Author Topic: rough idle, but runs great otherwise (Read 3186 times)

PaulC · « **on:** April 24, 2008, 01:39:46 AM »

This is more of a thank you to everyone on this board. I finally got my car running today after 5 months and without this forum it would have been much more difficult. I bought it last Nov and limped it home. It ran rough and showed an O2 sensor code, all the vacuum hoses were cracked and VC was leaking. I gave the motor a full tune up (except for testing the injectors but replaced the o-rings), fresh fluids, trans and diff too. Went for a spin tonight and it runs great. The idle is a little lumpy though and it hesitates when you tap the gas lightly, @ ≈1000rpm. I'm gonna replace the throttle cable and run some BG 44K with the gas. Anyways, thanks again and any advice would be greatly appreciated re the idle.

tim_s · « **Reply #1 on:** April 24, 2008, 01:47:21 AM »

you checked cam timing?

mkodama · « **Reply #2 on:** April 24, 2008, 01:59:53 AM »

That's good to hear. I very recently got a 318is and I probably have some similar work to do.

But as for the rough idle, what immediately comes to mind is ignition and vacuum tubing. Since you replaced all of your vacuum tubing, I would pretty much omit that as a possibility unless you left one vacuum port unused or a line slipped off.

Have you replaced your spark plugs and spark plug wires? The wires will make a slightly rough idle but fouled, broken, or used up spark plugs will definitely make very noticeable problems while the engine is running and are very obvious at idle. I just dealt with spark plug problems on my Landcruiser earlier today. Turned out to be just a really small problem with one of the spark plugs, but it prevented one of the cylinders from firing, which made for a rough idle and exhaust that smelled of unburned gas.

It's pretty easy to pull out the spark plugs, inspect them, and either clean them with a little brake cleaner if they are still decent or replace them with new ones if they look horrid. And alternatively, if you don't want to replace the spark plug wires ever again, for almost the same cost you can do a Coil-on-Plug conversion, a search for "COP conversion" will help you with that.

Cheers!

Cobra Jet · « **Reply #3 on:** April 24, 2008, 08:11:38 AM »

Paul - I would also check your ICV (idle control valve). This regulates air flow into the engine. When it starts to go bad, you will experience caold start issues, hot start issues, idle issues and sometimes drivability issues. It's really easy to remove & clean out. I would suggest removing it, cleaning it, lubing it and also performing an OHM test on it to make sure it's functioning 100%. These little buggers do get carboned up over time and will start to act up.

PaulC · « **Reply #4 on:** April 24, 2008, 01:08:17 PM »

-Haven't checked the cam timing. Looks like I have more research to do.

-All new vacuum lines are on there good and tight. But I'll doublecheck. Replaced the spark plugs and wires too.

-Will check the ICV as well.

When I had the intakes off and the fuel and vacuum systems apart I thoroughly cleaned everything in/out--intake box (sensor and flap), throttle (butterfly), intake manifolds and all electrical connections. I regret not replacing the temp sensors as well now (the $$ was adding up).

It starts right up cold after about 5 cranks and I set the idle @ about 800. There are no issues when accelerating and at WOT it revs smooth and the power feels good. (I'm just using the butt dyno but at 4000rpm it really comes alive) When parked, with the throttle held between 1000-3000rpm the rpms surge slightly up and down. And the idle is a little lumpy.

If I were to do it over again I'd delete the throttle heater plate, the vacuum hoses and most of the cooling system hoses. What a PITA. The money spent on hoses could have gone towards a catch can system and shorter studs like the ones discussed in Justin's 'mess under the intake' thread.

Thanks very much for the responses.

Wise Old Dog · « **Reply #5 on:** April 24, 2008, 01:23:44 PM »

The 2 manifold gaskets are problem areas for vacuum leaks. If you re-use these gaskets, they are prone to leak. Best way to check is a can of carb cleaner with the red nozzle attachment. Just spray small squirts around the gasket area to find the leak. You'll know when you find it. Works for the hoses as well. I had to tighten my manifold bolts a little tighter than I originally had them.

PaulC · « **Reply #6 on:** April 25, 2008, 12:30:33 AM »

I installed new upper and lower intake manifold gaskets and torqued both down a little tighter than spec.

While I had everything apart I cleaned out the ICV (#7 in the OEM diagram below, right?) as best as I could with with MAF cleaner. Just wasn't sure exactly what it was called.
http://www.realoem.com/bmw/showparts.do?model=AF93&mospid=47305&btnr=13_0307&hg=13&fg=15

The car ran better today. The idle is worse when its cold. Once it warms up it idles and revs much smoother. Over 3000rpm its smooth as butter. How long does it usually take for the DME to 'relearn'? (I've heard that e30's run weird for a while after having everything apart) The exhaust smells rich but the codes are clear.

Thanks again for all the great info.

PaulC · « **Reply #7 on:** April 28, 2008, 10:59:57 PM »

Just got back from an 850 mile trip to SE Iowa and the car ran great. I guess the DME just needed time. Best part was getting lost and zigzagging the hilly-curvy country highways, m42 screaming. This little car is really something.