Idle speed

[LeeB]

I'm sure someone can help me with an issue I have regarding idle speed. My W2 idles ok normally, drive around at 'town' speeds, no issues. I've noticed that if I push it over 4000 revs then the next time I return to idle it sits at about 1400RPM I.e. too fast. This doesn't change until I switch the car off and restart it, then all ok again - it doesn't need to cool down or anything for this to happen. Do I have an 'idle speed valve problem' or a 'throttle body' problem?

[16v_paddy]

You've got the common idling problem that can be caused by a long list of things ranging from faulty lambda, coolant temp sensor, throttle position sensor, idle control valve or even the speedo cable earth

[Chris197F1]

Welcome to club mate, im working my way through said list lol, only the earth cable left for me....

[LeeB]

Thanks, I guessed it wouldn't be that simple, ive seen lots of talk about idle speed valves and thought that perhaps that would be the obvious starting point. That said, as far as I can tell, my car doesn't typically run at the right temperature according to the gauge on the dash (so need to get a new therm anyway), so I'll start with that! Thanks, Lee.

[Coops]

nine times out of ten when its that set of circumstances (drive up the revs then it wont idle below 1400) its the road speed sensor earth on the top of the speedo cable. easiest way to sort it is whip clocks out, find the 3 pin plug at the top of the speedo cable.

then snip the black wire on the cable side of the plug, extend the wire from the speedo cable side (not the back of the plug) and use a small self tapping screw to fix it to a decent earth on the bare metal of the bulkhead behind the dash. or if you don't want to 'damage' the bulkhead just run the new bit of cable down to the earthing point on the sill near the drivers footwell.

[paush]

I cut into the speedo earth for mine years ago but a little while back I had the dash in bits and was curious why the original earth stopped working.

After much tracing I found that it is earthed from the back of the head unit, at least on my Williams 2.

I didn't clock it at the time that I'd only recently fitted a head unit and only used the thicker earth wire.

There was also a thin earth wire just lying there that used to be connected to the same terminal on the original head unit plug. I obviously didn't realise this at the time!

When I connected them both back together at the back of my head unit iso plug it fixed the original speedo earth.

[Coops]

plus one for Sherlock investigations

[paush]

Haha thanks buddy... You'd think with all this time on my hands for investigations I would have the bloody thing boosting on the road by now lol