"Air Visitor", not welcome in fuel system

[Pete]

A few weeks ago my engine would not start. To fix this, I had to bleed some air out (using the bleed screw in the Crescent fuel fitting) on my Univ. M25.

This was a real surprize, since I have not any any issues with an "air visitor", in over a years worth of weekends. I would be interested in knowing if anyone out there has any "theories" or ideas why air would show up now?

Thanks, PC

[Tom in Cambria]

If you've changed the fuel filter element recently you may have got some air in the system. If you haven't changed the filter element recently, you may have a build up on the filter element which would mean that the fuel pump is having to suck much harder than normal to try and bring fuel to the injectors. Places that didn't leak air when you have normal fuel flow might suck air when the flow is impeded. When you change the filter element the engine will often run for half an hour or so on the fuel in the line and then suddenly quit and have to be bled. It takes that long for the air bubble to reach the engine. If your filter element is clogged you will suck a vacuum and when you bleed it, it will run for a little while and then suck another vacuum and stop again. You may have sludge in your tank which is packing up your primary filter. How long has it been since you changed your filter element? According to my theory that would be your first suspect. After that look for weeping fittings/hoses on the fuel line which might suck air in a vacuum situation.

[DanaVin]

My friends boat has a Yanmar 35 horse engine in it.
It constantly stalls out or dies after running for awhile.
We've changed filters and he's had the tank cleaned.
If it runs at all, it'll run just fine until it coughs and bogs down again if it doesn't die completely. This running time can be hours at 2000 rpm.
We are buffaloed. Any other ideas?
He was going to change out the fuel pump this weekend so I haven't heard from him today.
We've gone through literally dozens of filters.
When we do get it started after one of these fits, it will often only run at enough rpm to do a knot or so. Very sporadic.
It's in a Hunter 376, 1996.
Help!
Thanks
Dana
CD25, 1977
sv "Gladys Erzella"
San Diego Bay

[Pete]

I suspected I had a clogged filter and have since replaced both my secondary and primary filters. I have some shakedown time on Friday, will see just how things run.

Thanks, PC

[Oswego John]

Pete,

While you're checking out your fuel system, read your fuel pump specs and then do a time/volume test to see if you are getting adequate fuel.

Check and see if your pump has a screen at the inlet or an inline filter. If so, either could be partially clogged.

Does your fuel pump have a diaphragm? Check it out to see if it is ruptured or leaking.

Good luck,
O J

[Tom in Cambria]

Dana,

Once when I was delivering a Coronado 45 and the engine quit I watched the mechanics do something which I filed away as a trick to remember. The engine was a Perkins 4-108. They rigged up a coffee can with plastic through hull with a hose barb on the end so that the hose size was the same as on the engine. They showed up at the boat with their can arrangement and just pulled the hose off the injector pump and put the loose end of hose from the coffee can on in it's place. We started the engine and were able to power over to the dock at their shop. It turned out the fuel tank had algae in it and had to be steamed cleaned, but the can trick was pretty clever. Your friend might rig up a can to serve as an aucilliary tank for testing purposes. If the engine would run on a coffee can of diesel you would know it isn't the injector pump or engine. Then you work your way back and hook the coffee can to the in port of the fuel pump. If it still runs you would know it isn't the fuel pump. If it doesn't, of course, you would know it is the fuel pump. If it runs on the coffee can but doesn't when the fuel line is hooked up you know it is in the line which feeds the fuel pump. By working your way back you can isolate where the problem starts.

I certainly sounds like a contaminated fuel tank. Did your friend actually take the tank out and clean it or just have the fuel polished? Sometimes fuel polishing won't do it by itself. I had my fuel polished and the problem soon came back. I finally took the tank out and cleaned the inside at the carwash. That solved my problem -- so far (knock on wood). After you replace the fuel filters how long does it run OK before it stalls again? Hours or weeks? You may have so much crud in the tank that it just immediately packs up the filters again. If you put the coffee can in where the fuel tank normally attaches and it still does it, that would seem to indicate it's not contamination in the tank, and, of course, the reverse is true also.

Good luck and let us know what he ultimately finds is the problem.
I've also seen the inside of hoses deteriorate although the outside looks OK. Then the wall of the hose collapses and cuts off the fuel supply although the outside seems to be OK. Hose is cheaper than mechanics so you might try replacing all the fuel hose as it couldn't hurt and doesn't cost that much to do yourself. Be absolutely sure the tank is immaculately clean as he's got the classic symptoms of a fouled tank or a bad fuel pump. With fouled tanks the engine generally doesn't cough though, it just quietly dies.

FWIW

[Oswego John]

Hi Tom,

Many's the time the old coffee can came into play around the shop. Very often it was used to prime an engine. Another thing we did was disconnect the output of a fuel pump and let the fuel dump into the can when testing the pump.

But there's another practical use with a container. Years ago a friend of mine ran an auto repair shop. Back in the seventies, before SUVs, gas was pretty expensive (for that era) and in short supply People were concerned about the mileage that their vehicle was getting.

My friend had a homemade plastic bottle that contained exactly one quart. It had a vented filler cap on top and a hose barb below. He would cap off and bypass the fuel pump and gravity feed the fuel from the container through a hose into the carburetor float entry. That was before bypass return lines.

He would set the trip odometer to zero, fill the quart bottle with gas and drive normally until the engine stalled for lack of fuel. Then the recorded mileage was multiplied by four whick gave him the average mileage that the vehicle was getting per gallon.

The reason that I bring this up is that maybe something like this could be used to determine Diesel efficiency or fuel consumption per hour at any given RPM.

FWIW
O J

[Mark Yashinsky]

both have fuel return lines that (usually) send excess fuel back to the tank. For the coffee can trick (or plastic bottle) to work for checking consumption, you have to return the fuel to the coffee can.
As for air in the system, its getting in somewhere. Just because a screen is clogged, if all connections are sound and all lines are good, no are should get into the system. It should just die from lack of fuel. Take a close look at all connections from the tank (any w/ hose clamps?), a close look at the hoses (old, brittle, chafed), all connections to the first pump on the engine, where its still pulling (vacuum) fuel.

[Marianna Max]

As for air in the system, its getting in somewhere. Just because a screen is clogged, if all connections are sound and all lines are good, no are should get into the system. It should just die from lack of fuel. Take a close look at all connections from the tank (any w/ hose clamps?), a close look at the hoses (old, brittle, chafed), all connections to the first pump on the engine, where its still pulling (vacuum) fuel.

From experience I can tell you this is true. I know nothing about engines but at one point I managed to finagle a trip on a 84' shrimp boat by pretending I knew how to cook (quick learner). Anyway, in the middle of the carribean our big ole engine stopped. This thing had a room of its own below decks. We all went down and pretended we knew what we were looking for (even the engineer who had also pretended to know about engines in order to finagle a trip out on the boat). The engine had coughed and sputtered before dying. Being a logical sort I said, hmmm, sounds like it was pulling air from somewhere. The engineer and captain looked at me and said, yeah, it did.

I reasoned that if air was getting in, fuel was likely to be getting out. So what I did was look at each joint in the fuel line (they thought I was crazy) and ran my finger along each one and when I came away with a bit of fuel at one joint I said "tighten that one". They again looked at me like I was crazy but the engineer did it.

After a bit of fussing to bleed the line, the engine started behaving again and did so for the rest of the month we were out.

I got a lot more respect the rest of the cruise.