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1996 Chevrolet Suburban 1500 Question: idle is low when driving feel like it is missing or slipping run rough.

 

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sumertime, Columbia, SC, May 22, 2013, 12:42
 Rookie

feel like it want to cut off but dont took to shop was told something is retarding my timing did diagnosticand really did not find anything said might need to change computer,they reset timing. It start good ide is at1 on the rpm but drop to5 when put in drive am able to drive but not smooth,any ideals what might be or need to ck before changing computer.

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    ChevyASE May 23, 2013, 02:21
     Journeyman

    Were there any codes stored? Missing and slipping are 2 different things. Missing (or rough running) is engine related. Slipping is transmission related. You say idling low. I assume this is at a stop? When does it feel like it is missing? Idle? Acceleration or under a load? Cruising? All the time? Does the problem happen when hot or cold or both? Does the Check Engine Light flash when it misses? Need more info please.

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    sumertime, May 23, 2013, 12:18
     Rookie

    I have to say slipping might be wrong word,more like missing, happen when cruising and at a stop just feel like it want to shut off, but never does, the check engine light does not come on or flash. Some hesitation when acceleration but seem to get up on speed OK. Took to shop because if I let sit up for two days it would be hard to stay crank and shut off at stops,check engine light came on then went off,shop said something was retarding my timing, they reset timing said if happen again minght need to change computer,timing has not did the same thing or cut off from where they set it, not running smooth. Work that i have had done Intake Manifold gasket Exhaust manifold gasket Valve cover gasket,new Distributor,Crankshaft Sensor,Heater control vale,Radiator and Thermostat,Transmission service,Catalytic Converter,Fuel Injectors clean Oil change,Fuel Filter,and much more truck has 240,000 miles just want to know what it might be before changing computer?

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    ChevyASE, May 31, 2013, 00:58
     Journeyman

    You say they set the timing? Why? if something is retarding the timing, you don't reset the distributor, you find the condition that is causing the problem and correct it. With that kind of mileage, you could have many possibilities. Do you have access to a scan tool (not code reader)? You need to look at the "knock" signal reading when the engine is running. the knock sensor is what tells the computer if it needs to retard the timing. If there is knock signal, then you need to start looking internally in the motor. Do a compression test. All cylinders should be above 115 pounds, and there should be no more than a 15% variance from any one to the rest. It is possible you have worn piston rings and if the compression test fails, you'll have to do a leak down test to determine the problem. There could also be high amounts of carbon build-up in the cylinders and this could cause some detonation which would be like "pinging" which will also generate a knock signal and cause the PCM to retard timing. I would not take it back to the shop you went to. if all they did was adjust the distributor, and then said replace the PCM, you need to find someone in your area more qualified. When you start the vehicle, does it crank for a long time before starting (have you noticed it taking longer and longer to start during cranking)? What is your fuel pressure? And if the Pressure is ok, what is the volume (flow)? A system with low fuel pressure or low volume will run lean, and this too will cause pinging and cause the PCM to retard timing. I would be hard pressed to condemn the PCM at this point. Needs more CORRECT diag first.

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    sumertime, June 03, 2013, 10:46
     Rookie

    Thank for your reply, I dont think it the PCM myself.Iam not a mechanic so if you would break down the knock signal,is it the same as missing? I will do a compression test next.Like I said not a mech but when Ileave radio on and let sit for a day or two it would crank up okay but wont stay crank have to press gas a few times to get it to stay on,white smoke come out the back,if it shut off, the battery light is on,turn the key to start everything goes off.All the work I have done, never change spark plugs or spark plug wirer,when driving feel like it want to cut off not smooth.Iam not sure what feul pressure is but I can say still get 17 miles to the gallon city 20 to 21 highway.Also will check some of the other things you said it could be, the truck ran fine until the poblem with timing,if anything else you can think of to check with the information I have given let me know.

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    ChevyASE, June 04, 2013, 01:30
     Journeyman

    Knock signal is not the same as missing. Engine knock is internal and means the knock sensor is detecting detonation or pinging (pre-ignition) and is sending a signal to the computer to retard the timing until the knock goes away. You can't hear it with your ears (most of the time), you need to use a scan tool to see if the PCM is getting a signal or not. No other way to tell. You also just said "white smoke". How much smoke? How thick? How long does it come out before you don't see it anymore? That could indicate a different problem all together. White smoke is usually related to the engine burning coolant. You really need to do the compresion test and let me know what happens.

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