Stumbling idle after tuneup

[Rtheriaque]

Greetings, Folks,

I'm back in the E30 world after a long absence. I just picked up a mint CA '89 325i.

The car ran beautifully but the only service history I had was the magic belt and water pump. To get everything to a baseline, I did quite a bit of work yesterday:

-Valve adjustment (looked like somebody did it cosmetically- all the adjustment holes on the eccentrics were lined up!)
-Plugs (Old plugs were correct, but worn and showed signs of some hot running)
-Cap/Rotor (BADLY pitted)
-Air filter (found that the previous owner had fitted a K&N, which I wasn't overly thrilled about but kept it)
-Engine/Tranny/Diff Oil
-Brakes

Anyway, I created a symptom: the cold idle tries to drop below 500 RPM. The motor catches itself, revs past 1100k or so, then drops back down. When under load, the engine is fine. Once warmed up, it steadies to a very low idle at about 600 RPM.

I went back over and verified everything I did on the ignition. I can't find any evidence of a vacuum leak, and when I create one (crack open the dipstick, for example) the engine almost dies entirely and runs horribly.

On my last E30, a 325e, my next step would be to turn the screw on the idle air control, but I don't see this on the i motor. Does anybody have any ideas?

Thank you for reading!

[EatsHondas]

When you were wrestling with the airbox there's a chance you could've cracked a hose or opened a vacuum/intake leak somehow. Leaks and a gummed up/defective ICV are the most likely causes of weird idle.

The scorched plugs are a sign of a lean condition, which again point to possible vacuum/intake leaks. How many miles on your new baby? Injector o-rings are known to crack and leak after all these years. How brittle was your valve cover breather hose when you removed it? Mine was like hard plastic and shattered like glass when I tried to bend it. The best way to find leaks is to have a shop smoke 'em out, or do it yourself.

[jlevie]

A sticky ICV or intake leaks would be my first suspects. The ICV can be removed and cleaned and a smoke test will tell if there are intake leaks.

[Rtheriaque]

The car is something of a time capsule- it's totally unmolested with 90k on the odometer (it works).

Hoses are supple. The intake boot was recently replaced and feels factory fresh. The breather is in good shape as well.

"Wrestling with the airbox" is an understatement. Last time I did that job was 15 years ago. The memories certainly came rushing back about how staggeringly stupid that design is. Playing with the distributor cap brought back similar homicidal hatred. There are so many nice engineering touches throughout the car that make jobs typically difficult on other cars far easier on the E30. Then, on the flip side, you have a few regular maintenance items that ratchet up the difficulty (or annoyance, anyway) to head-scratching levels.

I didn't check the injector o-rings when I was testing for vacuum leaks- I'll do that next.

[EatsHondas]

"Wrestling with the airbox" is an understatement. Last time I did that job was 15 years ago. The memories certainly came rushing back about how staggeringly stupid that design is. Playing with the distributor cap brought back similar homicidal hatred. There are so many nice engineering touches throughout the car that make jobs typically difficult on other cars far easier on the E30. Then, on the flip side, you have a few regular maintenance items that ratchet up the difficulty (or annoyance, anyway) to head-scratching levels.

I know right? I felt like an idiot the first time I replaced my air filter, there's absolutely no room to get to some of the airbox clips and I didn't know anything about e30's at the time so I didn't want to pull the box out of the car to make it easier. Changing the cap while swatting the plug wires out of the way and wiggling your hands around the coolant hoses trying to get my measly 1/4in 8mm socket in there... Lord. And all this time you're leaning over the side of the car destroying your hamstrings... Gotta love the learning process! ;)

[Rtheriaque]

Put the car on ramps, at least- it saves your hammies working in the bay!

[Rtheriaque]

Okay. Smoked the system, no leaks.

Also, I can't get the problem to come back.

So, to summarize, no problem before the work. After the work, stumbling idle. Now, the problem is gone.

Do these cars retune? The cap, rotor, and plugs were junk, could that have caused enough of a difference that however the car had adjusted made the good parts act like junk on the first startup?

[EatsHondas]

Okay. Smoked the system, no leaks.

Also, I can't get the problem to come back.

So, to summarize, no problem before the work. After the work, stumbling idle. Now, the problem is gone.

Do these cars retune? The cap, rotor, and plugs were junk, could that have caused enough of a difference that however the car had adjusted made the good parts act like junk on the first startup?

The ECU will "relearn" its environment if you disconnect the battery for a few hours and then reconnect it. If you had shitty ignition parts and replaced them with good ones, that very well may have been your original problem.

I don't know for sure, but if you had weak spark with the worn out/crappy parts, the ECU might have been smart enough to dial back fuel so it would run more smoothly, but with less oomph. Total guess, so it'd be cool if a guru chimed in. I have intermittent hard warm starting, but your idle should be constant. It's either crappy all the time or good all the time until you do something to fix it.

[dnguyen1963]

Not a few hours...disconnect the negative cable for a few minutes is all you need (actually, only about 30 s to reset the ECU).

[Rtheriaque]

Well, that's a relief.

I guess there's a moral of the story that you should probably check each repair as it's done, but once you've got the car in the air and level, it's not something you want to repeat a few times in a single day!