Question about 1998 Chevrolet K1500

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98 Chevrolet Silverado runs rough at half throttle and past.

My 98 Silverado Starts fine, idles fine, slight hesitation when you give it throttle but revs good untill about half throttle, then it will drop in rpms, at full throttle it really loses power and starts to back fire. I have replaced plugs, wires, cap, rotor, fuel pump, fuel filter, front oxygen sensors, map sensor. I have ran it without oxegen sensors, and pulled off exhaust pipes from the manifolds to see if cat was clogged. Nothing has changed how it runs. It will red line at half throttle then drop to about 4000 rpm at 3/4 throttle and 3000 rpm at full throttle.

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I have the same truck, same problem. After about three yrs. of chasing the problem i finally gave up. I tried GUNK engine flush during an oil change and beleive it or not it worked! It runs better than ever-no more stumbling, random misfire codes, or hesitation up hills. Apparently a sticky valve or lifter. Probably from coolant leaking into top of engine before i replaced the gasket. Try an engine flush.

Posted on Jun 14, 2010

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Try removing the EGR valve and clean inside of it and manifold

Posted on Feb 17, 2011

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Wondering if you tested the throttle position sensor and checked that all the fuses in the vehicle are good. You need to have the computer codes cleared to eliminate false codes you may have set while working on it, then go through a couple of drive cycles (about forty miles) of use, then have the codes read. I'm not an expert on your exact system but common to all is that all sensors work with each other and the computer needs a good signal from each of them in its "decision making" Mass air flow sensor and temp sensor included. MAF can be cleaned with carb cleaner and a Q tip, carefully wiping off the two wires that stretch across the inside.I often recommend either haynes or chilton as a reference but in electrical diagnosis chilton is far better...If you are doing the work yourself get one and follow the test procedures in there.
In the end, the manual is more valuable and cheaper than replacing parts 'till the problem goes away. When working with electrical, I often make notes on what I found during testing.. sometimes something that seems meaningless ties in with another item later on.
No reliable shop can make a diagnosis without the benefit of hands on testing. You shouldn't either.

Posted on May 12, 2010

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