• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

rough idle/misfire

csanchez

NAXJA Forum User
Location
SALT LAKE CITY
ok to start out i have a 1992 jeep cherokee with the 4.0. My jeep has developed a rough idle. The tach doesnt bounce it is more of a chugging..Since i have had a rough idle i have replaced the plugs, wireset, cap, rotor, neon injector upgrade, cleaned the pcv's, and cleaned the throttle plate. i recently did a compression test cold and all of the cylinders were about 90 psi. i am going to test with a warm engine soon. is there anything else i need to check? any help is appreciated
 
If all of your cylinders were around 90, my guess is there is something wrong with your test or your gauge. If your compression is in fact 90 you're motor is pretty worn out, 105 is usually the lowest allowed.

Remove all the plugs, prop the throttle plate open, use a fully charged battery and make sure the air release on your compression tester is screwed in tight (and the check valve is not leaking).

Open an old plug up to around 0.60, ground it good, an check your spark. You can usually hear a good spark and it it will be a sharp blue/white color. A fat yellow or even a yellow that splashes some is usually weak. Make sure the plug is grounded well and standing with your crotch pressed against the fender is to be avoided. :)

Double check TDC 1 and make sure your firing order is right and you have the number one on the correct pole on your distributor cap.

A bad cam position sensor or even a flaky CPS can do that. Is it running really rich at idle? Smoking out the exhaust?
 
So I cleaned the iac today and no change. I then went and checked compression with the same gauge. Last time I only checked a couple because I was in a hurry. Results weren't what I wanted to see cyl 5 had 0 and cyl 6 had 25 psi. I did a cylinder leakage test and unbolted the rocker arms to make sure the valves were closed and I had a lot of air coming from the intake. So I'm guessing I either have carbon build up or a warped valve does this all sound correct
 
I'm having trouble figuring out why two cylinders would be causing issues. If it was a bad valve, one cylinder would seem more likely.

Maybe try putting air into one cylinder and see if some comes out of the adjacent spark plug hole? Maybe the gasket is blown between two cylinders?

I had one valve that was bent, it looked like an upside down mushroom, it was a soup bowl shape instead of flat. A headgasket failed, the cylinder filled with coolant and bent the valve.

Another time I pulled a piece of gasket material out, that was stuck between an intake valve and the seat. I caught it early and there was no real damage.

If I was sure of my findings, I'd pull the intake/exhaust manifold off. You can see most of the intake valve. Maybe put a light coat of talcum powder on the intake, them blow some air into the spark plug hole.

I'm not one to jump to conclusions, I'd rather spend the time being sure, before I waste the time guessing and pulling unnecessary bits and pieces off the motor.

Except for the end studs and maybe the downpipe to exhaust manifold bolts, pulling an intake/exhaust is no big deal. I don't unhook any thing I don't have to, the fuel line, some connectors and vacuum lines, then unbolt it and flop it over onto a blanket on the passengers side of the motor. Just be careful with the harness and try to avoid kinks and unnecessary stress. I can have it off in half an hour and on in an hour. Two people to torque it back down properly is a big help, one to hold the socket on the bolt and the other person to work the torque wrench.
 
Back
Top