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1994 Chevy Silverado PU missing
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mgwest
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Jan 31, 2009, 3:06 PM
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1994 Chevy Silverado PU missing
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I own a 1994 Chevy Silverado C1500, 5.7l engine, automatic transmission, with all accessories, 137,000 miles. The engine idles at 625rpms in park and about 250rpms in drive. I have developed what I call a miss when idling in park or drive. The miss does not appear to occur when rpms are up under driving conditions. Usually occurs between 250 and 1600rpms. I have replaced the spark plugs, plug wires, coil, distributor cap, rotor, oxygen sensor, fuel filter, and air filter. I have cleaned the fuel body and checked the pressure of the fuel pump. The throttle position is off by about .02. I have checked for vaccum leaks with none located. Timing is correct. I am now out of ideas and need any suggestions that can be offered.
Any reponses would be greatly appreciated. Thanks to anyone who response.
Marc
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Loren Champlain Sr
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Jan 31, 2009, 4:42 PM
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Re: 1994 Chevy Silverado PU missing
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Marc; This is TBI, correct? I know that you stated that you checked for vacuum leaks, but take a very close look at the TBI base gasket. They usually blow on the back side. Also, look at the spray pattern on the injectors as it is idling. They should have a nice, even, spray pattern. You may want to take an intake manifold vacuum reading, also. If the manifold vacuum is low, it could be overfueling. What did the old plugs look like? Were they nice and white, or black and sooty?
Loren
SW Washington
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mgwest
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Jan 31, 2009, 4:58 PM
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Re: 1994 Chevy Silverado PU missing
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Yes, this is TBI. The spray is smooth and even. The old plugs had carbon build up but appeared to be dry. I am in going to take the plugs out again and look at them now to check for a possible leak in the manifold gasket. I have not checked the manifold pressure. Not sure how to do that...
Thanks for the quick reply...
Marc
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Loren Champlain Sr
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Jan 31, 2009, 5:12 PM
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Re: 1994 Chevy Silverado PU missing
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The manifold gasket is possible, but not likely. You'll need to aquire a vacuum guage and 'tee' it into a manifold vacuum source, such as MAP sensor. Take a reading at idle speed and another at 2500 rpm and post the findings.
Vacuum guages are relatively inexpensive and come in handy, from time to time.
Loren
SW Washington
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mgwest
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Feb 1, 2009, 5:11 PM
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Re: 1994 Chevy Silverado PU missing
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Little more info...I have not checked the manifold yet, but I did replace the spark plugs. All of the spark plugs were white. The #4 plug however, was white but caked up really heavy. It did not smell like antifreeze but unknown why it looked like that. Still had miss after replacing spark plugs. I changed the oil and it looked normal. I did not detect any foreign matter in the oil. Plan to check the manifold in the next day or so.
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Loren Champlain Sr
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Feb 1, 2009, 5:23 PM
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Re: 1994 Chevy Silverado PU missing
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Marc; I should have asked this first, but have you double and triple checked the firing order? It is very easy to get a couple of wires crossed. Driver's side is 1,3,5,7 and the pass. side is 2,4,6,8 numbering from front to rear. The firing order is 18436572, clockwise.
Loren
SW Washington
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mgwest
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Feb 1, 2009, 5:56 PM
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Re: 1994 Chevy Silverado PU missing
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Yes, the firing order is correct. The miss is not that bad. More like a hiccup every now and then. Not consistent either. Sometimes worse than others. Sometimes not there for several minutes. Thought it had something to do with temp, but sometimes it may not be there when warm or cold. That's why it's got us stumped.
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Loren Champlain Sr
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Feb 1, 2009, 6:46 PM
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Re: 1994 Chevy Silverado PU missing
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How many miles did you put on the engine before replacing the plugs this time? #4 could be on the EGR runner. Not sure. You should be able to look at the intake manifold and see which runner it is mounted to. But, if a leaky EGR, I'd think that the plug would be extra 'white'. Don't suppose you could take a pic of that plug? Several things are starting to enter this old brain. Valve guide seal leaking into that cylinder? Broken, or weak valve spring?Would really like to see that plug. Have you performed a compression test? Do you have any valve train noise? Weak lifter?
Loren
SW Washington
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mgwest
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Feb 2, 2009, 1:30 PM
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Re: 1994 Chevy Silverado PU missing
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The plugs had about 11,000 or 12, 000 miles on them at the most. Have not done a compression test yet, plan to do that when I test manifold vacuum. There is absolutely no valve noise. When listening to the engine it's quiet. Without the miss you wouldn't know it's running.
I have a picture, but can't figure out how to post it here....
Marc
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