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Keep this in mind. that something that is telling the motor to Idle high very well is the computer (ecu) it self. the prom in my ecu went bad, truck would cold idled at 1500 for 5min then returned to 550.ran great but idled high when cold. when warmed up and shut off for 10 min again would run at 1100 then return to550 changed ecu guess what, cold start up 1100 for 12-15 seconds idle down to550, drive till warm shut off for 10 mins. restart
1000 rpm for 5seconds, down to 550. Very happy to have changed the ecu.
get a ecu that still has the prom/calpak it came with from the factory you'll
be surprised how well it runs.
 

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I had a similar experience with a Chev Silverado, same year. It was still running badly after all sensors were replaced. there were several electrolytic capacitors in it that had leaked (electrolytics are infamous for this when exposed to time and heat). I repaired that ECU and the truck is running as designed. I'll rule out everything else on this S10 first before swapping the ECU. I already took the ECU out and opened it. Completely different design, must better construction, and no electrolytics. A component may be bad, but can't tell by looking.
 

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Well, my new idler wheel came in and I installed it. The truck idles much better now, after replacing the temp sensor in the manifold. Well, that's done:) But I noticed a different issue that I though had been resolved. I was getting ready to put the bed back on, and I see my spare is flat. I took it to a shop a while back and they cleaned the rim, applied sealer, and reset the bead. But it's flat again. they said the tire was good, but the rim is rusty. I knew that. It hadn't had any air in it for years. Well I reckon I need to buy a rim someplace.:(
 

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Did it pass the emissions test fine last time? have you done any modifications to the motor since then? Vacuum leak sounds like a possibility, could even be the TBI base gasket. I would check/tighten the TBI bolts. A can of WD40 or carb cleaner with the little hose on it can help locate a vacuum leak-spray a shot in the suspected area and if rpm's stutter or rise, you may have located the leak. A stethoscope or length of small diameter hose held to your ear can help you locate a leak as you test various areas-you'll hear a sharp hiss if you find a leak.
 
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