#4 plug miss and getting nice and dark

[mlunsford]

I have a 4.0L OVH with 104K miles. The engine does not have the power it once did and has miss. I have changed the plugs and wires a couple of times, currently running motorcraft plugs and wires.

I do not notice any consumption of coolant, nor any of the signs of water in the oil. No white smoke out exhaust.

No check engine light => no codes

Last night I check the 14mm intake nuts and the one closes to the 4 cylinder was slightly looser than the rest.

Reading the forum, I come up with 3 possible likely problems:
1 - Intake gasket leak. Is there a way to verify this with a vacuum gage ?
Could use some help here.

2 - O2 sensor - not sure how to verify ?

3 - cracked head - will do a leak down and compression test.

Any advice is appreciated.

 

[cr_the_machinist]

My 4.0 threw a #3 mis-fire code, barely ate any coolant, no coolant in the oil, and ran decent. Turned out both heads were cracked. It's a VERY common problem with the 4.0's. I took it into the local shop to get it tested. It's passed the pressure test and everything, they thought there wasn't a problem. I had them take the heads off and it was a whole different story.

 

[mlunsford]

How to verify cracked heads

Thanks for the reply, how did you verify that the heads were cracked ?

 

[mlunsford]

Sorry, reread

pressure test was ok, had to take the heads off to determine.

 

[Big Jim M]

Howdy muins

I have a 4.0L OVH with 104K miles. The engine does not have the power it once did and has miss. I have changed the plugs and wires a couple of times, currently running motorcraft plugs and wires.

I do not notice any consumption of coolant, nor any of the signs of water in the oil. No white smoke out exhaust.

No check engine light => no codes

Last night I check the 14mm intake nuts and the one closes to the 4 cylinder was slightly looser than the rest.

Reading the forum, I come up with 3 possible likely problems:
1 - Intake gasket leak. Is there a way to verify this with a vacuum gage ?
Could use some help here.

2 - O2 sensor - not sure how to verify ?

3 - cracked head - will do a leak down and compression test.

Any advice is appreciated.

Leave the heads alone untill there is some HINT that one or both are bad. So for you don't have even a SMALL hint! If only one plug is giving problems CHANGE IT and also the wire to it.. If the problem persists and you have multiple coils give it a NEW coil..
A change of the color of ONE plug could be a sign that the valve seal is leaking a bit on that one cylinder..Or perhaps the rings are allowing a bit too much oil into the cylinder.. But as long as it is running pretty good I'd favor leaving it alone until MOST of the valves need servicing.

You say it seems to have lost some power.. This could be from installing taller tires.. Or the cat or muffler is partially stopped up.. OR if you don't have the factory advised plugs in it.. You need the exact heat range of the initial plugs in that engine.
Big JIm

 

[mlunsford]

Compression / vaccuum

I did a compression test on #4 and shows 120+ psi
Vaccuum gauge at idle show 21in.

If O2 were bad would it through a code ?