RPM Loss Update - Expurts needed!

OK, DD gurus, here’s my latest. When we last saw our hero he was struggling to find his missing 200 rpm on the stbd engine.

To recap, I had been running the boat for a 3 hour trip from New Haven Ct to Westerly RI at about 2150 rpm (DD 6v92 making 435 hp) when my stbd engine dropped to 1950 and would not come back up. After docking I changed out the filters again and that had no effect.

I went thru the throttle linkage and the shutdown solenoid linkage and all were adjusted properly. I bypassed the floscan to eliminate that from the system. Suspecting the fuel pump, I read the manual and based on their recommendation, measured the return volume at 1200rpm for 1 minute. Flow was acceptable: recommended minimum is 0.8gpm at 1200; I was reading 1.1gpm.

In doing so, however, I did note that there was a substantial amount of foam (air) in the returning fuel. Suspecting a leaking line upstream from the pump, I first tested the good engine and determined that there was no air in its return flow. I then replaced all the inlet lines to the offending engine and ran the good engine’s supply line directly to the fuel pump of the bad engine, but that had no effect on the foam in the return line. All fuel lines upstream of the fuel pump are now new and solid. I am definitely getting good clean fuel (no air leaking in) to the pump.

Suspecting the fuel pump (cavitation?) I replaced it this morning, along with its fittings. I still have the same problem.

One interesting hint is that when I cranked the engine to prime the new pump, with the output line disconnected from the secondary fuel filter that it feeds, fuel spurted up out of that filter (i.e. back out the inlet of the secondary filer). That suggests to me that the fuel already in the engine is somehow being pressurized back into the filter. I’m not sure what to look for from here. Suggestions would be appreciated. Thanks.
Bob

What a mess!! You are still sucking a little air from somewhere. Not enough to affect fuel supply, but still giving air to the injectors input.
With the supply coming from the other engines line, That discounts the run to the tank, no? I personally think that you need to investigate further, ground thats already been covered. I would isolate given assemblies/sections of the run and cap them off and put a vacuum pump on it. They should hold absolute indefinitely. You may have a simple fitting with a hairline crack thats not visible to the naked eye. Even a crack in the Racor/threads.
The next test would be to remove and plug the foamers supply and run a temp hose from the good ones filters to the bad one. Start the good one first, then fire up number 2 and check the return for foam. Just another way of isolating sections. If you can run a seatrial while hooked up like that, at least youll see if you can reach full load speed. The one pump at 1.8 should be able to supply both.
The last test, altho tedious, would be to put a T and a valve in the secondary inlet. Run that one and crack the valve and check for air there. If that happens, then the leak is before the secondary. Actually, just after the pump is all pressure anyway. Just re-read the final line... Does the fuel return off the back of the heads have check valves ? That may explain the pressure but not the air. ws

I'm fairly confident that I am not getting air in the line to the pump. The port (good) engine shows absolutely zero air in the retun. The stbd is loaded. I hooked up the port fuel line to the brand new stbd pump, with new fittings, and there is no change whatsoever - it is still loaded with foam.

Bob, sounds like you know what you are doing, so this may be a waste:

"I then replaced all the inlet lines to the offending engine and ran the good engine’s supply line directly to the fuel pump of the bad engine, but that had no effect on the foam in the return line.."

So you extended the (non foaming) port engine fuel supply directly to the new stbd engine fuel pump.,.and still had foam in the stbd engine???? And also replaced all the stb engine fuel supply lines??

If I got that right, I sure don't get it either..

If not too much work, how about extending the stbd eng fuel supply (which may be suspect) line to the port engine fuel pump...if no foam in the port engine, then you KNOW the supply is good.....and the "leak" must be in the stbd fuel pump (even the input connection) or somehow after it.

good luck...

If its not the pump could be a bad injector sucking air.

That was my next conclusion... Actually compression air pushing back through the inlector. Does the foam smell like burned fuel? ws

"If its not the pump could be a bad injector sucking air."
"Does the foam smell like burned fuel?"

Interesting ideas.... an injector can get stuck open??

Would there be any accompanying tell tale signs...like smoke?? Would he lose only 200 RPM??

Thanks for the replies. I spent most of yesterday working on it to no avail. I like the idea of extending the fuel supply from the foaming engine to the good one. That would absolutely eliminate the fuel lines as a problem if the foam does not travel to the good engine.

I also suspect an injector problem. I discussed it with an Atlantic DD tech at some length on Friday and he told me that he has never heard of a 92 engine having an injector do that - 60 series do all the time, but not the 92. Still, something is causing it.

And, I'm getting pressure where I should not. The right cylinder bank fuel supply line is getting back pressure. When I removed both fuel supply lines from the secondary filter, and cranked the engine, the right supply line spurted fuel back out in the direction of the filter. The good engine shows no such tendancy.

I believe that the foam, loss of rpm, and back pressure are all related, and probably are the result of some sort of injector anomoly.

Once I switch the offending engine's fuel line to the good engine and absolutely eliminate an air leak as the cause, other than starting to swap out injectors, I really don't know what else to do.

One other thing - I did try to smell the foam, but the diesel smell is so strong I could not detect anything else.

I also shot all 6 exhaust ports with an IR thermometer. The left bank was consistently 129 - 131 degrees and the right bank 133 - 134 consistently (after running 20 min or so).

Maybe checking those exhaust port temps at low, neutral, RPM dockside (I'm guessing that's what you did judging from the low temps???) is not an adequate test to determine if one injector is faulty. I never checked any temps at idle, only at cruise RPM and found exhaust port temps within about 5 or 6 degrees via infrared thermometer.

Perhaps a check of exhaust port temps at normal cruise, under load, would detect a cylinder with low temp (faulty injector) better??? ...and my own readings were sensitive to angle and distance from the exhaust port. If yours are painted, be sure to read consistently either on paint or on "rusty" spots....that affected my infrared temperature readings several degrees also.

Dontcha love rocket science??? Rebs right... no load temp testing wont show a thing... Ya need load to make temps! OTH... can you take two fuel return samples (one from each) and let them sit in a glass jar (sealed) to settle out then compare fuel color, and if you shake them simultaneously, check for air or combustion gasses' solubilty... just a thoght before you start pulling injectors. Love those FM's !! ws

Good news - problem solved.

Since I was pretty sure the problem was on the right side of the engine, due to the back pressure on the fuel line to that side, I pulled the valve cover, and removed all the fuel pipe on that side. I disconnected the fuel line to the secondary so I wasn't pumping more fuel into the engine, and had my buddy hold the stop button and press the start button. The forward most injector shot fuel 3' in the air out of the inlet port, and bubbles came out of the outlet port. The other two injectors had no such symptoms.

I changed out that injector and finished reassembly. I put the return line in a bucket and fired up the engine. No air/foam observed in the fuel. Took a test ride and hit 2350 on both engines. Life is good.

My best guess is that the check valve in the injector failed, allowing compression back into the unit - that despite 5 different DD mechanics telling me that this never happens. I guess there's a first time for everything.

Quote:

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Originally Posted by rsmith

If its not the pump could be a bad injector sucking air.

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Ta Da

Quote:

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Originally Posted by rsmith

Ta Da

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Yes, we have a winner!! Give that man a Kewpie Doll!

Good Call.

Nice to hear a positive report from someone that actually does thier own work. Good going guys.

Those guys were probably Reliabuilt mechanics... those "never" fail either... ws

Good going....always good to get feedback on the final resolution....

I am still amazed that with all the things that can potentially go wrong aboard a boat, they end up being as reliable as they usually are.