'99 explorer 4.0 #6 cylinder misfire

[Itali83]

Hey guys, new to the forum but have searched a lot of the great information here. Here's the deal, truck has dead #6 cylinder. Has new plugs and wires from previous owner(bought it with skip) checked for spark and has good spark. Misses from idle to red line. Checked compression and here's the funny part, it only makes compression every 4th compression stroke, and when it does it has 130ish psi. Pulled valve cover to inspect the timing setup and found everything good. Tensioner and guide still intact. I then put shop air in cylinder to see if it was rings or valves, with both valves closed and 100 psi of air to the cylinder it will spin the motor until the cam opens a valve. Which puzzles me because if it isn't making compression. I wouldn't think it would turn the motor. It's not pushing any coolant like a head gasket or anything like that. I did inspect the cam and found #6 exhaust lobe has some hardfacing missing but still has full lift and the roller on the follower is a little chewed up but spins fine and no play on the shaft. Valve guide is tight and no broken springs. I just cant understand why it will make compression every fourth compression stroke(held my finger over the plug hole while cranking and felt it) compared to #5 right next to it and that cylinder is good. Thought of a cracked head or something but then #5 should be weak too. Any ideas? The motor has 173K on it and very clean inside. Oh, when I pulled the plug it was soaked with gas so I know the injector is working. Any help or ideas would be helpful, the truck is really mint and no rust which is s big deal up here in Maine. Thanks in advance.

 

[Itali83]

Anyone?

 

[stevenjr]

Me to but only with a 98

Hey guys, new to the forum but have searched a lot of the great information here. Here's the deal, truck has dead #6 cylinder. Has new plugs and wires from previous owner(bought it with skip) checked for spark and has good spark. Misses from idle to red line. Checked compression and here's the funny part, it only makes compression every 4th compression stroke, and when it does it has 130ish psi. Pulled valve cover to inspect the timing setup and found everything good. Tensioner and guide still intact. I then put shop air in cylinder to see if it was rings or valves, with both valves closed and 100 psi of air to the cylinder it will spin the motor until the cam opens a valve. Which puzzles me because if it isn't making compression. I wouldn't think it would turn the motor. It's not pushing any coolant like a head gasket or anything like that. I did inspect the cam and found #6 exhaust lobe has some hardfacing missing but still has full lift and the roller on the follower is a little chewed up but spins fine and no play on the shaft. Valve guide is tight and no broken springs. I just cant understand why it will make compression every fourth compression stroke(held my finger over the plug hole while cranking and felt it) compared to #5 right next to it and that cylinder is good. Thought of a cracked head or something but then #5 should be weak too. Any ideas? The motor has 173K on it and very clean inside. Oh, when I pulled the plug it was soaked with gas so I know the injector is working. Any help or ideas would be helpful, the truck is really mint and no rust which is s big deal up here in Maine. Thanks in advance.

Im having the same trouble just with a 98 model instead

 

[BIGTREAD]

...every 4th compression stroke? Really? Consistently no comp 3 comp strokes in a row and then one with compression!? Valve not seating properly, gets turned a bit each stroke and then comes around to seat well once...but consistently every 4th?

 

[Itali83]

Yes every 4th comp stroke. Crazy I know. I'm ASE Master truck certified and have seen my share of crazy things that no matter how much you think you know, it still surprises you. That's why I figured I'd ask the guys who work On them regularly. I work on big trucks for a living. I guess I'm going to pull the head and take a look. The truck is in really nice shape except for the skip. Thanks for the advice.

 

[BIGTREAD]

I've worked on cars in the backyard for 30 years but make no claims. This comp thing is quite interesting (as long as you don't need the truck in a hurry.) Here's an out of the box thought. You have to draw air in to compress it out. Is it possible that what should be a loading stroke is actually pulling a vacuum into the cylindar? Every forth stroke the intakes are actaully letting air in?

 

[drbenz]

It would be interesting to do a cylinder leakage test and see what you would find. You might also do the test then turn the engine over to tdc again and do it again to see if a sticking valve could be the problem.