Sleepless in St. Louis, 91 C1500

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  #1  
Old 09-15-04, 03:20 PM
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Sleepless in St. Louis, 91 C1500

Double start engine, hot or cold. Engine stumbles at upshifts (auto tranny).
At idle engine lopes. Put on new idle air control valve, no change. But, noticed if i take the IAC off and hold my thumb over the hole, the engine speeds up and runs smoothly and keeps running smooth after i take my thumb off the hole? Changed the O2 Sensor. New spark plugs. New fuel filter. New fuel injectors. Fuel regular diaphram kit installed. The Throttle position sensor seems to be ok since the motor runs funny when we disconnect it. The EMC seems to be happy showing code 12. And it does record trouble codes ok when other components are temp. disconnected. 156 k miles. We are out of ideas, help help help !!!!! Hopeless Harry.
 
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  #2  
Old 09-15-04, 03:44 PM
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Location: Pittsburgh,Pa
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Have you checked the ignition timing yet(don't adjust it)?Also a check of compression is in order perhaps the timing chain has jumped or is very loose altering valve timing.When you remove the IAC I hope it's unplugged so it doesn't unscrew itself.Also when you do this "test" does it idle real fast?Does the truck perform well at off idle speeds,pulling hills etc?
 
  #3  
Old 09-16-04, 03:14 AM
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you might also check fuel pressure should be 9-13 psi if its low the engine will usually surge at idle and may have other driveability problems such as hesitating.
if the pressure is below 9 try pinching off the return line if the pressure immediatly goes up to 15-20 psi it is a regulator problem usually the spring in the regulator, if it does not go up above 15 psi then the fuel pump is weak.
 
  #4  
Old 09-16-04, 07:39 AM
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Sleepless in St. Louis

Davo and Bejay,,thank you so very much for the new ideas to try. Davo, yes the idle does go higher than normal while my thumb is on the IAC hole. I think we goofed, since you said disconnect the wiring from the IAC, we didnt and the needle shot out of it, but, Haynes showed we could put it back by wiggling it from side to side as we put it back down to 1 1/8 inch. We can try our old one if we messed it up. And we will check the ign. timing. Bejay, we will check the fuel pressure and hose pinch. I will let you know. You guys are GREAT
 
  #5  
Old 09-23-04, 04:49 PM
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C1500 Chev Truck

Well we did a vaccum test and it is right on the money at 20psi and steady. We did the ignition timing light, it was right on the mark. We do not yet have tools for a compression test, nor a fuel pressure tester, 40 bucks. But, i am still baffled at that darn IAC Valve. I was messing around with it again and we just disconnected it to see what would happen. The engine starts and runs smoothly, although the idle is too high. It drives and shifts perfectly this way but will not idle down of course and when i take my foot off the pedal it will just keep increasing the speed. And of course when i hook the IAC back up,,,back to lousy idle and upshift stalling again. We even put the old IAC back in, just in case, same results though. I realize that there are many other do dads involved but it sure is strange that the IAC has such a negative reaction when i hook it up. I am sniffing around at the knock sensor and electronic spark control now, is that a waste of time since it has the ability to retard the spark if something doesnt work? In my shade tree days (i am 67), the old vacuum spark advance made a car run like poop if it didnt work although it didnt affect idling. Other than the pressure check and compression check, do you have any other thoughts on this. I do greatly appreciate any input i get from you.
 
  #6  
Old 09-25-04, 07:44 AM
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Davo, Carguyinva, and Bejay, thanks guys, problem finally solved. But i wanted to pass this back to you. Since the fuel pressure check kit cost as much as the new fuel pump, i put some gas in a windex spray bottle (and it didnt melt!), (laugh), anyway, the windex bottle sprays a fine mist and i used that to spritzer some gas in the carb as we were starting it cold and it turned on just fine, and i could keep the truck running ok as long as i kept helping out the fuel injectors with the windex spray. So we figured we would go for the gold and did, we dropped the fuel tank and put in a new pump and filter. The filter was black tarry looking and felt stiff. The truck NOW RUNS LIKE A NEW ONE. Heck it almost is with all the new parts we put on it. What part of ASK the car guys didnt we understand? hahahaha. well guys, next time we will ask and listen. Thank you all very much, you should be paid for this service and paid well !!
 
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