Just checked the spark plugs on my 3.9 V6 and most of them looked good except the farthest one back on each side nearest the firewall....covered with carbon. Why is this? Is it a compression issue? obviously the cyl isnt firing as well as the other four but the engine works fine...starts fine...mpg's still good but I am concerned that two plugs look radically different than the other four....any ideas? Thanks

 

check your compression and find out. Could be those two are running rich. Fuel delivery problem?

 

read a lot about plenum gasket leaks in the intake manifold on 3.9 and 318.you may have a small one at the rear. take off your throttle body and look down the intake with a flashlight at the rear runners to the back cylinders. if they look oily but the rest of the intake is fairly clean this could be the problem.

 

Thanks guys

 

your welcome, is the truck a 87-91 throttlebody injection because fuel delivery is not an issue with these, but if its a 92-96 multiport injection then nicegy525's option is probaly the best bet. i own a 89 so i tend to think in terms of throttlebody injection. let us know im interested in this one.