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I have an 2002 with 4.7L engine with mis-fire problem. Based on computer read outs I replaced plugs, fuel pump and injector and coil on #3 cylinder. and still having mis-fire.

Further checking shows low compression. 80-90#'s on on cylinders 1, 3, 5, and 7. I was told this was a leaking head gasket. My question is whether that is likely the problem, or maybe something else. If it were the head gasket I would expect it to be limited to one or two cylinders, not all four equally low.

I would appreciate your input. Is it a bad head gasket or maybe a valve problem?
 

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I'd do a leak down test next - it could be a head gasket, but like you said, you'd generally see leakage between 2 adjacent cylinders. Because this engine is SOHC, this could actually be a timing issue.

If you hear air coming out the coolant reservoir, somethings wrong with the head or head gasket. In fact I would have the head checked if this is the case.

If you hear air coming out the intake or tailpipe, first double check that the piston is at TDC, and if air is still coming out, you have a timing issue. Check the cam and timing chain on that side, making sure everything looks right. If it does, then I would start to suspect the valves.
 
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