Any advice on 3126 Caterpillar?

[David Mathis]

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.

[tkeithlu]

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.

[Lepke]

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.

[skipperpete]

I’ll take a shot at this. Are there any parasitic loads on the lazy engine that don’t 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 can’t 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 don’t suddenly heal. What are the printouts from the ECU indicating re operating parameters , any differences at all between port and stbd?
Pete.

[sesmith]

How's the exhaust. Could there be a buildup of carbon somewhere in the exhaust system limiting flow?

[donradcliffe]

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

[Jakebrob]

Did you ever figure this out? My 420 3126 is behaving similarly.

[David Mathis]

Two issues were discovered: Algae in the fuel and a restricted air filter. The mechanic failed to identify the air filter within the first two visits - certified Cat mechanic. In his defense, he said he was extremely busy and had other places to go, he was rushed. The tanks were pumped and cleaned. The fuel in the tank feeding the problem engine looked like Hershey's syrup while the other tank was crystal clear. Fuel filters weren't the problem, they were changed with no difference. It was discovered that the restriction was at the tank pickup. The air filter looked new on the outside but it was caked with yellow pine pollen on the inside. It was an aftermarket filter guaranteed to increase power, efficiency, and longevity. The engines now wears Cat filters. According to the mechanic, the old style breather compounded the problem due to oil mist getting into the air filter. The breather was upgraded. So far, no more problems.


Both fuel tanks were treated with biocide at the end of every season just before the boat was hauled for the winter (We're in upstate NY.). I've owned the boat for about 15 years so it seemed odd that one tank was crystal clear while the other was so nasty. The tanks were always filled from the same source at the same time.

[Jakebrob]

Thank you for the reply. I’m glad to hear you figured it out. That is odd one tank was so bad compared to the other. Mine draw from the same tank and I’ve tried swapping supply lines between pickups. I’ve got a couple things I want to try tomorrow.