97 328ic Rough idle when cold, surges high idle when warm

[C21no1]

I just bought my wife a 1997 BMW 328i convertible, it has a rough, shaky idle when cold and wants to die and sometimes does die. When its warmed up it surges from 1000rpm to 2000 rpm which makes stopping fun. I was told by the seller that it needed the idle air control motor, I pulled it out today and cleaned it with carb cleaner and a gun cleaning brush, tightened a couple hoses and checked for vacuum leaks but no change. The car had 15 codes including a P0505 and P1205 as well as some missfire codes and fuel trim codes and such. I cleared them and ran it for 45 minutes with no new codes, but still surges and runs poorly. Any ideas? Should I buy a new IAC? Is there a way to test it? I own my own shop, although I'm not familiar with BMW very much, but I do have tools and knowledge of American and Japanese cars.

Thanks

[C21no1]

Any ideas?

[jaredd5]

Try unplugging the maf and see if the idle stabilizes. Other than that i would check for a vacuum leak somewhere...

[C21no1]

I tried unplugging the MAF, it ran a little bit worse if anything. After I cleaned the Idle Air Control Motor, it moved freely, could it still be my problem? When cold it idles very rough and even makes a popping sound once in a while.

[C21no1]

I'm thinking its the IAC?

[C21no1]

I replaced the idle air control motor and the hose but now it revs to 3000 rpm when you first start it and then it drops to 800 and runs rough. I got codes for random misfires and fuel bank 1 and 2. It seems to be running rich, any ideas? I'm at a loss!

[Stephen Max]

An intake leak will cause rough running, especially when the engine is cold and the mixture needs to be rich. )An intake leak will also a warm engine to surge.) The lean mix will cause the dme to try to compensate by adding fuel, but if it has to add more fuel than normal, it will throw codes related to fuel trim. Check your manifold vacuum, if it is below about 17 in. hg. then you probably have an intake leak.

Here is another thought, which may be a red herring, but what the heck. Misfiring indicates you may have a problem with an ignition coil. The misfiring will result in unreacted oxygen in the exhaust, which the O2 sensors will detect and results in the fuel trim errors. The dme will compensate for the unreacted oxygen by enriching the mixture, which is why it seems to run rich. The fact that you have codes for banks 1 and 2 indicates that you have at least two coils that need replacing. But I would concentrate on finding a vacuum leak first.

[C21no1]

It runs on all cylinders at higher RPM's, but at idle it's rough, smokes like running rich and you can here it misfiring, popping. What's the best way to check the intake vacuum? If I had a bad coil, it would miss at all RPM's, but I may have a few plugs fouling out! Thanks!

[Stephen Max]

It runs on all cylinders at higher RPM's, but at idle it's rough, smokes like running rich and you can here it misfiring, popping. What's the best way to check the intake vacuum? If I had a bad coil, it would miss at all RPM's, but I may have a few plugs fouling out! Thanks!

You can get an automotive vacuum gauge from an auto parts store or Harbor Freight for pretty cheap, $20 or less. Find a vacuum hose that connects to the intake manifold, disconnect it and connect the vacuum gauge to the nipple. I can't think offhand of a specific one I've used in the past, but there should be a couple to choose from.

Smooth running at higher rpm doesn't rule out a bad ignition coil. The engine's inertia at high rpm will naturally smooth out rough running from a misfire. You should also do a compression check since this car is new to you, and take a good look at the plugs when you do.

[C21no1]

I will check it tomorrow, thanks!

[C21no1]

I disconnected the brake booster vacuum line from the intake manifold and connected my vacuum gauge to that nipple on the intake, I'm reading 4-5 in. hg. So I must have a pretty big vacuum leak somewhere! Any ideas where to look? I've checked all the visible obvious things.

Thanks

[C21no1]

I read that these cars have a sealed crank case and I had a pretty bad valve cover leak, so I replaced the valve cover gasket but the plastic was very brittle and it cracked in 2 spots as I was tightening it down, even though I was being careful! Now the idle is smoother and no popping or misfiring but after its warmed up the idle goes up and down from 800 to 1800, which makes it hard to stop!