1990 S10 4.3 - high idle/code 21

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Hello all,

My 1990 Chevy S10 4.3 (TBI) has a running problem.  The symptoms are as

follows:  Shortly after startup (immediately to 3-4 minutes), the idle
speed will suddenly climb to 1500-2000 rpm and stay there.  This is
usually accompanied by a check engine light (code 21 - TPS high
voltage) and a return to normal idle speed.  Sometimes the code will
clear while driving and the high idle will return.  At least once I
have seen it clear and return to a normal idle with no check engine
light.  More recently, after a bit of driving with the light on, the
motor will start to hesitate and want to stall, requiring some
persuasion with the throttle to keep it from dying.

I have found that resetting the ECU will often clear this up for a
short period of time.   I followed the procedure outlined in Haynes to
check the TPS output with the ignition on and engine off.  It read 4.8
volts across A and B, way above the 1.0 volt maximum.  The manual says
to perform the base idle procedure before replacing the TPS, so I did.
After doing this, the truck ran great for a spin around the
neighborhood, but just as I turned back into the drive the idle kicked
up again.  I did the base idle procedure again and measured the TPS
output before going for a drive: 0.8 volts - right where it should be.
Another drive and this time it set code 21 and began to stall.  The TPS

output was again around 4.8 volts after this drive.

One thing of note during the base idle procedure: the idle air valve
makes an unhealthy winding noise while in operation.  Furthermore, the
Haynes manual made it sound like this valve should eventually extend
and stop, which it did not seem to do even after waiting more than the
recommended 30 seconds.    Is this normal?

So, where to go from here?  It seems like the IAC valve is a
possibility.  Can this thing be bench tested?  I think the TPS is ok
since it does output the correct voltage at times, and it looks brand
new (guessing the PO installed it in response the the code 21).  I also

suspect the ECU, as the PO said a shop had told him the truck needed
one (at $1000, he declined).  Are the $90 remanufactured ECUs I saw on
napa online any good?

Any advice would be appreciated.



Re: 1990 S10 4.3 - high idle/code 21


I ended up replacing the IAC valve, which fixed the problem described
below.  It appears that this part can fail in a manner that affects the
TPS voltage seen by the ECU.  Hope this helps somebody.


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Re: 1990 S10 4.3 - high idle/code 21

I was having the same thing going on it was a 89 S10 4.3 it was the TBI
it was sucking in air at the bushings of the butterfly and throw verey
thing off it would run great then all at once high idle then die then
same times it would be really hard to start. Hope this might help. EARL

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