Revs drop low then spring up, sometimes stalling, when accelerator released

Hi I have a '98 Ford Telstar Mystique. All is perfect until the engine reaches normal operating temperature, at which point any time I release the accelerator the revs will drop down really low before springing back up to idle speed. Sometimes the engine will stall. It happens in gear, in neutral, at speed, or at a standstill. Any idea where I should look for the cause? Much appreciated!

I’d be looking at a dirty throttle body and idle air control valve

In fact, I would go ahead and clean the throttle body and replace the idle air control valve. A can of throttle body cleaner is only a few dollars, and the idle air control valve is also reasonably priced

Heads up . . . if you buy the idle air control valve from Ford, you’ll have to buy the gasket separately. Ford is strange like that. No matter what you buy, the gasket is always sold separately


That is true, but because the OP’s car is actually a re-badged Mazda, he might be able to get the IAC from a Mazda parts department, with the gasket included.

Thanks that’s where my suspicions have been lying. I’ve had no luck finding a replacementIAC valve unfortunately (I’m down in South Africa, the telstars/626’s are pretty uncommon - and of course old - so parts are scarce). Hopefully a good cleaning will do. I’ll test the valve too once I’ve got it out and if it’s buggered then, well, to the scrapyards I shall go. Cheers!

If this car has a bad or misadjusted dash pot it could cause those symptoms.

Do you have a heated iac valve . . . ?

Yes, there is a coolant line that runs through it

Hi all I though I’d post an update if anyone is interested. I managed to get my hands on two spare IAC valves (met a guy who’s been collecting old Telstars) and tried both but the problem of plummeting revs persisted. Then as I was playing with the throttle I noticed that if I don’t let it close all the way the revs don’t drop down to stall level. There is a switch that the throttle hits when closed (I can’t find it in my workshop repair manual and have no idea what it’s called or what it’s for) but I disconnected it on a whim and the problem is gone. Been driving fine without it for a couple of days. Only noticeable difference is it revs higher than usual on startup, just for a second, then settles down, sometimes swinging up again before settling down for good. Anyone know what that switch might be called/what it’s for? I’d imagine the TPS sensor would be good enough to let the ECU know when throttle is closed…

Clean the throttle and reconnect that throttle switch. That might take care of the problem.