Vibration at idle

[migreig]

I just purchased my second 944 turbo (1986) a couple of weeks ago and I'm having an issue that I'm not sure where to start diagnosing. When I start it and it's cold, it vibrates aggressively and feels like it's idling really low, but sitting at 800-1000rpm which seems about right. It gets better as the engine warms up but never completely goes away. I have another one that idles smooth as glass so I know this isn't normal for this car. I am going to smoke test it for vacuum leaks this weekend but what else should I check?

In all other respects the car runs great. Pulls hard, lots of power etc. Where else should I be looking for an issue?

Thank you for any help!

[944m3]

I would start with checking if your balance shafts are set/aligned correctly. Also check motor mounts. New OEM hydraulic mounts make a huge difference.

[migreig]

Good advice, I hadn't even thought of that. If the balance shaft(s) aren't timed correctly, would it be more noticeable at idle?

[barnwerks]

Balance shaft (out of time) vibrations generally begin above 2500RPM.
I would unplug fuel injectors one at a time to identify if one has less impact at cold idle. Also test compression for correlation.

[zooklm1]

How are the cap, rotor, and plug wires?

[944m3]

Balance shaft (out of time) vibrations generally begin above 2500RPM.
I would unplug fuel injectors one at a time to identify if one has less impact at cold idle. Also test compression for correlation.

I would imagine that if balance shafts are out of time the vibration to RPM correlation would depend on how far off they are. Vibrations are usually smoothed out at higher RPMs meaning you will feel it at idle. I’ve personally experienced out of time balance shafts after doing a timing belt job.

Could very well not be the issue but easy enough to check and rule out.

[migreig]

How are the cap, rotor, and plug wires?

The car is brand new to me and I don't have any info on when they were changed last. That said, everything looks okay.

[migreig]

I did some reading late into the night and I've become convinced that the issue could be motor mounts. The symptoms I have seem to align with other people's complaints I found online about motor mounts. Apparently failed motor mounts causing heavy vibration at idle is not uncommon at all.

Shame if that's the case. Porsche motor mounts are exorbitantly expensive.

[Wespa]

Porsche motor mounts are exorbitantly expensive.

True, but luckily better alternatives exist. I've been using these for over a decade now, couldn't be happier with them: https://9productsamerica.com/shop/ols/p ... ine-mounts If everything else with the engine is sorted out these do not really introduce any extra NVH imho. At least if the balance shafts are still operating.

[944m3]

Definitely a common problem. Yes they are expensive. But there is science behind them, many articles online if you’re interested. The hydraulics or fluid filled motor mounts make a huge difference. I’ve used aftermarket mounts and they feel horrible and lasted maybe 2 years. Some will say it doesn’t matter and they don’t feel a difference. If I was 18 I probably wouldn’t care at all. But I value my comfort now