'90 Escort GT, Rough Idle

Hello group,
148k miles, the car is idling erratic from normal, but consistent. Comes to a stop and it revs up to about 1750 for a short time and then
drops down to about 900, rough idle, check engine light.
I checked the vacuum pressure by tee'ing in the vacuum line between the PCV valve and the throttle body, the reading on the guage came in at 15 PSI @1000 RPMs, but not sure what it should be.
I looked in the haynes and chilton manuals but could not find any manifold pressure spec listed. Does anyone what the vacuum should be at 1000 RPMs for this car?
I can hear a hissing sound near the firewall, but so far have not been able to find an open vacuum line. It might be coming from inside the air bypass valve, not sure, maybe need a stethoscope. I tried listening to egr valve with a spare vacuum line, but did not hear any vacuum hiss.
Any ideas, TPS, EGR, air bypass solenoid, O2? Getting a KOEO 41.
Thanks, Matt
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I suspect that you have a vacuum leak somewhere. Could be a hose, or a device that uses vacuum (like EGR valve or heat riser, etc.)
At 148,000 miles and given the vintage, you probably have more than one.
Check your EGR valve with a vacuum pump and gauge with the engine off and see if it moves proparly and doesn't leak vacuum. Do the same with everything else that uses vacuum.
Also check for air leaks, clean your MAF sensor, check your coolant temperature sensor (contacts get corroded easily).
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scott snipped-for-privacy@my-deja.com (Childfree Scott) wrote in message

I was thinking it was time to pick up a vacuum pump.
Does the cooland temp sensor sends a signal to the idle air control valve? I think I remember reading this somewhere, just not sure.
Also, the cooland temp sensor is not the switch used to turn the electric fan on (in the thermostat housing), it's the one screwed into the head or block underneath the intake?
Thanks, Matt
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You have 3 cooling temperature sensors, this is what they are connected to: 1. dash gauge 2. cooling fan (relay) system 3. computer, which in turns has an output to the IAC valve.
tyr http://www.feoa.net
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