3126TA won't go over 1,200 RPMs

**Dani-Lu** · 10-18-2012, 05:53 PM

Yesterday, boat was running fine. Then after idling around for a while, I went to throttle up and the starboard engine would not go over 1,200 RPM. Changed primary and secondary fuel filter. Racor filter was black. Took boat out and it ran beautiful for a minute or so and then the starboard engine started loosing RPM´s. Checked oil filter and it was a little dirty but plenty of clean area to let fuel through. Tried switching fuel tank selector to other tank, thinking maybe the pickup was getting clogged, but no change, still could not get starboard engine above 1200 RPM. When I do give more throttle the RPM´s go up a little but seem to drift up and down. Boat runs fine at idle, shifts into reverse fine. Even at 1,200 RPM she seems to drop up and down 100 RPMs or so. Even at idle I feel as though the starboard may be getting a little less fuel than the port side, based on the synchronizer gauge, but it could have been that way before... Could this be a bad fuel pump? What would be the next logical thing to check? Should I try installing new air filters? Helllllllllllp!!!

**fwebster** · 10-18-2012, 06:46 PM

This could be an injector seat leaking. On your engines, the injectors are fed off of a fuel gallery in the head. If an injector seat fails, the leaky seat allows combustion gasses (air) past the injector and into the gallery and essentially turns off all the cylinders fed after the leaky injector seat. I would guess that cylinder #4 or #5 on the stbd engine. There is an easy test for this problem.......temporarily replace the fuel return line with some clear tubing then watch the tubing for air bubbles in the returned fuel. This is a 2 man job because many injector seats won't leak at low speeds and need some rpms to break down.

The solution is to have the injectors reseated which requires reaming the seat with a special reamer........in other words, Call Caterpillar.

**Dani-Lu** · 10-18-2012, 07:00 PM

Frank,

Thanks as usual for such a quick detailed reply.

A friend of mine talking me off the ledge just asked, why wouldn't we check the fuel pump first. He is an ex auto mechanic, but admits never worked on diesels.

Jeff

**Dani-Lu** · 10-18-2012, 08:41 PM

Frank,

Can the test you alluded to be done at dock in neutral? Not sure if this is important, but there is no problem with RPM's in neutral.

Jeff

**fwebster** · 10-18-2012, 10:17 PM

Jeff,

Those engines do not have a lift pump. The distribution pump is the only one and it is not nearly as likely for that to cause your problem as a bad injector seat, but it is an engine and all things are possible.

I start with the most likely issue which is the test for air in the return line. It usually takes some load on the engine for the injector seat to leak in the early stages....later on you may not get the engine to crank on 4 or 5 cylinders. Also, you need to hope I am right.....the injection pump is about $2800; reseating all 6 injectors takes about 6 hours or $600 + travel time + expenses.

**ImpulseIII** · 10-19-2012, 06:39 AM

Jeff,

Any chance someone loosened the knurled nut that act as as an air bleed for the manual fuel pump to purge air from the fuel lines? We had a problem similar to what you describe, and it turned out to be just that simple. Pilot error when changing the secondary fuel filters. Once the nut was closed off, boat ran fine.

**Dani-Lu** · 10-19-2012, 09:33 PM

Originally Posted by ImpulseIII

Jeff,

Any chance someone loosened the knurled nut that act as as an air bleed for the manual fuel pump to purge air from the fuel lines? We had a problem similar to what you describe, and it turned out to be just that simple. Pilot error when changing the secondary fuel filters. Once the nut was closed off, boat ran fine.

Frank H.,

I will take a look at the knurled nut. But, no one has done any work on my engines since launching in March. Unless i hit it by accident checking oil and fluids, which I doubt, since the nut is probably tucked away near the secondary fuel filter, I guess.

Spoke to Frank W. today and I am hoping I got some bad fuel. I say this because after I changed the primary and secondary fuel filters, she ran fine for maybe 5 minutes and then bogged down again to less than 1,500 RPM. i am thinking that she ran fine with the 1 gallon of clean fuel I put in the 2 filters and once she burned through the clean fuel the bad fuel bogged her down. I will be shocking my fuel this weekend as per Frank's directions on the home page and take her for a spin with clean fuel and see what happens. I am hopeful this is the issue, rather than some of the more costly possibilities.

Jeff

**HUMPH** · 10-19-2012, 09:45 PM

Good luck, keep us all posted!

**Dani-Lu** · 11-11-2012, 09:15 PM

Hey guys, got a little distracted from boat over the past couple of weeks with hurricane sandy and then a noreaster. Finally got power back tonight.

Just before the storms, I put 911 and biobor in the fuel tanks. Today I filled boat up with diesel fuel. Wasn't easy to find fuel on the water since most stations were demolished. I am embarrassed to say that I allowed the fuel to run so low! She took 146 gallons on port and 126 on starboard! I will never allow this to happen again. The first thing I repair in the spring are my fuel senders!

Took boat out for a test run: Boat ran excellent and then bogged down at almost WOT. However, she ran perfect at 2300 rpms and below. My thoughts: based on her now running well up to 2300 rpms, I am thinking the new fuel has helped but the filter is probably clogged again, which I will check next weekend. Thoughts and comments would be appreciated!

**fwebster** · 11-11-2012, 10:17 PM

Bad injector seat on cylinder #6.

Your fuel is supplied to the injectors in a fuel gallery. A leaking injector seat allows combustion gasses to enter the fuel gallery and the gasses are compressible so one leaking seat will kill the cylinders after it. Sine you are only losing about 500 rpm, you have only one cylinder dead. This is a rough approximation, but it proves the point: 2800 rpm/6 cylinders X 5 cylinders firing = 2300rpm. What may confuse you is the fact that when an injector seat begin leaking, it will first only leak when the engine is inder load and working, but the rpms where the cylinder will drop out will gradually decrease until you cannot exceed 2300.