skipping problem with 3126 Caterpillar

[David Mathis]

My 1997 Wellcraft has twin 420hp Caterpillar 3126 engines with several thousand hours but not nearly enough to need a rebuild. The injectors are mechanical rather than electronic. The starboard engine lacks a little power (barely noticeable other than when watching the synchronizer) getting the boat on plane but then seems to run fine for two to four miles before it starts skipping and the power slowly decreases as the skipping gets more severe. After a few more miles, the power is down so much that the boat gradually slows and comes off plane unless the other engine is run at wide open throttle. The fuel pressure has been checked and is at factory specs, the blow-by test was run with numbers similar to a new engine, all injectors were replaced and the seats reamed, the solenoid voltage was checked then the solenoid was removed and tested to ensure it wasn't allowing the fuel shutoff valve to partially close. Everything has tested normal. Fuel filters were changed even though the Racor gauges read less than one psi of vacuum. The Racor filter can has no air in it. A sight glass was installed in the fuel line to check for air leaks. The fuel flow appears normal. When the engine begins to decrease power, the throttle is pushed forward which increases the boost. This seems illogical. When the skipping gets to the where the boat must be slowed, the boost is up to about 27 pounds. (Normal cruise boost is about 16-20 pounds.) When brought off plane, the skipping stops and it runs perfectly at hull speed (7- 8 knots) then, after a couple miles at hull speed, it can be brought back up on plane for a mile or two then it starts skipping again. Temperature and oil pressure are normal. No unusual mechanical noises are detected. The oil was changed with no detectable fuel smell nor was it over the full mark. Even the transmission was checked and found to be normal.

Any suggestions?

[GRIT]

Well, it sounds like it's heat related. I'm not that familiar with the MUI Cats, I worked on trucks, and construction equipment, and we had a couple HEUI engines, and they could be a PITA.

You've done a good job of eliminating the most obvious things it could be, and I was going to recommend you look for a worn high pressure oil pump, until I re-read your post and realized you had an MUI engine.

I assume you've not seen any smoke, during the loss of power, or when you let off the throttle? Have you checked the wastegate? I only ask since you're showing high boost when calling for more power. It's an odd issue, for sure.

When you say skipping, do you mean it's missing a beat? Like one cylinder is missing a power stroke? If so, can you monitor each cylinder's temp with a laser thermometer? Try shooting at the exhaust manifold at each cylinder, when it starts to skip bad, and see if you find one cylinder that's slightly cooler (or hotter) than the others. If you find one, you may be looking at a bad valve, perhaps just going bad. I'd bet on an exhaust valve. Since your injectors are new, the valves are pretty much what's left. If the exhaust valve was bad, you might see a slight increase in pressure from the turbo, but the wastegate should be able to handle that, and keep the pressure below say 18 without issue.

You've done an excellent job.

Good luck.
Paul.