M20 slow idle - threads read and ICV/bluesensor/TPS ok

[manvell]

Hi

I'm pulling my hair out with this one - 1987 325i Sport (MTech 1) will only idle at 650 rpm hot or cold. This can't be described as lumpy, but it isn't as smooth as it is at 900rpm where I would prefer it to be.

It doesn't stall, it accelerates fine (shifts actually!), not hesitant, average 32.8 mpg at 80mph so it looks ok on paper.

However, when it starts up it hunts aggressively 500-1200rpm (ish) about 4 times before settling on an unsteady 650rpm.

I have two of these cars so can swap bits about and it is not the ICV, the ECU, the TPS and the driving performance and the plugs suggest the mixture and therefore the AFM are ok.

I chipped the ECU (Ants wild one) to raise the idle to 900rpm but it is stuck at 650rpm

No apparant air leaks as opening the oil filler cap introduces a big air leak and the thing nearly stalls, I've removed and refitted all the vacuum hoses, inlet gaskets etc...

Wind the idle stop up a bit as far as possible without opening the TPS, but the rpm doesn't change much until the TPS opens and the revs go way past 1000rpm.

Just fitted a new blue sensor (genuine BMW VDO part) despite the old one testing ok and no difference. When cold, removing the connector while the engine is running shoots the revs up past 2000...

Lastly- schrick 272/272 cam. Cold tappets set at 0.25 a couple of times and also 0.3 just in case. Idles ok but not smooth and shifts when driven but not as it should. It's got an original Hartge branch manifold on it but from what I understand this tuning is mild and the car should still idle?

So, why do I care - because the idle at 650 is reliable but lumpy enough to shake the exhaust about, over straining the manifold studs (new) and rattling the exhaust like a bag of spanners.

Help?

Thanks,

a.

[Jhonno]

they idle pretty much that low.. mine certainly does

[Jhonno]

oops didnt read bout the chip.. .errr

[Simon13]

hmm stange the cam will be having some effect for a slightly lumpy idle but she should tickover a bit higher more like 700-800 if it was on a standard chip.

my alpina idles a bit lump and the cam is less aggressive than yours!

Is your exhaust properly secure throughout? or is it really lumpy?!

[Turbo-Brown]

Thought the idle speed was supposed to be about 700rpm too, although I can't find reference to that in the hand book or Haynes manual now!

Don't adjust it without checking what it's actually for, but I thought there was a screw on the AFM that had something to do with the idle, although that could be completely wrong!

[Dan318-is]

make sure you air flow metre is adjusted properly using a bear machine/gas analyser, both my idle faults have been to do with these!