1994 Ford Probe idle issues

AMOPOWER

MEMBER
1994 FORD PROBE
4 CYL
2WD
AUTOMATIC
159,700 MILES

Need some help from some advanced mechanics out there. Got an EEC-IV controlled 94 Probe, automatic. It starts up and idles cold at 2000rpm, then slowly (~3min) drops to normal - 750 or so. The trouble is, when I am idling and if I tap on the gas and let go, it will go back up to that 2000 and stick there for a few seconds, then drop quickly back to normal idle.
I am struggling to figure out what is causing this temporary stick. I am lucky to have a breakout box, so I was checking various sensors while reproducing this behavior, and I only found that the Spark output (timing?) Was changing during this behavior.
TPS voltage range is within spec. EGR holds vaccuum. Throttle body is clean.
Overall idle vacuum is low though - 12in mercury.
I am wondering if the throttle plate screw is open too much, letting too much air in which in turn is decreasing my vaccuum, yet the idle is still OK because the computer is compensating correctly (perhaps by decreasing the IAC duty cycle?). The timing change delay is confusing to me though, it's almost like the computer's got a delayed reaction when switching from Part-throttle strategy, and warm idle strategy. (Assuming that me tapping the gas is what EEC-IV would cosider part-throttle strategy, and me off the gas (throttle plate closed) is warm idle strategy.

3 Replies

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Have you checked the idle air control valve? Also, has the EGR been checked?

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Monday, October 18th, 2010 AT 1:48 PM

AMOPOWER

MEMBER
2 POSTS

EGR is fine, it opens when the ECU commands it to, and closes when it doesnt get the command (i know this by monitoring the EGR valve position voltage).
The only thing I seemed to notice is that the IAC duty cycle does stay high when the idle stays high, then when it "catches" back to normal, the IAC duty cycle also drops down. To me this means that the ECU is falsely telling the IAC to stay high when it shouldn't, but I dont know what is causing this. The IAC is opening as it is being commanded to (so its not sticking), but what the hell is causing it to be told to stay high for a little bit, then drop to normal several seconds later. (Picture yourself sitting at a stop sign or in your driveway, tapping the gas and letting go. You would see the tach go up but them immediately go back down to idle rpm in a "normal" car. With mine, it will stay at 2000rpm for a good 5-10 seconds before it decides to go back down).