92 Aerostar idle surging

[Popjack]

Read the codes no faults found,checked MAF and cleaned and OK.Same for the IAC.Found no vacuum leaks.
Starting form a cold start surges and drops to low rpm almost to a stall when in Park.Place in Reverse and idle smooths out.Turn ignition off and try to start again and very difficult to start almost like there is no gas.Once it starts it again has surging idle.After engine warms all is ok and runs just fine.
Puzzled by this,any ideas here would be appreciated.Thanks
Forgot to mention I did check TPS and had smooth voltage range from .88v to 4.45v Think its ok.

 

[BrooklynBay]

It could be the O2 sensor or the TPS.

 

[Popjack]

Thanks BrooklynBay.That O2 sensor is on my suspect list.Will take a look when weather warms up.

 

[K80]

If by surging you mean over 1000 RPM then lower, then higher, etc, then that sounds like a fuel line issue. When it gets cold out, I put HEET into my tank....about a fourth of a bottle every time I get gas as well as a bit in my back up gas can. If I don't use HEET, I have to turn the car on but not start it, wait two seconds, turn it back of and on again, etc until I can hear the fuel pump running smoothly. Once the fuel pump has been running for a while, it stays hot on its' own.
If by surging you mean it stays steadily high ( say 1500 or so) until the engine gets warm, that is normal behavior. The engine temp sensor tells the ECU when the engine is hot enough to switch to a slightly leaner fuel mixture to save gas. It also sets the idle lower to stall the torque converter and reduce vibration while idling.
Now, I've never heard of an engine temp sensor failing intermittently, but only while cold. Same with just about any other sensor in the car; they either work or they don't, hot or cold. It is mechanical things which exhibit the greatest sensitivity to thermal differentials. Therefore, if temperature seems to be a factor, I would bet on a mechanical difficulty.

 

[Popjack]

By surging I mean 900-1000 rpm down to almost stall and backup again.I was beginning to think it was mechanical also but what ?
Can anyone tell me if O2 sensor is bypassed untill engine warms up?
What puzzles me it runs perfect when gear.Travelling along there is absolutely no problem whatsoever.

 

[BrooklynBay]

Check the resistance of the engine coolant temperature sensor when it's cold.

 

[Popjack]

Problem solved !!!

The ECT resistance checked out as per the table posted elsewhere on this site.I was running out of ideas so I took my focus back to the IAC.Started from a cold engine again and surging still there until I pulled the electrical connection,idle evened out but bit lower in rpm.I thought this is probably normal.
PCM was telling IAC to do this based on info it was receiving from the sensors,or was it.I decided to heat the IAC with a heat gun.Started engine again after plugging electrical back to IAC and idle was normal.
Bingo IAC is the problem after all.

Replaced with new IAC and cleared KAM (keep alive memory)by pulling negative batt cable for 15 min. so PCM could relearn and create new idle tables.

This puppy runs like a charm now!!

 

[BrooklynBay]

Thanks for the update!