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Old 12-22-2002, 07:38 PM
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Post bad throttle body?

I am hoping someone can help me with a problem I have been having with my 93 GMC. First off, it
is a 1500 with a 5.7 auto. My problem is that
when the engine comes to operating tempurature
the engine seems to miss if you sit at idle for
over 30 seconds roughly. when you take off at
full throttle the the engine maintains that miss
but if you let off to half throttle the engine will run just fine nice and smooth. I have replaced all vacuum hoses and checked for leaks
and have found none. I was thinking it is the throttle position sensor. I would appreciate any input on this. Thank you.

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Old 12-23-2002, 12:00 AM
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You can test the TPS fairly easily with a digital volt-ohm meter. I am assuming that you have done the usual checks to the ignition and found nothing? Are you getting a check engine light? A '93 should still be OBD-I so scanning for codes should be easy (not 100% sure how Chevy's do it though).
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Old 12-23-2002, 07:10 AM
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How bad is the miss, is it like a dead cylinder? I would look long and hard at your ignition system, have you tuned it up recently? A bad plug wire can cause this problem. If you haven't tuned it up, perform a secondary ignition tune up, including, cap, rotor, wires and plugs.
IF the miss still exists, MAKE SURE YOUR TBI BASE GASKET IS IN GOOD SHAPE, THESE THINGS ARE NOTORIOUS FOR THE GASKET BEING DETERIORATED.
Check your engine's compression in each hole, look for consistency in your readings, any low cylinder can cause a miss.
A faulty throttle body is rare, but, watch each {2} injector's spray pattern at an idle, and as you increase the throttle, if one of them looks like it's not spraying as much as the other, the injector screen in the TBI could be plugged up on one injector. If your injector spray pattern looks good, then I would dismiss the notion of a faulty TBI. As others mentioned before, have your truck scanned and look for codes, check EGR function also, as a sticking EGR valve can create havoc, but it should probably kick the check engine light.
The TPS sensor will cause maladies, but if you are suspecting any of the components on the TBI unit, IE-TPS sensor, IAC unit, it's sometimes about as easy to go and purchase a good used, known good TBI unit, sometimes as cheap as 40 bucks for the whole thing, which is cheaper than the TPS sensor itself, and it will have all of the components on it.
ONE OTHER THING, MAKE SURE YOU HAVE ADEQUATE FUEL PRESSURE, IF IT'S UNDER 10 PSI, YOU HAVE A PROBLEM, MAKE SURE YOUR FUEL FILTER IS NOT PLUGGED.
VERIFY, SPARK, COMPRESSION, FUEL PRESSURE AND VACUUM LEAKS FIRST, THEN GET MORE IN DEPTH WITH YOUR TROUBLESHOOTING.
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