hanging idle when coasting....

DemonGT

Founding Member
May 24, 2002
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Sparta,WI
my car idles fine...but when i drive it...and im coming to a stop light...i push the clutch in and the RPM's hang at around 1500 then a moment before coming to a complete stop it will drop down?? or as soon as i come to a stop it drops? what could cause this? is this the TPS or IAB...i do have a ford idle plate behind the IAB...mabe ill try removing it..need suggestions
 
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i would guess you are describing the VSS(vehical speed sensor). there is no problem going on. it is suppose to keep the idle up a little to help with up and down shifts. with a stock motor many don't notice it. when you throw in a modded motor it becomes more noticeable.
 
but i mean...i was going down hill on a road so i just coasted for like four blocks down hill with the clutch in and it did this?? also..where is the VSS at? its on the tranny isnt it?
 
the effect of the VSS besides for the cruise control and other various EEC things is to keep the idle up, while the car is in motion. to test it, when car is idleing in the driveway, blip the throttle a few times, or even hold it at 1500, 2k for a couple of seconds. let off of it and then the rpm should then drop back down to whatever you have the idle set at. next, take it for a drive. just do a couple of 1-2 shifts,then push in the clutch or just put it in nuetral. the rpm should stay up above 1k. when you come to a stop, after a brief moment the idle will come back down. reason is the VSS is reading that the car has stopped, and thus not needing the extra rpm's, it goes back to idle mode. another test is go on the hiway or a long road and get speed up and then coast as long as you can/want. as long as the car is moving the idle is going to be a little high because the VSS is detecing motion. if you try to "blip" the throttle like in the carb car days of warming up a car, the idle will still be high. but when you come to a complete stop, then the idle will come back to normal. if this is what is happening, nothing is wrong and the VSS is doing it's job.
 
what jerry said is what its doing..just seems to be keeping the RPM's alittle TO high...close to 1500rpm....what should it be hanging the rpms at? seems to me it was like...1100 or something...will try some carb cleaner on the IAC...

any other info on if i can unhook the IAC with the idle plate?
 
DemonGT said:
what jerry said is what its doing..just seems to be keeping the RPM's alittle TO high...close to 1500rpm....what should it be hanging the rpms at? seems to me it was like...1100 or something...will try some carb cleaner on the IAC...

any other info on if i can unhook the IAC with the idle plate?
It will depend upon mods, base idle setting, etc. 1000-1500 RPM rolling idle is right in the ballpark, IMHO.

If you have a vac leak, that can create a hanging idle too (I think someone mentioned that, but it bears repeating). good luck.