1997 Lincoln Town Car: engine surges

I've got a 1997 Lincoln Town Car, Cartier Series. I notice that when idling in the Drive Gear, the car sometimes surges ahead, occasionally with force a bit stronger than
the force with which I've got the brakes holding it back. And sometimes, the engine dips really low, almost to the point where it seems like it's going to stall, which thankfully it doesn't.
One other pattern: the car only does this when I've got the heater or A/C on.
What could this be and how might I repair it? I took it to Midas and they adjusted the idle, which helped but didn't ultimately solve the problem. TIA.
-The Derfer
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There IS no adjustment to the Idle Air control. What they PROBABLY did is either adjust the throttle stop screw... a NO-NO.. or spray some intake cleaner in the IAC.
NEVER LET THOSE MONKEYS under your hood.
If you have some metric wrenches, you can go to NAPA, get the part and replace it yourself. I dont recommend cleaning it. Effect wont last long.
While you are at it, you might google on how to clean your Mass Air Flow sensor. Needs done periodically, never mind what Ford says.
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maybe its related to A/C compressor which obviously takes some power from the engine. mine 4.6 does the same, its normal i guess. Without A/C it idles fine, i have new ignition wires, new spark plugs and MAF cleaned, and air filter new too. Dont know what else could affect the idle speed, its controlled electronically, no any adjustments,
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On 29 Oct 2006 16:21:37 -0800, snipped-for-privacy@gmail.com wrote:

As is the case with the OP, your problem is also likely an IAC problem. If the IAC is working properly, you will have to pay very close attention to ever know when the a/c comprerssor is in or out. Many times, the IAC can be cleaned by soaking the valve end overnite in a good solvent. You can also just bolt in a new one from a dealer for about $80 or aftermarket for about $50. Either way, the IAC is the likely problem. You should never adjust the idle stop screw. That controll the engine base idle. If you adjust it to cover up an IAC problem, the IAC will not be able to properly control the idle even if it is replaced. Setting the base idle must be done "by the book".
Lugnut
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lugnut wrote:

Isn't it necessary to recalibrate this after replacing the IAC valve anyway?
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snipped-for-privacy@gmail.com wrote:

No... has nothing to do with the IAC, except to keep IAC air control within proper range. If they adjusted the stop screw to increase idle speed, the IAC might hit the internal end of travel when starting in very cold weather. With resulting hard to start symptom.
Once IAC is replaced, get a tach and check for approx 550-600 rpm when engine is up to temp and IAC connecter off. If higher than that, adj to 550 rpm or the rpm stated in book. That rpm is for v-8 only.
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On 29 Oct 2006 17:58:31 -0800, snipped-for-privacy@gmail.com wrote:

Not unless someone has already screwed around with the idle stop. Idle stop adjustment also changes the base for the TPS. The only thing you need to do in a routine replacement of the IAC is make sure the throttle body is clean. You can get the procedure for a good service manual- you may find one at your local library. The MAF also needs to be cleaned occasionally - this is a good time for that maintenance. Once you do this, the codes need to be cleared. A scanner is the best way to do that but, disconnecting the neg battery cable for 10-15 minutes will also clear it. If you clear the codes this way, start the engine with all accessories "off" and don't touch a thing other than the starter. Let the engine idle til warm and move it among each of the trans selector positions several times allowing time for it to stabilize in each. Drive the car around normally for a few miles to allow the PCM to relearn the engine operating parameters. You should be good to go at that point.
Lugnut
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The Derfer wrote:

Clean or replace IAC valve. Take it to someplace like AutoZone where they'll pull the engine trouble codes, for free (except Caifornia).
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