Need help with troubleshooting bad engine vibration or miss!

[Valleyordnance]

Hi guys,

I bought a 1992 M998 with the 6.2 out of Barstow for a buddy. Truck had a bad water pump on it, we swapped that out and got it running. It also needed one glow plug that we changed.

At idle the motor shakes badly, like it's "missing" on one or two cylinders. Cracked open 1 injector at a time at idle to see if the RPM's dropped, and all of the injectors seem to be firing. I had some known good injectors and swapped out the 4 on the pass side (easy to get to) to see if that made a difference, still no dice. If it was a gas motor I'd do plugs, wires and distributor. There is no smoke to speak of coming from the exhaust. Also checked & bypassed the cold advance switch (we're in AZ and its 90 degrees today)

Could this be a timing issue? I know that the timing is supposed to be done with the snap on tool. but can a guy "fake it 'til you make it"?

What else could it be?, I'm stumped!

Thanks!

Chris

 

[86humv]

Check the balancer on the crank pulley area.

 

[Valleyordnance]

Looked at it - Nothing obvious.

Seams more like a fuel issue to me

 

[Action]

check the motor mount rubbers.
or the fan for missing or broken blades.

 

[Valleyordnance]

check the motor mount rubbers.
or the fan for missing or broken blades.

Fan's fine, Motor mounts are OK. Engine does not just vibrate, it runs rough as well. It's like it's missing on one or more cylinders. Other than injectors what can cause this? I've now swapped the known good injectors into the drivers side and there is no change.

Chris

 

[Action]

24 year old fuel?
air filter clogged up? remove it and start motor.

 

[infidel got me]

Hit each cylinder with a heat gun, if you find one that is way colder than the rest--- there it is

 

[cwc]

You can also locate a dead or weak cylinder by spraying some water or lube/solvent on the individual exhaust pipes and looking for the ones that evaporate slower.

It might be time for a compression test...

 

[cwc]

Working with gas motors, I've seen cases where a cylinder head gasket leaks between two cylinders; the cylinders still contribute, and show an effect if you pull the plug wire, but the motor runs rough.

I recall working with a Ford 351W once, the head gasket was blown between two cylinders adjacent to each other on the firing order; when the first one would fire under load it would cross over to the second and fire it 90 degrees early, making a pretty dramatic knock!

 

[Retiredwarhorses]

air in the injection system..bad Injector pump. Bad fuel....do a compression test.

 

[3000gt]

air in the injection system..bad Injector pump. Bad fuel....do a compression test.

Yep
My money is on injector pump

 

[Valleyordnance]

Borrowed a compression tester from a buddy and did a compression test. #2 cylinder is dead.

Pulled the valve cover off the drivers side and one of the bolts that holds the rocker arm assembly was broken. Pushrods from #1 and #2 cylinder intake valves ARE MISSING....

Pulled the head and found one of the pushrods down underneath by the lifters.

Pulled the oil pan as we could not find the other pushrod, it was in 4 peices in the pan..... Not really ground up though. Figure it must have slid thru the center galley and got chopped up by the crank.

Can you believe this motor RAN????

 

[Retiredwarhorses]

Borrowed a compression tester from a buddy and did a compression test. #2 cylinder is dead.

Pulled the valve cover off the drivers side and one of the bolts that holds the rocker arm assembly was broken. Pushrods from #1 and #2 cylinder intake valves ARE MISSING....

Pulled the head and found one of the pushrods down underneath by the lifters.

Pulled the oil pan as we could not find the other pushrod, it was in 4 peices in the pan..... Not really ground up though. Figure it must have slid thru the center galley and got chopped up by the crank.

Can you believe this motor RAN????

lucky for you there are a lot of takeout motors due to the scrapping going on....I have a 6.5 rebuilt here if your interested.
but probably easier to source locally.

 

[dmetalmiki]

I'd still check the piston, if OK, the valves. If OK put some new push rods in..Who knows? My curiosity would be really kicking in though....Win or loose. (The expense!)

 

[Awol]

Seeing how you have nothing to lose, I would throw two pushrods in it and see what happens.

 

[Valleyordnance]

Seeing how you have nothing to lose, I would throw two pushrods in it and see what happens.

2 pushrods, New head gasket, Exhaust and intake gaskets and a rocker cover gasket later the truck runs great!

Thanks for the help