Diesel Fuel System Question

[Pembquist]

The engine on my CD 28, its an MD7A, will on occasion hiccup and drop rpms, a few times it quit when the throttle was reduced to idle but would restart. I drained water from the seperator but that hasn't really made a difference. The last time I took a close look at it I decided to run the engine a little to draw any water up out of the fuel tank into the seperator before draining it. The seperator has a clear bowl and I could see foam entering it from the supply hose so I assume air is leaking in at the hose fittings. When I shut down the engine and opened the the seperator cock at the bottom there was a vacuum which drew in a lot of air through the drain. I never got a gravity drainage from the seperator and always had to use a vacuum pump to draw out fuel from it but this was the first time air was sucked up throught the drain, it seemed like a pretty strong vaccum in the fuel system. My thinking is that the supply line or tank pick up tube must be occluded or something as I don't understand how the seperator could hold such a vacuum with it being supplied by a vented tank. The seperator is only a foot or so above the tank. Before I go to the trouble of pulling out the tank and getting it cleaned do you fellas have any advice on this problem.

I'd also like some advice on fuel system plumbing, the PO used iron pipe T's in a couple places with plugs in them I assume to facilitate bleading, but I'm not sure, the last such T is directly before the injectory pump and after all the filters. Should I be worried about rust from this T making its way to the injector pump? The idea of backing out and replacing a plug on this fitting worries me as my understanding is that injector pumps are really touchy when it comes to bits of crap, is this arrangement normal? Would Brass be a better choice of fitting? Should the hose go from the injector pump to the fine filter without any intermediate fittings?

Thanks for any advice.

[mgphl52]

Wow! That's some crazy plumbing, IMHO...

IIRC, the lines from the engine mounted fuel filter to the lift pump and then to the injector pump are all solid tubing with banjo fittings. Sounds like some one has 're-engineered' this system. If that's the case, it may be difficult to find the correct parts. I have heard of place in the Northeast that has a fair bit of older Volvo parts, but cannot remember the name... Never the less, I would highly recommend getting rid of the iron pipe! (I do have some 'black iron' pipe on KAYLA, but only to connect the exhaust flange to a stainless steel Yanmar-style mixing elbow.)

As for the vacuum, have you checked the actual vent on the tank? Lots of critters can seal a vacuum, including dirt daubers.

[Paul D.]

Agreed on the black pipe ix nay. ABYC Fuel systems standards: see page 3-4

https://law.resource.org/pub/us/cfr/ibr ... 3.1989.pdf

On the fuel filter/suction problem, I would go about diagnostics in this order

1. Clear outside fuel tank vent, vacuum etc. If you have access to interior end, try to blow it out and confirm it is unblocked
2. Replace water separator filter and try
3. Run motor without the tank in the feed - Make some sort of fuel tank hooked up directly to the fuel/water separator
4. Try with the water separator out of the system as long as you are still filtering your fuel
5. Double check fuel lines to see if they are collapsing at any point

Good luck on solving this and getting back to smooth motoring (only when there is no wind though!)

[Doug Jesseman]

I would simply change the filter. It is probably full of crud and the system is sucking in air at extreme pressures.

[Pembquist]

OK I believe at least one problem is fixed. I tried vacuuming the vent line at the outside hull fitting, made no difference except that now when the engine ran it would drain the fuel bowl on the seperator,(I assume its just a coincidence not cause and effect.) I disconnected the fuel line that goes from tank to seperator at the seperator and blew into the tank. I could tell it was clogged then free so....I pumped out the tank and took the tank and the seperator home and cleaned out the black gunk inside the tank and clinging to the fuel sender. Cleaned out the seperator, got a new filter for it, put everything back, added some scarily toxic sounding biocide to 10 gallons of new fuel and it all seems to run better.

Upon reexamination I realized I was mistaken about one of the iron T's, it is not between the fine filter and the injection pump but is infact between the injection pump and the tank return. I am guessing that the PO could not find metric fittings for the green painted fuel lines with swagged banjo bolts so improvised, (not sure but it is what I suspect.)