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1993 Chevrolet 1500 Pickup Question: Rough idle but otherwise runs fantastic

 

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maniac121, Panama City, FL, November 05, 2009, 14:21
 Rookie

Truck runs great when driving down the road and idles smooth for a couple minutes after coming to a stop. Then it develops a "stumble" that is very pronounced. I'm fairly confident it isn't ignition as the distributor, cap, plugs, wires, and coil are new. Grounds that I can find under the hood check good. O2 sensor is new. TPS is new and and at .6v at idle. MAP sensor is new. Spraying starting fluid around the intake doesn't reveal any vaccum leaks. Throttle body is clean. Fuel pressure is good. EGR ports are clean, but I'm unsure how to check and make sure it seals closed.

Other than at idle truck runs great and gets 17+ MPG on the interstate. What have I missed?

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    patrick mannion from Greg Solow's Engine Room, November 05, 2009, 14:32
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     Master

    You seem to have been very thorough and logical in your tests. I would look at the Oxygen sensor for clues. Look at the voltage, looking at it using an oscilloscope is most accurate because you can see voltage change pattern and cross count time. Voltage should cross between low to high .1 to .9volts when driven extreme lean or extreme rich. If the EGR was causing a vacuum leak the voltage would be "stuck low". I wonder if the fuel injectors are worn out. On a throttle body you can rev up the engine and shine a timing light down the throttle body and look at the atomization or see how it sprays at idle. I guess even listen to the injector using a mechanic's stethoscope (SEARS $15) or use a long screw driver put the metal edge on the body of the injector and place the plastic handle up next to your ear. The injector should open and close with a sharp "tick" not a dull "thud". On a port fuel injection car at least you have a basis of comparison by listening to other injectors. I wouldn't rule out a vacuum leak, Many automotive repair shops have a machine that generates an inert non flammable low pressure smoke that looks similar to cigar smoke. The intake manifold is sealed off, and smoke is put into the engines intake manifold. The smoke then fills the engines crankcase and if there are any vacuum leaks from gaskets, hoses, or housing smoke emits from the source of the leak. Some mechanics will use propane or aerosol carburetor cleaner, with the engine running they will spray the cleaner or propane around the intake manifold and vacuum hoses. A leak will cause a "lean mixture" condition, when the flammable fuel (carburetor cleaner or propane) comes in contact with the source of the vacuum leak the engine idle speed will raise as the engine has received "fuel". this method of finding a vacuum leak works but is not recommended as it can easily cause a fire.

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    maniac121, November 06, 2009, 05:33
     Rookie

    Inspected the injector pattern with a timing light last night and the pattern appears to be good. Injectors have ~80K on them, truck is up to 195K. It turned cold last night (I'm in Florida), and I now have an additional symptom.

    With the A/C off and idleing for an extended time the truck develops a cyclical idle. I don't have a tach, but my ears tell me it yo-yo's between 300 and 1000. Turning the A/C on bumps up the idle speed and corrects the cyclical idle; the "stumble" reappears.

    IAC was replaced when I cleaned the throttle body, swapping the old one back didn't change anything.

    Still working on a scope to watch the O2 sensor voltage. My digital voltmeter moves to fast for me to deduce anything from it.

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  • Answer #2

    Visitor, October 16, 2010, 18:39

    if its a 350 tbi then it could be the idle air control sensor..if clogged with carbon or not adjusted right it will die..

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