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MG MGF Technical - Tiresome erratic idle problem

Idle around 1500, usual story: drops to 900 with a throttle 'blip' but then climbs again with the occasional foray to 2000+. Fan always on.
Coolant temp sensor replaced (needless change: checked the resistance of the old one in hot water- ok. Checked wiring from brown sensor to multiplug: ok.)

Cleaned out the carbon from the IACV.
Car is a 1996 VVC, with standard plastic throttle body & K&N filter.
Can't see how a warped throttle body would affect it as at idle it is closed? Car has been 'in storage' 12 months, started first time :-) but the problem predates the storage.

What next???
Charles Dundas

Contact cleaner/scrub with an old toothbrush on all the ECU pins and sockets. For certain the brown temp sensor input is open circuit/high resistance if the fan is staying on, and probably others.

Thanks, will give it a go when the rain stops (mid August probably!)
Charles Dundas

What type of K&N filter? If it is held on with a worm-drive jubilee clip, then there is a chance that the plastic throttle body is warped if it has been over tightened at any stage. Unfortunately it is something that is not readily identified by visual inspection.

However, if the radiator fan is always on, then there is another problem, unrelated to the throttle body. You've already done the checks to the loom and water temperature sensor, but it does appear that the problem persists - the MEMS is appears to be "seeing" a very high resistance across the water temp sensor, hence triggering the fan. This doesn't explain the erratic idle unless the electrical fault is intermittent...

I think I'd check the loom and sensor/connector out again - and clean all connections with contact cleaner spray to be sure.

Hope you track this one down soon Charles!
Rob Bell

I had a similar problem with my VVC. Changing the brown temp sensor made no difference. Neither did a replacement throttle body.

A trip to an ex MG dealer with a working diagnostic computer did no good until they read through their old technical bulletins and found the problem - VVC solenoids. Apparently these give the same fault as a broken brown temp sensor and it is not picked up by the diagnostics. Two years later the problem has not come back, and the fix was not overly expensive (in the UK that is)
Brian Highe

Nice one Brian - definitely one for the check lists! :o)
Rob Bell

thanks- a bit lax lately- toyed with the TPS but to no avail.
I'll take a look at this this weekend.
Charles Dundas

Well I think (at least hope) it may be solved.
Disconnecting the outboard side VVC solenoid plug (black) seems to cure it with a cold idle 1100-1200 rpm and no radiator fan!
Does this seem right?
Charles Dundas

Not sure why this would work. Does the problem return as soon as you reconnect the plug?
Rob Bell

1100 to 1200 rpm is 250 to 300 rpm too much so I suspect the VVC solenoid isn't the root of the matter.

This thread was discussed between 09/05/2010 and 29/06/2010

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