ken86

I have a 1989 F150 with the 4.9L inline six with 175,XXX. I am hoping someone can help me solve this annoying problem with the sixth cylinder. At a cold start the motor runs excellent but as it reaches full operating temperature it misses on the sixth cylinder and can be felt through the whole truck but seems to be less noticeable at higher RPM's when driving. it is only the cylinder and as you can see from the pictures the plug becomes very fouled but dose fire on a cold start. I have installed new plugs and replaced the sixth one a second time already, also new spark plug wires have been installed recently. After ruling out an ignition problem I believe I am getting oil entry from somewhere or possibly a vacuum leak. Should my next step be to perform a compression test when the engine is warm? When it is cold It tests within spec with the other cylinders. Am I having a stuck valve issue?

Matthew Burr

definitely do the compression check when warm. but it does look like something is entering the chamber and fouling it

sdmartin65

I have same buildup on plugs, my 6 fouls a set of coppers in a little over 5000 miles. My solution is MC single platinum plugs, still get some buildup but they keep firing fine. I have yet to foul a single platinum plug.