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Old 20-09-2014, 14:47   #1
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Unable to Power up Engine above 2800rpm

I have a yanmar diesel that I can't get above 2800 rpm (cold), and that will diminish over time to about 2500rpm. I believe its a fuel problem. I've changed the primary and secondary filters, and just today noticed bubbles in Racor separator when operating. Could it be that air is entering a fitting in fuel line and I'm loosing suction?
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Old 20-09-2014, 15:14   #2
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Re: Unable to Power up Engine above 2800rpm

There was a recent thread about bubbles in a racor... turned out it's normal.
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Old 20-09-2014, 17:15   #3
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Re: Unable to Power up Engine above 2800rpm

Air in the system may do as you said.

I found this when a lifting pump on a Perkins Sabre got a bad leak.

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Old 21-09-2014, 06:55   #4
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Re: Unable to Power up Engine above 2800rpm

I will spare you the gritty details of what I went through on my boat.

TRUST ME if you think it is air replace the hoses and clamps. One little irregularity can introduce air and be the source. It is just more cost efficient to replace the hoses. Of course the cheap German in me cut off the ends of the longer runs and saved the middle sections.
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Old 21-09-2014, 08:23   #5
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Re: Unable to Power up Engine above 2800rpm

have twin 4jh2e yanmars port side runs out over 3200 under load stbd runs out 3200 with out load under load only to 2200 after warms up also ,, naturally local mechanic says pump ?? but will change lines asap as they are old anyway ..let me know your results and i will post mine as well
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Old 21-09-2014, 08:39   #6
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Re: Unable to Power up Engine above 2800rpm

Our 4JH-TE would not get above 2800 rpm-cold or warm. The mechanic found 3 of 4 injectors bad, and the injector pump was not putting out sufficient pressure. We sent it off to be rebuilt (cruising in Turkey), and it came back that it had been reassembled without gaskets(!!!) during the major rebuild we had done in the Philippines about 2 years prior. After the injector pump re-rebuild and replacing 3 of 4 injectors all is well. You could say it was a fuel problem....fuel getting to the cylinders. But eliminating a possible air leak with new hoses sounds a lot quicker and cheaper to start with.
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Old 21-09-2014, 09:10   #7
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Re: Unable to Power up Engine above 2800rpm

Quote:
Originally Posted by dougklay View Post
I have a yanmar diesel that I can't get above 2800 rpm (cold), and that will diminish over time to about 2500rpm. I believe its a fuel problem. I've changed the primary and secondary filters, and just today noticed bubbles in Racor separator when operating. Could it be that air is entering a fitting in fuel line and I'm loosing suction?
I had a similar problem with my 2QM20 and it turned out to be a plugged exhaust system. The mixing elbow was completely packed with carbon. Once I cleaned it up power and RPM came right back. Check that as well as air leaks on the fuel side.
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Old 21-09-2014, 13:19   #8
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Re: Unable to Power up Engine above 2800rpm

Hi, I had exactly that same problem with a yanmar 30 and it went all the way down 'til it wouldn't rev above 1500. The solution was amazingly easy to fix.
It had started after an air leak from a slit under a jubilee clip on a water intake hose. That caused a lot of black exhaust smoke and therefore an unusual amount of coking building up. I fixed that but soon after noticed the revs weren't revving as high. This became the new problem and was incredibly easy to remedy. Before you change all the hoses take off the stainless steel exhaust manifold coming straight off the rear of the engine and into the hose that leads to the exhaust. It is an ss water jacket and has a slight downward kink in it, and 4 bolts holding it on. Remove the hose from it first then undo the four bolts. Chances are the tube in the manifold piece is blocked with carbon. Mine was down to a pencil thin hole allowing the exhaust out. Chip it off with a screwdriver to clear the passage. Bolt it back on. That should be the problem fixed. Maybe an oil change helps if it was all coked up. It's a chance to change the gasket but I got away with my old one. Engine revs freely.
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