Engine Revs Surging

[mrh123]

My Perkins 6.354 has some sort of issue that causes it to surge in revs with a vroom, vroom, vroom type of sound (my girl friend described it as such so I thought it worthwhile adding that ). I am at quite a loss as to what is causing it. It occurs between about 1100 rpm and 1500 rpm in other words she idles fine and will run above 1500 rpm quite well, though I wonder if the revs are disguising the problem. I have checked the fuel line for air leaks and changed the filters and all seemed well, but now after only a few hours running its back. If it wasn't for the fact that I can run the engine up though the problem area I would think it was fuel related (which it might still be), so I can't understand what is causing it to surge up and down. I did also think this was an off load issue as it seemed fine under load but not out of gear, but thats wrong as it does it in gear as well.

Its almost like it has a malfunctioning load governor like a tractor that tries to maintain constant revs under varying loads, but in this case the load is constant but the engine governor thinks its not.

Thoughts/suggestions on what I might do/try?? If you think it is fuel delivery could it be the lift pump diaphragm?

Cheers,

Martin

[DeepFrz]

You might try running a short line from the input to your lift pump to a clean container of fuel to see if that changes things. If it does, then the problem is upstream from the fuel pump, if it doesn't then the problem is on the engine.

A partially plugged vent line or a partially plugged screen on the pickup tube could possibly cause this problem.

[mrh123]

Not a bad idea. It was also suggested to me that a vacuum gauge on the lift pump supply line would tell me if it was having issues with fuel supply. The only problem with that is I am unsure what normal would be. Zero or some level of vacuum?

I will see if I can try your idea by disconnecting the out line from the first filter as its a flexible hose so could be feed into a Jerry can of clean fuel.

I don't believe I have any in tank screens as I have been busy cleaning out the starboard tank for repair as it was leaking. Quite clean inside which was good to see.

[belizesailor]

I agree, certainly sounds like a possible vaccum leak. They can be a pain to isolate. Have you made any changes to the fuel system prior to this problem occurring?

You should see some some level of vacuum between lift pump and tank (that's what the lift pump does for a living after all). Don't know exactly how much, but specs should be available in shop manual for this engine or maybe someone here knows.

If working with good clean diesel then you could, as suggested, just plumb straight from lift pump to a container of very clean diesel. If the surge goes away then there is a fuel delivery issue somewhere between lift pump and tank.

[DeepFrz]

On the vacuum gauge reading its hard to say when the engine isn't operating properly what the baseline should be. Your baseline should probably be 1" or 2" inches of mercury (right after a filter change) however a large lift from the tank may cause it to be higher. I think that any reading under 10 psi would not cause a problem (dirty filter) but your engine service manual may differ from my suggestion. If you install a vacuum gauge install one with the drag needle so you can see what the highest vacuum reading was.

http://www.proboat.com/attention-to-...auge-tool.html

[SkiprJohn]

Another issue is that your governor might be "hunting" which is caused by an idle speed set too low. If the air in the fuel line issue does not resolve it then try setting your idle speed just a tad higher.
kind regards,

[mrh123]

Well I am a step closer to solving my engine surging problem. I removed the lift pump yesterday, to check if I had a cracked diaphragm, which I dont, but what I did find surprised me. I found that the lift pump has a gauze filter, which must of never been looked, at as it was quite clogged with a brown sludge, as was the top chamber of the pump. So I cleaned the chamber out and left out the filter as I felt one prior and one after, the large paper element types was plenty of filtering. Of course in the process of doing this I dropped a nut off a fuel line support clamp under the engine, darned if I can find it. Anyway my surging is much much better but not gone, which is quite disappointing. I still get a 50 rpm change in revs at 1500 rpm and the cycle is not constant, she will run at 1500 rpm for a while then start the slight surge for a while then back to constant running. The time between constant and surging being quite random. It is way better than it was but still not gone. Any more ideas or thoughts. I have yet to try a clean jerry of fuel a the source, so I may still have a flow from tank issue.

[Astral Blue]

Originally Posted by mrh123

Well I am a step closer to solving my engine surging problem. I removed the lift pump yesterday, to check if I had a cracked diaphragm, which I dont, but what I did find surprised me. I found that the lift pump has a gauze filter, which must of never been looked, at as it was quite clogged with a brown sludge, as was the top chamber of the pump. So I cleaned the chamber out and left out the filter as I felt one prior and one after, the large paper element types was plenty of filtering. Of course in the process of doing this I dropped a nut off a fuel line support clamp under the engine, darned if I can find it. Anyway my surging is much much better but not gone, which is quite disappointing. I still get a 50 rpm change in revs at 1500 rpm and the cycle is not constant, she will run at 1500 rpm for a while then start the slight surge for a while then back to constant running. The time between constant and surging being quite random. It is way better than it was but still not gone. Any more ideas or thoughts. I have yet to try a clean jerry of fuel a the source, so I may still have a flow from tank issue.

I've seen these gauze filters on several lift pumps. I remove them and place a coarse inline filter before the pump, depending on whether your main fuel filter is after your lift pump.

I would suggest a filter/water separator before the lift pump -- as it would eliminate the possibility of an inconsistent stream of fuel delivery to your engine.

Is your throttle control at your injector pump? If so, keep an eye on it when your engine is at the RPM where the fluctuations are most likely to occur. If you have an issue with the throttle cable holding its place, you will see movement of the lever at the injector pump. Just a thought...

[DumnMad]

I had a similar thing with my new boat, turned out the Borg-Warner was very low on oil.
Topped it up and it was fixed.

[markpierce]

The only time this occurred with me was with a clogged fuel filter (here in process of changing).

[JSSailem]

Originally Posted by DeepFrz

On the vacuum gauge reading its hard to say when the engine isn't operating properly what the baseline should be. Your baseline should probably be 1" or 2" inches of mercury (right after a filter change) however a large lift from the tank may cause it to be higher. I think that any reading under 10 psi would not cause a problem (dirty filter) but your engine service manual may differ from my suggestion. If you install a vacuum gauge install one with the drag needle so you can see what the highest vacuum reading was.

The Vacuum Gauge Tool - Professional BoatBuilder Magazine

I can confirm Deepfrz's comments. I installed a new Racor500 and added a vacuum gauge to the filter to engine line. When new the indication was "-2" inches. We ran the engine 35 hours with known dirty fuel in the tank. On our return to port the engine began to surge about every 30+ minutes. It was a short 200-300 rpm increase. On checking the gauge the reading was "-12" inches. Pulled the filter, installed a new one. 30 minute easy task. Here is an image of the cloged filter.