the Mm I did use solids on my 928 with out any issues. Wondering if the 4-Cylinder cars are more prone to this!

Exponentially so. At one time I knew and understood the physics as to why, but that's been lost long ago (power stroke, rotational mass, blah blah blah). The short of it is that a straight 4 is terribly balanced (secondary vibrations, not primary). It's why you rarely see a 4 cylinder bigger than 2.0L. Somewhere back in those physics I've forgot, the problem becomes a big issue once you pass the 2.0L size. Porsche's answer was the balance shafts (not their invention, it was invented in 1911 and Mitsubishi patented it in the 70's and I'm pretty sure I've read Porsche paid them to use it).

The 2.5, 2.7 and 3.0 liter engines Porsche used are some of the biggest (GAS Engine) 4 cylinder ever used in production cars. Not THE biggest, but up there. (3.2L is the most widely excepted king of the non-diesels/passenger car engines) Bigger 4's were used in tractors and other industrial applications where vibrations weren't a huge issue, or where RPM's were lower.

Er.... sorry for the history lesson. :eek: Didn't mean to do that. But yeah TL;DR V8 and I4 are totally different beast on the vibration front. I was surprised as well when I first got into these cars.