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Old 05-08-2011, 05:57 PM   #1
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Intermittent RPM drop at consistant throttle position.

Hi All

My boat (twin perkins 4.236 diesels) has been developing a strange rpm drop on the starboard engine. It is slowly getting worse and happening more often.

Right now the rpm drop happens about every 20 - 40 seconds and lasts about 2 or 3 seconds. The starboard engine will just drop down about 150 rpm for 2 or 3 secs then recover to whatever it was running at and repeat this over and over.*Occasionaly the*drop will be up to 300 rpm but this*larger rpm drop is maybe 1 out of 15 times*of the usual 150 rpm drop.

It occurs at a variety of running rpm from 1200 to 1800.

Anybody got any ideas where to start?

Thanks

Roger

*
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Old 05-08-2011, 06:02 PM   #2
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RE: Intermittent RPM drop at consistant throttle position.

Air leak into the fuel system. Have you changed filters or done anything else lately to the fuel system?
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Old 05-08-2011, 06:07 PM   #3
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RE: Intermittent RPM drop at consistant throttle position.

Hi Keith

The only fuel related thing that is a possibility, is the CAV fuel pump on that engine was leaking a bit around the shaft for the shut down control. That leak has stopped recently. I thought maybe that was a good thing but perhaps it is letting air into the pump. I believe but do not know for sure that that part of the pump is only under the minimal pressure of the lift pump.

Otherwise no changes of filters or other fuel things.

Roger

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Old 05-08-2011, 06:56 PM   #4
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RE: Intermittent RPM drop at consistant throttle position.

if it is not the fuel filters,may you have a plug in the fuel line someplace???
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Old 05-09-2011, 12:37 PM   #5
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RE: Intermittent RPM drop at consistant throttle position.

You could install a clear in line fuel filter to see if you have an air leak for abvout 15 bucks.* *Also check you filters to see if there is air in the canister.** I installed one in the Webasto Fuel line to find/see the air leak.
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Old 05-09-2011, 04:06 PM   #6
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RE: Intermittent RPM drop at consistant throttle position.

You can also check the tank vents and make sure they are clear...or simply open the fill caps while underway and see if this solves the problem.

There also may be a flex fuel line collapsing internally (from delamination). If the lines are old it would be worth while changing them anyway as a PM item.
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Old 05-09-2011, 04:19 PM   #7
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RE: Intermittent RPM drop at consistant throttle position.

** You might want to install vacuum gages on your fuel supply lines on the engine side of the filters, it wouldn't hurt to have them between the tank and filter also. With this set up you will be able to note changes as the filters become dirty or for that matter to help in firguring out problems. They are available from Granger supply for less than 10 bucks each.
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Old 05-09-2011, 05:42 PM   #8
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RE: Intermittent RPM drop at consistant throttle position.

Maybe off the wall but does the voltage change with RPM?* I have had a similar problem with a gas engine caused by a voltage regulator that pull the alternater in and out.* Would be easiest to see with a good digital meter.

*
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Old 05-09-2011, 05:49 PM   #9
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RE: Intermittent RPM drop at consistant throttle position.

Just sit at the dock and your rpm will be constant.

Ok bad solution, bringing you back to reality
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Old 05-09-2011, 07:52 PM   #10
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RE: Intermittent RPM drop at consistant throttle position.

We went through this about 3 months ago with.... Remember?
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Old 05-09-2011, 08:00 PM   #11
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RE: Intermittent RPM drop at consistant throttle position.

Quote:
Phil Fill wrote:


You could install a clear in line fuel filter to see if you have an air leak for abvout 15 bucks.* *Also check you filters to see if there is air in the canister.** I installed one in the Webasto Fuel line to find/see the air leak.

I see several great ideas but I like Phil's the best. If you are introducing air somewhere it is VERY hard to prove. Think of anything before your fuel transfer (thumb lever) pump as suction, and anything after as pressure. A leak on the pressure side will not affect your speed , but it will obviously cause an external leak.*** a leak on the suction sorchid the pump may leak a little with the engine shut off, but will draw air into the system when running. Air compresses, diesel don't. By installing clear tubing as close to the fuel transfer pump as possible, you will be able to see air bubbles if any hose/ fitting/ etc. from the clear hose back to the tank is drawing air in. Keep in mind, you can still be pulling air at the fitting going into the pump and possibly even the diaphragm IN the pump but of course wouldn't see bubbles. In that case, your engine oil would also probably be diluted.
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Old 05-10-2011, 04:48 AM   #12
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RE: Intermittent RPM drop at consistant throttle position.

"If you are introducing air somewhere it is VERY hard to prove."

Only if you haven't spent $15. and installed a refrigeration armored sight glass.
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Old 05-10-2011, 08:49 AM   #13
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RE: Intermittent RPM drop at consistant throttle position.

The air leak was like champagne bubble for a few seconds every minute to two minutes.* It would have been very hard to find/see with out the filter.*I left the in line filter on the line after the new big main filter.* The Webasto did not mind the small amount of air, it was the air*over a period of time filled the filter.* The webasto clean polishes the fuel as it draws more fuel than it uses, just like some engines.* I don't think the DD 671 would have notices as it draws about 60 gph, but uses 2 to 5 gph.*

*

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Old 05-23-2011, 04:05 AM   #14
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RE: Intermittent RPM drop at consistant throttle position.

The easy way to actually FIND an air leak is always a small tank , high in the boat that pressurizes the system.

This and a roll of paper towels and observation will find 98% of air leaks.
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Old 06-09-2011, 08:58 AM   #15
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RE: Intermittent RPM drop at consistant throttle position.

Thanks everyone for your help.

Some thought it was an air leak. A few thought it was a filter problem. Everyone was right !

The engine had a small fuel leak at the fuel pump where the shut off lever shaft enters the CAV pump. Once the filters got clogged enough that the lift pump could not keep positive pressure in the CAV pump the leak reversed. Instead of fuel weeping out around the shaft air started entering the pump.

I have changed the filters, the engine runs fine and my small leak on the fuel pump is back. Now I have to decide if I am brave enough to take the top off the fuel pump and replace that O ring.

Thanks again
Roger
34 Californian
Two Easy
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Old 06-09-2011, 08:33 PM   #16
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RE: Intermittent RPM drop at consistant throttle position.

Quote:
Mappy wrote:
Thanks everyone for your help. Some thought it was an air leak. A few thought it was a filter problem. Everyone was right ! The engine had a small fuel leak at the fuel pump where the shut off lever shaft enters the CAV pump. Once the filters got clogged enough that the lift pump could not keep positive pressure in the CAV pump the leak reversed. Instead of fuel weeping out around the shaft air started entering the pump. I have changed the filters, the engine runs fine and my small leak on the fuel pump is back. Now I have to decide if I am brave enough to take the top off the fuel pump and replace that O ring. Thanks again Roger 34 Californian Two Easy
*Well done Mappy!

Somewhere on the board I posted a cutaway of a real CAV pump- might help if you take on replacing your seal!
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