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Old 02-06-2021, 15:00   #1
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Any advice on 3126 Caterpillar?

One of the 420 hp 3126 Caterpillars in my 43' Wellcraft started losing a slight amount of power a couple year ago. It didn't have full power when coming onto plane but ran fine with no issues at 2300 - 2400 rpm cruise with about 18 pounds of boost. Caterpillar claims this engine will run all day at 23 pounds of boost so that isn't the problem. Last year, after about 5-7 miles at cruise speed (abt 24k), it started skipping, lost power, and came off plane. After a couple miles at hull speed, it would plane again but resumed skipping after maybe a mile. It will run at hull speed all day without missing a beat. So far: Fuel filters were changed then the turbo was changed after one mechanic said it was damp on the inside after which the intercooler was removed, cleaned, and tested 20 pounds of pressure with no leaks. All fuel connections were checked to be sure no leaks. All injectors have been changed. The governors were changed between the engines which resulted in no change to either engine. The fuel pressure was checked at idle and cruise and found to be normal. The blowby was checked and found to be the same as a new engine. Both engines were run at full power with the port side turning about 2900 (normal) while the starboard, the problem engine, maxed at about 2600 with the last two inches of throttle movement making no difference. No odd colored exhaust gasses are present, the temperature, oil pressure, and other gauges indicate normal. The props were changed which made no difference. No odd noises, leaks, or hull speed issues were noted. Bags of money are being spent on this problem with absolutely no improvement.

Any suggestions?

The new mid 40s with outboards are looking better every day.
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Old 03-06-2021, 09:14   #2
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Re: Any advice on 3126 Caterpillar?

I toyed around with an airflow constriction between the intercooler and the engine itself, but then decided that that was not likely because partially burnt fuel would be giving you black smoke.

Compression sounds OK.

Timing would give you smoke, too.

I'm back to a constriction in fuel flow, such that you are getting enough for 2,600 RPM but not any more.

Quote "The fuel pressure was checked at idle and cruise and found to be normal." Where? lift pump or injector pump?

Have you blown out the fuel lines? You had to replace the filter. A pressure reading in any hose/pipe is not an indication of flow. It could be full pressure with no flow at all.

These are wandering thoughts, but all that comes to mind after several minutes. Good luck with it.
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Old 05-06-2021, 13:22   #3
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Re: Any advice on 3126 Caterpillar?

Could be a gasket on a fuel filter not completely sealing and allowing air into the system. Or an air leak somewhere in the fuel line.
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Old 05-06-2021, 16:18   #4
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Re: Any advice on 3126 Caterpillar?

Iíll take a shot at this. Are there any parasitic loads on the lazy engine that donít exist on the strong engine such as stabilizer pumps or bow thruster pumps? Just a thought but one thing that might explain a subtle loss of power would be a head gasket blown between adjacent cylinders........ this can be very gradual and does not show up as crankcase pressure, just a loss of power that increases over time as the crossleak progresses. A compression test will confirm or eliminate this proposition but I prefer the information derived from a leakage test.
I canít explain why you dropped off the plane but then picked it up again, my theory implies a slow linear loss of power over time, not a sudden brief loss..... compression leaks donít suddenly heal. What are the printouts from the ECU indicating re operating parameters , any differences at all between port and stbd?
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Old 05-06-2021, 19:08   #5
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Re: Any advice on 3126 Caterpillar?

How's the exhaust. Could there be a buildup of carbon somewhere in the exhaust system limiting flow?
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Old 05-06-2021, 21:10   #6
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Re: Any advice on 3126 Caterpillar?

Anybody take a IR temperature gun to both engines as the engine starts losing power?
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