ive got the shakes

ok so i have a 99 4.0 with 114k miles on it that ive had for the past year and a half. when i bought it it had an engine vibration at idle that is still bugging me. so piece by piece ive been attacking it with parts. it starts right up, has never died out on me nor does it burn oil or coolant. it has good power and runs good once i give it gas in any gear but at idle it has this shaky feeling , kinda like an intermitent misfire and you can see the needle shake just slightly. it has new champion plugs, plug wires, cap, rotor, msd ignition coil, new elbows and lines, battery, cold air intake thermostat, and other non engine related parts like water pump and radiator. ive also cleaned the throttle body and the IAC. but the shake is still there. it does it only at idle wether im in park gear or neutral. i was thinking O2 sensor but from what i understand they dont do anyhting when you first turn the vehicle on (does it then also). could it be a CPS? a bad harmonic balancer? i also did the 784 injectors but that did nothing. even seafoam did nothing. any ideas. no check engine light on and no pending codes. if it helps out when i turn off the jeep, as i turn the key to shut off, the motor will shake for a half second as it shuts off. thanks

Great job of attacking it. How are your engine and trans mounts? What is your idle speed?

cruiser54 said:

Great job of attacking it. How are your engine and trans mounts? What is your idle speed?

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I've looked at the motor mounts and they seem ok. My idle speed is normal, the line right before the 1000 mark so I would say 700-750? The rpm needle isnt jumping all over the place but I do see it fluctuate a about 2mm up and down. Other times it stays steady but the shake is always there. I think if it was the motor or tranny mounts it wouldn't cause the rpm needle to fluctuate right? The shake is like a stumble to be more accurate. sometimes it does it every 2 seconds and other times it does it like every 5 second

Possibly the TPS. Cheap and easy to change. Don't always get a CEL for TPS and never for a CPS. Maybe just do one at a time and chalk it up to maintenance.

cruiser54 said:

Possibly the TPS. Cheap and easy to change. Don't always get a CEL for TPS and never for a CPS. Maybe just do one at a time and chalk it up to maintenance.

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I'll try that next. Thanks

I don't know about the 99, but with my 96 it seems to learn bad habits after a while. If I periodically disconnect the battery and then reconnect (I usually wait at least an hour) and then go for a drive, it seems to idle and run a lot better.

The O2 sensor may not do much at start up, but eventually it will have input into the programming. I just replaced my O2 sensor and my idle really smoothed out. One of those times I made multiple repairs, I also replaced the intake gasket and plugged up a, for sure, exhaust leak and a maybe vacuum leak. So I'm not sure the O2 sensor was the main culprit.

First off I'd try re-torquing the intake bolts. Some of mine were pretty darned loose, when I went to remove them. The end studs are always dangerous, I wouldn't go anywhere near the (I think 24 ft lbs,) specified torque, maybe 15 ft lbs. (no kidding). Look up the tightening sequence and specified torques, I don't even try to memorize stuff like that, I always look it up.

a compression check might be in order here as well. You could have a valve that's not seating correctly and causing a rough idle.

Shorty said:

a compression check might be in order here as well. You could have a valve that's not seating correctly and causing a rough idle.

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But would that only show its face at idle? It runs great while giving it gas

Might also have a dirty injector or 2. No CELs though? If compression were low or an injector crapping out, you'd think it would throw one of the "misfire" codes.

frankrizzo said:

But would that only show its face at idle? It runs great while giving it gas

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valve stems have round ends 'cause they need to rotate during operation. You could easily have a valve that seats fine everywhere except idle, and you can also overcome a weak vacuum signal with RPM.

I'd still look at engine compression as a cost effective method of diagnosing idle problems.

Shorty said:

valve stems have round ends 'cause they need to rotate during operation. You could easily have a valve that seats fine everywhere except idle, and you can also overcome a weak vacuum signal with RPM.

I'd still look at engine compression as a cost effective method of diagnosing idle problems.

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Not a bad idea. Somewhere around 99 they had valve seating issues. but why no misfire CEL?

cruiser54 said:

Might also have a dirty injector or 2. No CELs though? If compression were low or an injector crapping out, you'd think it would throw one of the "misfire" codes.

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I did the 784 injectors with no change. so the injectors are clean. And no check engine light. Also just ran a can of bg44k. No change

Frank, you've done all the right things as far as I can see.

And, you've done things systematically and thoroughly.

I know when my buddy's 99 had a valve seating problem that he got a CEL. You don't get a cEL. Low output from CPS at idle? TPS faulty at the lower end of the scale?

cruiser54 said:

Frank, you've done all the right things as far as I can see.

And, you've done things systematically and thoroughly.

I know when my buddy's 99 had a valve seating problem that he got a CEL. You don't get a cEL. Low output from CPS at idle? TPS faulty at the lower end of the scale?

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thanks cruiser

You're not nuts or anything. You just haven't gotten to the bottom of it yet!!

cruiser54 said:

You're not nuts or anything. You just haven't gotten to the bottom of it yet!!

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ha. I'm getting there