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1991 silverado 3500: die..the engine is warmed..untill..idle fine…

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Have a funny problem with...
Have a funny problem with a 1991 Siverado 350 5 speed trans. The engine when cold starts wonderfully, ides for about 5 seconds and wants to die. It will do this for a while until the engine is warmed up (NOT operting temp, but warm) It will then go into fast idle (1500 or so RPM) untill op temp is reached and will idle fine. Power is down and will ping if throttle is stepped on (but not always) I replaced plugs, wires, cap, rotor, IAC. After about 30 or so minutes after engine has run, when shut off, it is hard to start. In fact, I had to take air lid off, sit with foot on throttle with ignition off, then take foot off throttle and it would barely start....any suggestions?
Submitted: 8 years ago.Category: Chevy
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Answered in 15 minutes by:
6/27/2010
Chevy Mechanic: ironmike, ASE Master tech replied 8 years ago
ironmike
ironmike, ASE Master tech
Category: Chevy
Satisfied Customers: 3,452
Experience: 30 years ASE Master L1 GM certified
Verified

greetings,

I would go right to the TP sensor.

  • Connect digital voltmeter from TP sensor connector terminal "B" (BLK wire) to terminal "C" (DK BLU wire).
  • With ignition "ON," engine stopped, the TP sensor voltage should be less than .85 volt if more than 85 volt verify free throttle movement. If still more than .85 volt, replace TP sensor.
  • good luck

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    Customer reply replied 8 years ago
    Ok, I tested voltage by backprobing wires, got .6 volts and as throttle was opened steadily climbed to 4.89 volts. Tested map sensor and ECT as well with same results (MAP at 4.89 volts with engine off, 1.21 volts with running) ECT was at 455 ohms after truck shut off for a while and at 220 ohms around operating temp. I was afraid to test O2 sensor for wires too close to exhaust and didnt want to break sensor. Truck has a funny sputter around 2000 or so RPM like spark plugs fouled...but can let throttle down or up from there and is fine. Possible Distributor issue or EGR???
    Chevy Mechanic: ironmike, ASE Master tech replied 8 years ago

    I would check the EGR for carbon causing it to stick open and next I would think about a new ignition module or even a reman distributor if original.

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    Customer reply replied 8 years ago
    Tried new module...no luck. Friend of mine told me he had similar problem in '90 Suburban. Turned out to be a leaky intake gasket on his vehicle. There is quite a bit of oil sludge build up on intake around EGR mount side of intake. Tried to re-torquing intake bolts, no luck. I dont have a vacuum gage to check vacuum @ idle, so will have to obtain one to test that theory. I was even pondering if fuel pressure regulator may be culprit. I replaced fuel filter with no luck, but somehow doubted that was the issue. Im no certified mechanic by any means, but common sense can go a long way! Any other input would be EXTREMELY helpful :)
    Chevy Mechanic: ironmike, ASE Master tech replied 8 years ago

    did you try spraying around the intake with propane or carb cleaner. If the idle picks up or gets smooth you have a vacuum leak. Check around the throttle body too.

    what do the spark plugs look like? white is lean

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