D311 Engine-Need some guidance.

Wild guess, point of injection, timing off a little and that's the weakest pump.

I'm guessing the 250 psi compression test result is as high as you could read with the tester you had????? Serously doubt the rest of the cylinders would fire if they were anywhere close to 250.

I think you will gain more from a proper compression test and then if it shows a low cylinder, a leak down or really just a leak test with compressed air into the cylinder with piston at TDC of compression stroke will tell you more about where the leak is.

If all cylinders have approximately equal, but low pressure, then maybe camshaft timing, but just seems unlikely for that to cause a single cylinder to drop out preferentially. Something like amount of valve seat and valve grind, piston surface/type would have to be different about that one.

OH.................could be something in the intake blocking air flow, though I would think of that causing a misfire at full speed instead of preferentially at low idle. Have found some strange stuff in engines before. Worst was a large chunk of rag in the intake runner of a D333. It went alll the way to the far end of the head from the air filter pipe. I'm guessing those guys never figured out why the old dozer sounded like that!

You don't mention injectors..........have you swapped any between cylinders or replaced them?

How recent was the rebuild? Has it run long? Problem show up on first startup or has the tractor run well since the rebuild.

It would be pretty easy to take off the valve cover and check camshaft timing as the valves "rock over" at TDC of exhaust. Also could measure lift to check camshaft wear. That is another possibility as far as the injection pump camshaft goes. You could have the lifter set correctly, but there still be enough wear on the lobe that the pump wasn't getting a full stroke.

One last idea, it really doesn't sound like your description, but are you sure the pumps are correctly timed to the rack when they were installed. I've heard a D320 start up with the pumps out of time on the rack, and it was really rough.

I was going to hit on what ccjersey put towards the end of his post above, which would've been pump plunger to rack timing. But the other thing I was thinking (and this is way out there, but I suppose it could be possible) is that since the geared segments on the pump plungers are only clamped to the plunger rods, they could've been "twisted" out of their proper position if they were siezed in the pump bores at one time and the fuel rack was forced through a certain degree of its fore-and-aft travel by some sort of leverage application (prybar, screwdriver, etc.). But for this theory to work out, it would mean that three plunger segments would've had to twist out of position a bit and become slightly "advanced" and one stayed where it should be. So instead of having one weak cylinder and three normal, you would actually have three too strong and one normal. Also, I didn't read in the original post if all four pumps and plungers had been changed out at the same time, which would totally take all of this right out of the equation.

Anyway, it's just something I thought of when pondering how the engine would appear to be running normally under load but not at idle, and how possible governor operation might be opening all the pumps up just enough for the one alleged lower volume plunger to open enough to actually start contributing power. It could also explain why other known good pumps haven't changed the condition, assuming that you've only done the change-out on the weak firing cylinder. But again, if you've already swapped all four at the same time and still have the same condition, then disregard all this.

Edit: One last thing, are you sure you have the correct pump and plunger assemblies on the three strong firing cylinders? It could be possible that they've been replaced at one time with pumps from a D315 or other bigger bore engine, which may affect fuel output/volume? Just thinking some more, I'd really like to know what you find if you get it sorted out.:noidea:

Hi AC,
all good suggestions to check given above.
Given that your changing its' positition dead cylinder problems are not being improved by injection pump change outs and seem to be confusing in their symptoms, I have encountered erratic running due to injectors leaking combustion gasses back into the fuel gallery.
Suggest also getting injectors tested and/or exchanged with known good ones as a test.
Cheers,
Eddie B.

Not that I know anything...
But could it be possible that your valve clearance is a little snug on the #4 cylinder.
Regards Chris

One I forgot....a case of weak valve springs and/or improper valve spring installed height. Easy to check, drive a big screwdriver into the coils while running and see if that cures the local problem.

Thank you all for yor responses. When we moved the pump that was supplying fuel to #4 (missing) to #1 and moved #1 to the #4 position, and the missing moved to number #1, we thought we had it, because the problem followed the pump. This last Saturday we replaced the pump with a CAT OEM pump and nothing changed. We then used a pump taken from another D2 sitting next to the grader and nothing changed even with a known good pump. When you drop the blade or drive it up hill, the governor action will allow it to run on all four cylinders.

We have moved lines, nozzles, etc. to further isolate the problem, but nothing that we did last Saturday made a difference, thus the note for some better ideas.

Thank you all for yor responses. When we moved the pump that was supplying fuel to #4 (missing) to #1 and moved #1 to the #4 position, and the missing moved to number #1, we thought we had it, because the problem followed the pump. This last Saturday we replaced the pump with a CAT OEM pump and nothing changed. We then used a pump taken from another D2 sitting next to the grader and nothing changed even with a known good pump. When you drop the blade or drive it up hill, the governor action will allow it to run on all four cylinders.

We have moved lines, nozzles, etc. to further isolate the problem, but nothing that we did last Saturday made a difference, thus the note for some better ideas.