It's that time of year again (Unsolved from last year)

[SnakeyesTx]

Well, its getting cool outside finally down here in Houston; been waiting for this for about 3 months now but an ugly monster started poking around again and is making my car run pretty close to god aweful. Last year I had enquired about this and the cause and solutions were sparse. Hopefully by now someone else may have been through this issue and can say "Oh yeah, its this!"

Under normal conditions where temperatures are over 80 degrees F outside this almost never happens. Usually in cooler weather or when it rains outside (so the temperature dips a bit) does this occur.

I'll try to explain this in story form a little bit using a trip to work as an example (much like this morning).

Start the car, release the parking brake, head on my merry little way to the quik-e-mart about a mile away. Leave the car running to let it continue warming up (the neighborhood is still moderately safe to do that suprisingly enough), grab my 1-liter tea, pay, walk outside, and the car does one of two things when I get there :

* Idle is bucking god awefully between 850 down to maybe 200 rpm then the ECU recovers banging it back up to 850, and repeat. The fluctuation is rapid, not a slow change in rpm.

-or-

* Engine stalls because the ECU couldn't recover

Okay, no biggie, get in the car, restart the engine, see the check engine light on (this code always comes up o2 sensor, I guess its a generic code for what ails my engine - I had replaced the o2 sensor and the relay last year when this all started as a precaution) I start driving along with the CEL on, and once the slushbox is in 3rd or 4th I basically pump the pedal a few times (sometimes a dozen times) while I'm rolling along around 40-50mph and the light will shut off and I continue on my way.

Pass a few green lights, weave through a little traffic... all is well.

Slowing down to a red light. Stop. Sometimes engine is bucking when I get to a stop pretty bad where the back end of the car dips with every pulse making me look a little foolish at lights. Sometimes the idle is smooth at the stop. Either way, if I'm stuck at the light for more than 60 seconds or so, the CEL comes back on, the idle goes bonkers again, sometimes stalling the engine. If it doesn't stall, I pop into neutral give the engine a rev or four and the light usually goes away and the idle is trying to normalize again. If the light turns green before I can clear the light, I continue in Drive but now the car feels really sluggish until I do the 40-50 pedal pulse thing to clear the light.

This could happen at every red light, which is pretty annoying.

Now, this never happened past last year. The engine was replaced last August because of a bad head-gasket and head crack. The old engine ran fine, no lights, no issues for 15x,000 miles or so and averaged 22mpg with the a/c on. This new engine at best gets 14.9mpg in more city-style driving (rarely do I get to drive on the highway for very long to effect overall milage). I wanted to suspect the throttle position sensor (as it effects idle speeds on some vehicles, and even shift points on some chryslers), but it doesn't explain why it only happens in cooler temperatures outside. So... what gives?


-Almost non related note : Acceleration and power seems to be a LOT better when the air is cooler outside and I don't have the CEL on or being stuck at lights for the idle issue-

[Dave M]

Demons.

I chased a similar thing for a year (almost identical symptoms, including the realtionship to temperature). Finally thought I had it licked by fixing a bad connection at the fuel pump, but it has since returned with the cooler weather this fall. Mine is definitely not as bad or regular as yours sounds. Its an annoying hesitation only while warming up and only occurs every few days. Livable, but still annoying.


Mine threw a throttle position sensor code. Even though it tested OK I replaced it. No difference. My CPS and ECU have been replaced as well. I know it doesn't help you much at this point, but if I can figure it out, maybe it will solve your problem. Oh, I just discovered a disfigured coil pack (the plastic around the connection to the plug is dissintegrating. I may get a new one and see if it helps.

Dave

[pingu]

I once had similar symptoms when my Idle Control Valve (ICV) was duff - it wouldn't idle properly and hunted idle speed up and down. Have you tried that avenue?

[Ross]

Idle valve sounds logical but I can't make the correlation to temp.
Idle valve is ~$100 and a PITA on the M50. Doable without removing the intake but a bear. A vac leak could also be the prob so maybe do the intake and the valve at once.
If you want to try the TPS I have a known good one you are welcome to try on it.

[SnakeyesTx]

I once had similar symptoms when my Idle Control Valve (ICV) was duff - it wouldn't idle properly and hunted idle speed up and down. Have you tried that avenue?

I've been considering tearing down under the intake and checking it out, but it's that cooler weather thing that keeps making me thing its some kind of electrical issue causing this. I know there's a large hose coming off the intake elbow that I believe does down to the ICV, someone suggested I spray a bit of seafoam down there to try to clean it up since that line has extreme vacuum pressure.

[Ross]

The hoses are all in pretty tough spots if you haven't already seen for yourself.
Cool weather, in Houston, you kill me.
I'd say a fair chance of some of those hoses being fried from the those Houston summers.

[Dave M]

I've also swapped a known working ICV and cleaned the crap out of mine while trying to diagnose this (to no avail). I'm also leaning toward an elecrical gremlim or lack of solid ground somewhere.

Dave

[Barney Paull-Edwards]

Changed your fuel filter lately????????????????????????

[icesoft]

Maf?

[e34.535i.sport]

My friend had a similar issue with the jerky motion off the lights mainly when booting it in his e36 320i (m50) recently without (I think) the relation to temperature - we changed his fuel filter last weekend and found it hadn't been changed since 1992... Not good. It was pretty jammed up to say the least. Put the new one in, along with a tank of fresh fuel and some injector cleaner and the issue seems to have disappeared! Anything's worth a go when your in that sort of pain. Good luck man.