m42 hunts a bit when cold on idle

[mattIS]

as subject describes what my m42 does a bit of when cold.it seems to be fine when warmd a bit.also when you rev in neutral it drops down b4 going back to 850.is this just a trait of m42's or is there some areas i can look at.i know the m3 motor has some sort of idle control valve that can cause richness.it has newish injectors,clean filter,new plugs etc.maybe leads?

[Speedtouch]

Check the cold start temperature sensor resistance, which I gather is under the inlet manifold on M42s.

Check it at the ECU plug (pin 78 on the 318is):

http://www.e30zone.net/e30zonewiki/inde ... CU_Pinouts

[mattIS]

do i set multimeter on ohms

[Speedtouch]

Yes, 2 kilohm range should do.

[Brianmoooore]

Meter must be connected between 78 and 43 (pins are marked on the ECU, not on the plug).
2k ohms range will not be high enough for a cold sensor. You'll need the 20K ohm range for that.
Don't damage the buckets in the ECU plug with the meter probes!

[mattIS]

Meter must be connected between 78 and 43 (pins are marked on the ECU, not on the plug).
2k ohms range will not be high enough for a cold sensor. You'll need the 20K ohm range for that.
Don't damage the buckets in the ECU plug with the meter probes!

so im checking for no resistance?

[Speedtouch]

No, a high resistance when cold (around 3.5 kilohms, say) dropping to a low resistance when hot (around 600 ohms, say).

[Brianmoooore]

600 ohms at full worling temperature would be well out of spec. and faulty.
Hot resistance should be about 330 ohms, but has to be measured quickly after the engine is stopped, as it soon cools off.
I once bought a very cheap E30 that was off the road because those that had tried to fix it had failed. Had a slight hot running problem - hesitation and blackish spark plugs.
Blue temp readings weren't far out, but removing it and measuring it while being heated in a pan of water alongside another blue sensor proved it was the problem.

[Jookeone]

Hi Brian,

After reading the Wiki on blue temp sensor testing I see it gives the figure of 330 Ohm for a hot engine as you mention. It then goes on to say "If your Blue Temp Sensor has not changed its resistance significantly, then it is at fault and should be replaced."

This leaves a lot to be interpritted, as to how much variation to expect on the given figure. At the time I tested it I considered it to be a significant change, although thinking back 600 Ohm's rings a bell. I will test it again tomorrow. Would you suggest anything above approx. 400 to be faulty and perhaps the cause of my cold start idle issues?