Cold Revving

[Drewe30e]

I tried searching but didn't find anything that was exactly like my situation.

Lately when I start my car when its cold, the idle sits at around 900 RPM. This is OK although my car never ran that high during warmup normally. However, after warming up a little more at 900 RPM, my idle will start to rev between 900 and 2100 RPM. Eventually after warming up some more my idle returns to its normal warmup state of about 600 RPM where it will stay until the next time I start the cold engine.

ICV, throttle body, MAF? Any ideas?

[AndrewBird]

Cold start valve or possibly coolant temperature sensor. If it doesn't do it when warm, you can pretty much rule out the ICV, MAF and throttle body.

[Adrian_Visser]

better check out the coolant temp switch and thermo time switch too since they are what controls if and for how long the cold stat valve comes on.

[nando]

is this an eta, 325i?

E30s don't have fucking MAFs!!

[Drewe30e]

E30s don't have fucking MAFs!!

Wow, touch a nerve? Sorry, I meant volume air flow sensor.

Its an eta, by the way. Its in my sig.

[Liam]

Something is causing a base CO% to be too rich or too lean (after warm up the O2 sensor compensates).

Check Fuel pressure, Intake air leaks, NTC (Coolant temp sensor) etc. If everything checks you could try adjusting the AFM air bypass screw.

[Drewe30e]

Back from the dead...

Checked the coolant temperature sensor and the old one seemed to have good resistance values. Just to be safe, I replaced it anyways. Still having the same problem.

Quick visual inspection of the air intake shows its fine. Rubber is still soft, but I can't be certain.

What's the best way to check fuel pressure?

[nando]

Wow, touch a nerve? Sorry, I meant volume air flow sensor.

Its an eta, by the way. Its in my sig.

actually it measures pressure, not volume (which is what a MAF does, and why it's annoying). It then uses air temp to calculate volume, like a MAP based system does. :)

you kinda need a gauge to test pressure, you might try a local autoparts place.

However if the FPR is bad it will let the pressure bleed out of the fuel rail, so what you can do is start the car and let it run for a minute or so, then shut it off. Remove the fuel pump relay and let the car sit for ~15 minutes or so. Try to start it - it should start and run, and then die (since the FP isn't running). If it won't start, then the FP is letting pressure bleed out of the fuel rail.

[Drewe30e]

actually it measures pressure, not volume (which is what a MAF does, and why it's annoying). It then uses air temp to calculate volume, like a MAP based system does. :)

you kinda need a gauge to test pressure, you might try a local autoparts place.

However if the FPR is bad it will let the pressure bleed out of the fuel rail, so what you can do is start the car and let it run for a minute or so, then shut it off. Remove the fuel pump relay and let the car sit for ~15 minutes or so. Try to start it - it should start and run, and then die (since the FP isn't running). If it won't start, then the FP is letting pressure bleed out of the fuel rail.

So, in the end, it measures volume by way of pressure. I feel as though we're arguing semantics.

Thanks for the heads up about the FPR. I'll try to check that out this weekend.

I'm trying to get the car running perfectly before I sell it.

[nando]

it measures volume by measuring pressure, yes, but not directly. It has to use intake temp and calculate the volume based on physics. the problem is IAT sensors like to heatsoak, so it's not always accurate. A MAF on the other hand directly measures volume, there are no corrections or calculations that lead to errors. Not semantics at all.

[Drewe30e]

it measures volume by measuring pressure, yes, but not directly. It has to use intake temp and calculate the volume based on physics. the problem is IAT sensors like to heatsoak, so it's not always accurate. A MAF on the other hand directly measures volume, there are no corrections or calculations that lead to errors. Not semantics at all.

Ha, I understand physics and the gas equation...I'm an engineer.

Back to the issue at hand.

New updates: Yesterday on my commute home, the engine did its same revving while cold for about 30-45 seconds and then settled down. However, after sitting in some stop-and-go the engine began to rev between 600-1200 rpm (while warm). Today, I started my car, it immediately revved up to 1800 rpm and sat there for 30 seconds (no revving, just holding high) then settled out. It drove perfectly fine on the way home after warming up.

So, is this still looking like FPR or more like the TPS, ICV, etc.?

[Drewe30e]

Bump...any ideas?