surging/vibration at idle

[davem]

I know this is very difficult to describe through prose but I'm going to give it a try anyway. This is for my E30 (91', 325i, auto-tragic).

I notice this when I first start the car up, and when I'm at a stop light (so idling situations). The car vibrates and engine sounds like it's throttling up and down causing a subtle surge. The rpms hold steady at 600 rpmson the tach, a little bit higher (700/800) when the engine is cold until a few minutes later it settles to 600. I'm not sure if this is just normal, I drove a few different E30s when I was shopping for one but didn't really notice it, although I'm not always as observant as I'd like to be. The car was like this when I got it. Driving around the engine operates as one would expect and is smooth.

I've changed/replaced: plugs, O2 sensor, breather hose, air intake boot (all new obviously). I thought the surging was an idle issue which prompted me to clean the ICV and replace the cracked air intake boot. In retrospect my rpms were always constant so I never really had any idling issues to begin with. The car is in good shape for its age and the engine mounts don't look like crap or anything.

[llll1l1ll]

I've noticed that when my car is warming up, it'll have a lopey idle - almost as if it has a nice cam.

It just goes away once it's warm.

7-800 rpms on start-up is just fine and 600 is the normal idle speed for your car.

[davem]

Yeah, my idle is normal. I think my exhaust has a leak or something, gotta check it out later.

[jlevie]

My first suspicion is that you still have intake leaks. Find a shop that can run a smoke test on the intake. There can be other causes of this, but until intake leaks can be ruled out there is no point is per suing the alternatives.

[hotballs]

Or just replace everything vacuum related including your valve cover gasket and you should be fine.

If it's not a vacuum leak then maybe a sensor like the coolant temp sensor which is cheap and easy to replace.