2002 4.0 Ranger - Misfire reported in cylinder 1 but low compression in cylinder 2.

[MinnRanger]

Hi All. This is my first post on this forum but I'm on my 3rd Ranger. Wish I'd found you guys earlier. My currant Ranger is a 2002 4.0 automatic with just over 100000 miles on it. It started running rough then died after I got gas so I thought it was a gas problem but it reads 60 psi for fuel pressure.

When I start it cold it runs good for a few minutes then it runs rough and dies. The Computer says P0301 Misfire Cylinder 1... Cylinder 1 compression is at about 128 while cylinder 2 is at about 95. I'm thinking its a head gasket but theirs no white smoke coming out the tailpipe.

Could a stuck or broken valve cause one cylinder to report a misfire while the one next to it has the low compression? I see other people saying that they're getting around 190 for compression in the 4.0's so maybe my measly 128 is trying to tell me that theirs more of a problem then I think there is?

 

[RonD]

Welcome to TRS

To do a compression test you should remove all 6 spark plugs and then test 1 cylinder at a time
You need to get good "crank speed", to get higher numbers, you can only do that with no compression strokes except on the one you are testing
4.0l SOHC should be about 175psi with a dry cold test

Yes 128 is low as is 95
Your timing chain gear may have slipped on one side
Best thing is to test all 6 cylinders that will tell you more

After doing the dry test you can then do a WET test if you want
Have a bottle of motor oil and a straw
You wrote down the results of the Dry test right?
Dip the straw about 1/2 way into the oil and put finger on the top, pull it out and put spark into spark plug hole on #1, take finger off to let oil flow out into that cylinder
The oil will seal the rings better
Now re-test #1, it WILL go up, but by how much will tell you if its rings or valves that are losing compression


Cold start and the shutting down after warm up could be failed ECT(engine coolant temp) sensor, but I would expect a code for that

 

[MinnRanger]

Thanks Ron. It’s pretty cold here in Minnesota right now so I’m just doing one test at a time to narrow down the issue a bit. I hadn't considered doing the wet test but will get that in too. So with the wet test, if the compression readings don’t change, I can pretty much consider that the rings are good and the issues probably in the top end somewhere?

 

[RonD]

WET test always goes up, but yes if it doesn't go up much then valves are the source of the lower compression

BUT.........................you really have to do more than 2 cylinders on a 6 cyl engine, and because its OHC, at least one on the other side
You need some type of baseline/average for that compression gauge, your battery and crank speed

128 and 95 means you have 2 bad cylinders

Your gauge may read low, cold battery cranks engine slowly

Metal rings against metal walls and metal valves sitting on metal seats DO NOT hold air pressure very well or very long
So crank speed really matters

 

[MinnRanger]

Thanks for the info and advice. I plan on testing #3 next time it’s warm enough to get out there and then l‘ll do 4 5 and 6 on the other side. I’ve disconnected the plug wires from the ignition coil and the relay from the fuel pump so I can give it a good long crank on each cylinder.