Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
I have a mystery misfire on my 1997 2 door tahoe witht the 5.7l vortec v8. It drives great on the streets, no hesitation in the throttle, lots of power and no signs of a misfire. As soon as I get up to 60+ mph on the freeway, it will start misfiring on cylinder 6, shaking the whole truck and throws a P0306 and P0300 code. It will then slip the TC so I can't get it into full lock or else it will just keep misfiring under load (especially when towing). I have a TopDon scanner tool that I used to set the camshaft posting retard when I installed a new distributor ( gear was pretty worn). I've also replaced cap and rotor 3 times, ignition coil 2 times, ignition coil control module 2 times, plug wires, all 8 spark plugs, fuel filter, crankshaft position sensor, transmission speed sensor, and both upstream 02 sensors. It still rides great on the streets, but nothing made my misfire on cylinder 6 at 60+ mph go away. When I tested the fuel pressure I get about 55 psi KOEO and about 48 psi at idle. I thought the fuel pump was okay because when I'm driving through gears the psi is a constant 60 psi, even when it's misfiring. I'm thinking about just replacing the fuel pump but I'm tired of firing the parts cannon at my beautiful truck. Oh yeah, MAF and O2 sensors all check out and fuel trims are normal and I upgraded the injector to the electronic spider about 5 years ago. Any help would be much appreciated.
You will want to test valve spring tension and look at push rod ends for that cylinder. this will determine if you have a valve floating from weak valve springs or a collapsing lifter\bent pushrod. also check and readjust rocker arms.
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