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Actually it's a Mercury 'Tracer', which is the luxury Escort
It has the 2.0L SPI engine.
No code light showing but when warm sitting at idle for 10-seconds it starts to lope dropping to 600 up to 900. When AC is on that is exaggerated to almost stalling. A trusted mechanic friend said it was most likely the IAC (Idle Air Control). Looks to be $80+. I have a Haynes book on it that has no testing procedure. Just replacing. Of course I would rather not dump money on an un-needed part (again). Is there a way to test these to be sure that is the problem? Would just disconnecting it's cable tell me anything?
Thanks
 

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It is possible the IAC is the issue, however lots of things can cause a surging idle. A large vacuum leak will also cause the same symptoms.

It is possible that disconnecting the IAC may just make the motor stall out as it could be compensating for a massive vacuum leak somewhere else causing the loping idle, as opposed to being the issue itself. A large vacuum leak should cause a lean code to pop up, but it may be just under the threshold where it would cause a check engine light.

Having a scan tool with live data capabilities would help you narrow down the possibilities, if your fuel trims are extremely lean at idle (adding fuel), but improve when accelerating the engine, you likely have a vacuum leak as the engine will produce less vacuum as you raise the RPMs. You would also be able to see what the engine is calling for on the IAC duty cycle (e.g. the engine is ramping it up and down trying to compensate), or if something else is acting funky.

Hell, a dirty throttle body can also cause some wacky things to happen at idle.
 

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I had a similar issue on my '97 and decided to just start with a standard tune up. When I pulled the plugs I was surprised the thing was still running at all based on the condition of the leads. After new plugs, wires, etc. The idle was much improved. Not perfect, but pretty good for 270k. Might be worth a quick check if you haven't yet.
 
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