E7 Mechanical Engine Miss

[67RModel]

I am running a fully mechanical EM7 in a 1994 RD690 dump truck. It is a bone stock, one owner (before me), all original unit with 335,000 miles and 11,500 hours on the clock. It has some type of miss while under light throttle input from 1100 - 1300 rpm. For instance, if you are slowing down to make a turn, downshift, and your rpms fall within that range and you apply light throttle to maintain that speed/rpm it will make and audible missing sound. If you lift completely out of the throttle and let the trucks momentum carry itself it will go away. If you apply more throttle (say half or more) it will accelerate and go away around 1300 rpm. If the truck is on a downgrade or coasting in that rpm range under its own momentum with no throttle input at all it will not happen. It only happens when the throttle is lightly depressed in that certain rpm range. Any idea what it could be or tips on how to troubleshoot it. It appears the pump has never been apart as it and the timing gear cover still have the security wires and pressed lead fasteners still intact. Its drivable as is but annoying and obviously not right. For reference this is an EM7, 250 horsepower, governed at 1750rpm. Thanks in advance.

[Ahubbard]

I had the same problem on my em7-300.  It was right around 1200 Rpm. Foot lightly on throttle.   Would miss or sort of “gallop” I kind of describe it. Almost felt like it had way to light of a return spring in the throttle, but that wasn’t the problem.   Eventually over time it got worse and did it in a broader rpm range. And even got to the point of doing it under load some when I let it get real bad.   I changed the pump for a reman because the diesel shop thought it was in the governor.  Swapping the pump cured it. 
 

can’t say if you have the same problem as me but sure sounds it.   Would only do it when for instance I would coast down into a 35mph zone without downshifting, and then just give it slight throttle to keep it moving through the zone at about 1200rpm or so.  Didn’t do it on the coast down, or when I gave it real throttle to speed up.  Just that light pedal push at that rpm. 

[Joey Mack]

the injection pump may be the issue,  myself.... I would run the rack, then drive it to duplicate the problem, if it's still there after that, I use a temperature gun to see if there is a big difference in exhaust port temps...  im talking 30+ degrees difference.  you can also crack fuel lines individually to see if the engine changes sound.  As you listen, if a cylinder doesnt react like the others, then ... good chance you have isolated the problem/symptom...   Jojo