This is good stuff!!

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Oct 6, 2007
Vessel Name
Vessel Make
1983 42' Present Sundeck
One of the hardest problems on a diesel engine to locate is an air leak into the suction fuel line*allowing air into the fuel system. You look for external leaks, tighten fittings, look for loose filters/ gaskets, etc., then you run the engine- sometimes quite a while- before air accumulates again and shuts the engine down. Been there- done that.........
Today I was making a repair on a Bobcat skid steer loader for a customer. The Kubota diesel was starving for fuel- filter was clean, etc.-I found a crushed fuel line that had vibrated beneath the pump mounting. Rerouted/ shortened hose- problem solved. I thought. Wrote the repair order up- picked up some supplies for home at the customers hardware store*before I left. The machine died again. Possibly some air left in the system from the repair. Bled/ started again. Ran 5 minutes-DIED again!!
Here is the good stuff.
This fuel system used a marine type hand primer pump to bleed the system after a filter change. Suspecting a small leak in the brimer bulb- I replaced it with a clear in line gas filter I had in stock on my service van. I mostly just needed a way to remove the primer bulb and reconnect the lines.
Restarted the machine....
Guess what I could see -- Blub, Blub, Blubbing clear as day in the filter housing? You guessed it- air bubbles. I realized then that this is a GREAT way to prove/ disprove air being pulled into the sysyem. I had a failed plastic pick up tube in this case, replaced it, started engine, no bubbles. Problem solved.
When you think about it- you could strategically place a clear filter just prior to the transfer pump, at a fore or aft tank, etc.. Anywhere on the suction side of a diesel fuel system.
I realize that a 135 Lehman with copper fuel lines and SAE fittings would not be as easy to tap into as the little Kubota. But if I had a hard to find fuel problem I am sure I could apply this technique. Thoughts?
For years I have suggested the instalation of a refrigeration "bronze armored Sight Glass" at under $20 in 3/8 line size , so any fuel can be monitored for bubbles.

They work great in our bus conversion , but are not needed on LUCY as the USN was smart enough to provide a gravity fuel system.

The bronze units are OK for 200psi , far more than most primary fuel pumps can produce. They have std SAE fittings , and can be installed in 5 min.


-- Edited by FF at 05:38, 2008-03-26
With all due respect, forky and FF, seems to me you would just be introducing more potential problems(places where air could get in) instead of solving them. But I am FAR from being an expert.
Thats a good point. I guess I forgot to mention that this is simply a troubleshooting process- to be removed after the problem is located/corrected.

FF's sight glass is (I believe) designed to be put on the pressure side of the fuel pump. So if it were to leak, it would possibly seep a little diesel and be pretty obvious. And since it has no gaskets, it's pretty unlikely to leak.

You might also install it on the return line from the injection pump - which should be bubble free when running. Even if somehow a little leak were to develop that allowed air into the system when the engine was shut down, air in the return side shouldn't give any problems.
CAV type filters in a glass or simply a glass water trap go before the priming pump and should be standard on any marine or stationary engine. They are invaluable to detect water contamination and of course to show air leaks. Their downside is that they break if you hit them with a tool accidentaly. However glass bowls and CAV filters are extremely cheap due to the large number of applications.

-- Edited by Marc1 at 03:29, 2008-03-31
What the heck does CAV stand for anyway? Anybody know?

I had those dual CAV filters on my Lehman. I scrapped them and replaced with a Racor spin-on with a see-through bowl at the bottom. MUCH easier to change and spot any problems. By the way, I sold the old unit easily, as lots of people had stripped out the threads on the top bolts, tightening them so much trying to stop air leaks.
I have never been able to find out what CAV stands for. However, it's been a division of Lucas since 1926.

We changed the dual, multi-part cannister filters on our engines for the Racor (Parker) spin-on adapter. Two adapters needed per engine. Makes changing the secondaries a whole lot easier......
According to a website CAV stands for
"The company was formed by Charles A. Vandervell (1870-1955)"
That is about the last thing I would have expected

-- Edited by Steve at 23:10, 2008-03-31
Sorry guys-
But the glass bowl at the bottom of*your filter assy. is a fine way to see water trapped- but it ain't gonna show you air being introduced into the fuel lines prior to the fuel transfer pump. Air floats up. If you had a CAV filter assembly that was clear on the top it would work.

A CAV type filter with a glass bowl flows the fuel down thrugh the center and into the bowl and then up through the filter element. When it is ture that the air will tend to go up, small bubbles of air will be visible in the fuel flow providing there is a resonable flow of fuel and that the amount of air is large enough to be visible in the form of bubbles.
Howver your idea is not wrong. There is nothing stopping you from mounting two water trap, one the normal way and the second upside down to trap air in it.
Most of the OTS air traps are to remove the bubbles from bypassed fuel returned to the tank.

Trucks with smallish tanks that want many hundered of HP are the customers.

On a 3gph boat its far cheaper to FIX the air leak.

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