I have a 1986 M1008 with a J code 6.2 that has 45k miles. The engine starts and runs great and idles smooth. The problem is a vibration that is at light throttle and fast idle speeds. It feels as if the truck has solid motor mounts. It must have been that way for a long time because the air cleaner lid holes are enlarged and cracked. I have replaced motor and trans. mounts. The rubber on the harmonic balancer looks like new. I converted the truck to a NV4500 5 speed transmission. The vibration was the same with both flywheels but the manual trans. transfers the vibration to the transfer case enough to make it rattle at acceleration and decelleration. Because of that the truck sounds like a bucket of loose bolts. I have checked the fuel system for air bubbles and it is OK. On warmup all of the exhaust ports seem to heat up at the same rate. I hope someone can help so I don't break a crank.


Thanks, Perry.

How bad is the vibration. When these engine's have a miss because one cylinder isn't firing it's noticable at idle as a jump in the engine.

The vibration does not feel like the engine is misfiring. The engine never shakes. The idle is smooth. I have worked on a 400 small block Chevy with the wrong flywheel, (the balance weight was missing), the vibration does not feel like that either. The 6.2 has a sharper and faster vibration. It feels like the exhaust pipe is hitting the frame or metal to metal contact on the motor mounts but it is not. The rearview mirror blurrs from the vibration. The vibration transfers through the transmission and makes the transfer case rattle loudly on accelleration and decelleration. If the pop off pressure on the injectors was not set evenly and caused one or more cylinders to be out of time, would that cause this problem?


Thanks, Perry.

Stanadyne introduced several updates to older 6.2L DB pumps to cure rough idle and other problems, no telling how many updates have been applied to yours until you send it to have it rebuilt.


It is possible that the injectors have become unbalanced, normally the springs weaken, spindles and valves wear, but at 45k they should be alright.


Is it possible that whoever had the truck before you advanced the pump timing too far? If so you would hear too much detonation clatter.

The pump is set on the mark, I tried to advance it but it made no difference. I might install a set of short style injectors because I plan on putting a 6.5 turbo system and intercooler on it. I might also install a mechanical 6.5 TD pump.


Thanks.

I have replaced the injectors and it did not change the vibration. One thing that is odd, from the appearance of the bolts, it appears the engine mounting frame brackets have been changed to small block brackets. When I replaced the motor mounts, the 6.2 mounts would not fit. I had to use new small block mounts, they are not made as heavy as the 6.2 mounts. I wonder if the extra weight of the 6.2 causes metal to metal contact inside the rubber mount.
Thanks.

The 1986 engine mounts are the same for the 6.2L diesel and the 350 gas (at least for the C30 one ton dually).

The Anchor part number for a 86 K30 4x4 with a 5.7 is 2292. The part number for the same truck with a 6.2 or a 7.4 is 2395. I just verified this on the Advance Auto Parts website. The 2395 mount is taller and the bolt pattern is different from the 2292. I have no idea why someone would have changed the frame mounts, but they also swapped the original Dana 60 front axle for a 79 model Dana 44.
Thanks.