I have a 1999 E150 4.6 with a miss at low RPM runs very smooth on the highway but with a bad miss in the city (sort of has a popping sound). Coils, wires, plugs, and injectors are good. Code comes up cylinder misfire #3, #4. Compression test #3 has 15 PSI. It's not using any rad fluid. How can I checkwhats causing the low compression?

thanks

James

 

On a 4.6 engine, bad piston rings are real common...could also be a valve related problem. Do a compression check on the cylinder(s) that read low again, then squirt some motor oil into those same cylinders and repeat the test. If the numbers go up quite a bit, you have a piston ring problem...if the numbers stay about the same, you have a cracked piston, head gasket, cracked block, cracked or warped head or a valve issue.

(The oil will help to temporarily seal a bad ring, but won't seal bad valves, bad valve guides, cracked heads, etc)

Since you describe a "popping" sound and didn't mention excessive oil smoke or oil consumption, though...I'm inclined to beleive you have something wrong int he valvetrain...and the "popping" you hear is compression escaping past the problem, either into the intake or exhaust.

 

You are right, It was a bent valve. The valve is bent just enough for the rocker to fall out.