3126 Running rough after warmup.

dtfeld

Water Contrails
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Jun 5, 2016
5,519
Milton, GA
Boat Info
410 Sundancer
2001
12" Axiom and 9" Axiom+ MFD
Engines
Cat 3126 V-Drives
I let the engines warm up like normal (about 10 minutes while untying), and then a 5 minute idle out to the channel. Then I attempted to power up on plane. Initially both engines accelerated normally. Then about about 1500-1700 RPM, the port engine started to lose power. I pulled both back to idle and out of gear.

I would describe the feeling like a gas engine that was having ignition/missing cylinder. Once put it in idle/neutral, I attempted to push the port throttle up, but same symptoms, as the engine would not accelerate past 1000 RPM and was running rough.

I was thinking fuel...but

- Racor's were just changed within a couple hours and bowls are clean. On engine filters were changed at the same time.
- No Smoke (once warmed up, a little white when first started).

I shut the engine off while I pumped the head.

On the way home I started it and put it in gear and ran it about 900 RPM. Symtoms not appearant.

This has happened 2X like this...
 
My similar situation was the hand pump on the engine. It was loose and I think it was sucking air into the fuel line. I pumped until I heard fuel going back into the tank and then I pumped some more, flipped the switch and screwed the pump knob down and tight. Problem solved in my case. Best wishes for you!
 
My similar situation was the hand pump on the engine. It was loose and I think it was sucking air into the fuel line. I pumped until I heard fuel going back into the tank and then I pumped some more, flipped the switch and screwed the pump knob down and tight. Problem solved in my case. Best wishes for you!

That makes sense!
 
Sounds like fuel starvation to me. Did you prime the Racors manually when you changed them?..........same question on the Cat secondaries as well.

If you failed to turn off the priming pump and it is leaking, you should smell raw diesel in the bilge.
 
Sounds like fuel starvation to me. Did you prime the Racors manually when you changed them?..........same question on the Cat secondaries as well.

If you failed to turn off the priming pump and it is leaking, you should smell raw diesel in the bilge.

I filled the Racors with fresh diesel before replacing the top, and I used the on engine pump to prime the secondary filters. On the port engine, I did not get the priming valve back into the correct position after the filter change and had a small diesel leak, but I corrected that immediately. This has happened the last two times out. Previously, the boat ran flawlessly for a couple hours.

I agree with the fuel starvation. My list of places to "look".

1. Reprime the engine.
2. Reseat the priming valve.
3. Ensure Racor is full/tight
4. Reseat the secondary fuel filter.

Thanks gents!!
 
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There are usually 2 fuel delivery problems..........

1. Fuel starvation where fuel doesn't get to the fuel pump, and, therefore, not to the engine, and
2. Air in the fuel system.

The effects on the engines are different: With fuel starvation, the engines will gradually slow down until they reach the rpm where the fuel system does supply the fuel demanded by the engine. At some point, the engine will slow down enough so it dies. With air in the system, the engine will miss and run rough, then eventually die because air is compressible and the injectors won't atomize the fuel at the injector. With 3116/3126's, once you kill 2 cylinders, the engine becomes air bound and it dies. You will need to purge the entire system to get things right again.

If I were in your situation I would open each fuel filter carefully and slowly and examine how much fuel is in the fuel filter. Any air on top of the fuel and you probably found your problem. On the Racors, it is very easy to install the flat rubber gaskets with a twist in them.....if that happens, they cannot seal and will suck air, so it is worth checking. The Cat fuel priming pump has never been a favorite of mine and I don't use it. The plunger can leak, you can leave the lunger loose and it leaks air, the off/on ball valve may fail to seal. If its knurled knob gets loose on its shaft, you may never again figure out where the pointer should point, and if it is left open, fuel is supplied to the priming pump all the time which is another source of leaks.

Good luck with it..........\\

Frank
 
Frank, what do you do to prime the filters on the motors if you don’t use the plunger?
 
My fuel source is the marina and they sell ValveTech fuel that is double filtered. I keep a small plastic fuel container on the boat for priming filters. For the Cat filters, even though Caterpillar recommends against it, I fill each filter with fuel from my container, let the filters sit a few minutes to allow the fuel time too soak into the filter elements, top off the filters then carefully thread them onto the filter housing. Next, I advance the throttle 1/2 way, then start the engine.........hold the starter switch down until the engine fires, then gradually bring the idle back to the idle stop as the air is purged. It never takes more than 10 seconds to purge the air trapped at the top of the filter element.

Disclaimer....I know what Cat recommends. Cat's priming pump is a poor design (the only thing on my Cat engines that I don't like) and I have never been able to successfully prime the engines without making spilling diesel fuel everywhere or getting air in the system. I have 40 years+ experience with all sorts of diesel equipment and I know how to handle fuel. If you are married to the Cat priming system, then more power to you!
 
Cat primer systems sucks. I filled two oil mats worth of fuel redoing this. What a PITA!!
 
I think I’ll give Frank’s method a try this Fall.
 
I have replaced one of my primer pumps but other than that they work like they're supposed to without any problem or mess...
 
I have replaced one of my primer pumps but other than that they work like they're supposed to without any problem or mess...


After replacement, does your spit fuel at all ?? Mine spit fuel from around the shaft when pulling it out and a little pushing it back in.
 
After replacement, does your spit fuel at all ?? Mine spit fuel from around the shaft when pulling it out and a little pushing it back in.

I wonder if there is a replaceable seal or O ring in there.
Anyway, I think ai still like Frank’s method. I’ve been using my pumps but they take a long time and a lot of pumps.
 
Cummins says the same thing only fill them by turning the key on 100 times. I use Frank’s method and following the change, when I hit the starter, you would never know anything had changed.

Bennett
 

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