4.0 rough idle with leaking intake valve

I just bought a 97 XJ with a 4.0. The idle was rough and the previous owner said they did lots of short drives in it. The engine also seemed to be running cold so I pulled the thermostat and found it was a 160 degree unit, which I replaced with a 195 and the engine runs at the proper temperature now.

I did all the typical first steps towards fixing the rough idle: new plugs, wires, cap, and rotor, new oil and filter, and new air filter. But it still ran rough right at idle, on acceleration things seemed fine.

I ran a compression check and the #3 cylinder was at 120 while the others were 145-150. I then did a leak-down test and discovered that #3 read 63% while the next cylinder over was at 75% leakdown (a homebuilt tester so just consider the ratio). Listening for air escaping on #3 I opened the throttle and heard a very distinct sound, whereas the same test for the good cylinder (#2) revealed no such noise).

I also pulled the spark plugs and the good cylinder (#2) has a nice gray coating, while the bad cylinder (#3) has a dark gray coating after 400 miles.

I've already run mopars combustion chamber cleaner through, and then seafoam.

What should I do next? A few more seafoam treatments? Or do some normal driving hoping it's carbon build up on the valve that will burn off? Pull the valve cover and check the valve stem for normal wear (and not a half moon from a sticking valve?) Or pull the head and have the valve fixed?

Thanks in advance!

check for abnormal wear on the valve stem or possibly a broken spring. it may have a burnt valve, where it should be replaced. i dont know how many miles you have, but a alve job could be beneficial

Could try some MMO in the oil and fuel, and then just drive it for awhile and wait and see.

Did you check the vacuum? If you have a sticking valve the vacuum will have a severe fluctuation, as when the valve sticks/doesn't stick. If the valve can't fully seat that would be a steady or more consistent drop.

Thanks for the recommendations!

And yes, I should have said how many miles: 150K :looney: