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Old 08-02-2010, 02:36 PM   #1
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Default Silhouette Intermittent Slightly High Idle Rpm

This has been going on more a few months. Idle in gear usually is around 750 with warm engine but there are times it is 1000, 1100 and as high as 1300. If I shut car off and restart idle is back to 750. I suspect a sticking IAC but the other day I got a code for low circuit voltage for TPS (P1022 if I recall correctly). I cleared code and haven't seen it again. What do you think it is, a bad IAC and/or bad TPS?
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Old 08-02-2010, 03:38 PM   #2
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if you have a scanner, hook it up the next time it happens and look at TPS%. i've had this happen before and every time, the TPS% was reading 2.35% or more with the throttle fully closed. i believe it had to do with the ECM'* 5 volt reference signal being less than 5 volts and then the ECM rescaling based on bad data, which caused fake idle readings. the engine fought against the brakes quite a bit too... no throttle from a dead stop would get the car up to 30MPH quickly just from the ECM commanding the IAC opened further and more advance due to being at "non-idle".

i never had luck with shutting off and back on though, i always had to pull both ECM fuses for 15 seconds or so to make it go away, though it only seemed to happen when my MEMCAL wasn't fully inserted into the ECM.
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Old 08-02-2010, 05:09 PM   #3
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Thanks, I'll check out the readings when it does it. I don't know why I didn't think of that but I have a lot on my table at the moment which explains that.

So if I see a high TBS% would that mean bad TPS or bad PCM?
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Old 08-02-2010, 05:17 PM   #4
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could be either, both or none.

none: the 5 volt reference wire, or any of the connections it makes, is getting flakey and will intermittently not be 5 volts, or another sensor which uses the same 5 volt line is drawing too much current and dropping it down.

both: well, it'* possible, and there could be many causes.

either: if the TPS is becoming inconsistent, then when the throttle is closed, it may not drop down to the normal voltage that the ECM expects at closed throttle. look at TPS voltage when it'* acting normal and odd as well, it may provide some clues. if the ECM can't keep a good regulated +5V line, then other sensors may start acting funny as well.
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Old 08-02-2010, 09:30 PM   #5
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Thanks! That clears things up a bit.
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Old 08-08-2010, 03:41 PM   #6
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Finally got scanner on it and here is what I found. When it was idling at 1200 RPM the throttle % was 2.7%. I turned it off and idle was where is is normally and it showed .4%. I think I have a bad TPS. Any thoughts?
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Old 08-08-2010, 03:46 PM   #7
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couldn't hurt. if it'* not the TPS itself, check the wiring, and then suspect the PCM.
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