Odd Engine Behavior

[modifiede30]

My car is begining to act a bit strange. After my track day Monday, I started to notice a bit of hesitation at speed. This only occurs at a constant speed, and it seems a bit worse when its humid or raining. Its not a severe hesitation but occasionally, it feels like someone grabed the crank and held it still for a split second making it feel as though the engine's about to cut out.

At the same time, I'm noticing that more often than not, the idle is loping in unison with my A/F ratio gauge at idle. Now this I see as very odd since the A/F gauge isn't supposed to be able to read anything accurately unless at full throttle. You can tell that the lope to higher idle is consistent with a rich meter reading.

Lastly, before the car is warmed, the A/F meter reads perfectly stoic, and idles very smoothly. Again, the A/F meter shouldn't be reading anything.

The entire engine has about 3500 miles on it at this point. The 02 sensor is brand new but I've always noticed they tend to burn out a bit faster with the headers but this quick seems odd.

Nothing has been checked yet diagnostically, but I'll begin the process tomorrow, likely starting with the O2 sensor, AFM, and thermo-time switch. Anyone have any other suggestions?

[rs4pro3]

Are you even still running a Thermo time switch. What engine harness, ecu and stuff are you using?

[modifiede30]

Its a 1990 ecu and harness. I thought this was still somewhere in the engine. What else would cause this aside from a bad O2 sensor?

[JeffBij]

Its a 1990 ecu and harness. I thought this was still somewhere in the engine. What else would cause this aside from a bad O2 sensor?

How do the plugs look? Any wet, or heavy carbon. Is the ecu running the stock chip or have you changed it out?

[modifiede30]

Checking plugs tomorrow, but they're only a couple months old and the engine has only 3500 miles on it. The chip is stock, but I've made several chip changes over the last few weeks, probably 4 changes. Could the ecu be bad after these chip changes?

[JeffBij]

Checking plugs tomorrow, but they're only a couple months old and the engine has only 3500 miles on it. The chip is stock, but I've made several chip changes over the last few weeks, probably 4 changes. Could the ecu be bad after these chip changes?

The plugs can be a good indicator in diagnoising the problem. Have you tried disconnecting the battery for a while to clear ecu? The BMW ecu will "learn" from the way the car is driven, air conditions, etc. Sometimes you have to let it unlearn some bad habits.

ECU could be going bad, it does happen, but I wouldn't assume anything at this point.

With such a "new" engine, have you checked the valve adjustments?

[modifiede30]

I'll check the plugs and disconnect the battery tomorrow when I'm working on it. The wires are new as well so that's not a problem but I'll check they're condition since they're so close to my headers. The valves were adjusted just before Monday's track event. I'm pretty confident its not the valve settings but if nothing else works, that might be the next thing I do.

[bimmer8604]

kinda sounds a bit like what my car was doing when my vaccum line from the FPR to intake manifold crapped out on me. Just my 2 cents.

kyle

[nando]

you checked your engine codes lately? it could be an O2 sensor.

[modifiede30]

Well, did all the possible diagnostics on the O2 sensor, checked the plugs, vacuum lines, etc. Everything checked out okay except one of the throttle body intake vacuum lines (from the brake booster), seemed pretty loose so I wrapped it in teflon take to see if that may be a part of the problem. I ran it for a few minutes and the idle was definitely smoother, with no loping. However, if I keep it at a constant position of about 3000 rpm, you can hear the engine running smoother each time the A/F ratio meter reads rich as it moves back and forth. When it swings back to lean, it sounds more unsteady. I can't figure this one out, totally puzzled by it. Does anyone elses car seem to do this time of steady/unsteady engine tone? Its a pretty subtle difference but its really annoying me.

As for engine codes, I've not checked but my check engine light hasn't come on since I installed the new engine. When I turn the ignition on without starting it, it just stays solid. Not sure I'm getting any codes.

[iXer]

Re: Odd Engine Behavior

My car is begining to act a bit strange. After my track day Monday, I started to notice a bit of hesitation at speed. This only occurs at a constant speed, and it seems a bit worse when its humid or raining. Its not a severe hesitation but occasionally, it feels like someone grabed the crank and held it still for a split second making it feel as though the engine's about to cut out.

My car acted this way when the contact surfaces of the fuel or main relay were bad (fixed them both at the same time). I cleaned them up and have had no hesitation since.
I had occasional hickups but finally the car suddenly just died, knocking on the relay (in front of the left front shock tower) caused the engine to stumble. It was definately an electric problem.

/Mattias