primer pump only pushes air- how to purge air from diesel generator? - PeachParts Mercedes-Benz Forum

bigblue · #1 10-18-2009, 02:45 PM

I apologize for this being a non-mercedes specific diesel tech question, but I am a somewhat regular on this forum and know there are plenty of well educated individuals who could help me with my issue. I am having fuel starvation problems, after hours of continous running it will randomly start to bog down and then stall.

i have an older non-idi diesel generator. It has a mitsubishi 4cyl turbo engine from 1980's vintage Mitsubishi canter (australian truck) engine. It has 2 screw on fuel filters and a primer hand pump on the injection pump.

When this happens and I try to re-prime it, usually the hand pump doesn't feel to have any resistence which is telling it is only pushing air into the injection pump. i immediately replaced the hand primer with a new bosch spare (for my 617 mercedes) hoping it was the issue. So I figured to post a question here since this engine shares at least a few common components with our mercedes diesels.

I think gravity might be my culprit. My fuel tank is about 4 feet below the primer pump (as well as the rest of the engine) and the only way I have ever had any sucess it to loosen the hand primer, and rig up a setup to pressurize my fuel tank until I notice fuel coming from the hand primer, quickly tighten it, and pump, which is not easy to do. My fuel hoses are a little old but seem to be intact. I'll replace them.

Does this make any sense or am I way off? Can anybody suggest a solution? I don't have too much experience with diesel generators, or anything non-automotive. My business depends on reliable operation of this generator.

kerry · #2 10-18-2009, 03:55 PM

I'm guessing that once the fuel level in the low tank drops below a certain level, the lift pump can't lift the fuel anymore, causing the problem. I'd try raising the fuel tank.

Junkman · #3 10-18-2009, 05:37 PM

Your system needs to be closed. I have had a rustic (read primitive) water system at what used to be my primary residence in the hills of TN. Water is pumped (actually sucked) from a holding tank up hill into another tank. The 2nd tank is pressurized and supplies water when the faucet is opened. A leak is the only thing that caused (in 30 years) the pipe between the downhill tank and the pump to lose its prime. Your sytem is similar.

Tank+++pipe+++Pump+++pipe+++Valve

Diesel911 · #4 10-18-2009, 06:37 PM

From the sound of it ounce you get it going it works; you just have trouble getting it going.

It could indeed be that the Fuel Suppley/Lift Pump is at the very limit of the lift it can provide.

I do not know what is availble where you are but in every Autoparts Store here they sell Electric Fuel Pumps (solid state type not the ones with a Bellows pump) from $40-$50 US. If you put one of those down by the Tank Outlet I think the problem would be solved.
I believe the valves in the Pump would also keep the fuel from leaking back and losing prime.
[Similar to above you could install a large manual Priming pump down at the level of the Fuel Tank to push the Fuel up for starting.]

A none electric thing that might work is if you put a 1 way Check Valve down by the Tank Outlet to keep the Fuel in that 4 foot of Hose from leaking back and losing prime evertime you shut down.
I do not know how long Hose would hold up doing this but you could simply clamp off the hose down by the Tank Outlet; if nothing else just to test and see if it will work.

Less likely solutions.
As the other post sort of indicated if you could seal off your Fuel Tank an pressurize it until the Engine gets going and the Lift Pump can Lift would work. Depending on the surface area of the Fuel in your Tank it might not take much pressure but a lot of air volume.

Also similar to what other post suggested concerning raising the Fuel Tank. You could provide a small Fuel tank for start up (keep it filled from the return Fuel going back to the main tank).

I like the Electric Fuel Pump idea best (I have 3 that I have aquired over the years and have found various handy uses for them).

Diesel911 · #5 10-18-2009, 06:38 PM

Another note: Other inline Fuel Injection Pumps I have seen have Air Bleed Screws on the Fuel Injection Pump Housing. Does yours have this feature?

TMAllison · #6 10-18-2009, 08:30 PM

Quote:

Originally Posted by bigblue

I am having fuel starvation problems, after hours of continous running it will randomly start to bog down and then stall.

Fuel tank level, air entering lines, or filters.

I'd go with Kerry's solution first as long as there isnt any wetness anywhere.

bigblue · #7 10-19-2009, 09:00 AM

Wow what great input. I did some work on it yesterday.
The hoses were slightly old and the previous owner of the generator put 2 ball valves on the feed and return that were a little loose. In the interest of cleaning up that and eliminating something else old in the troubleshooting I cut out everything and replaced both the feed and return lines with new hose direct from the tank to the filter. I found one hose was clamped over male threads. It wasn't leaking out any liquid but possibly could have been letting air in? It now has a proper barb. After doing that, pressurizing the tank to prime (still requires a second person- something I don't like to have to do) was much easier and quicker and the generator started up quicker than it had been.

I also measured the height of the fuel lift. 42" from bottom of tank to the hand primer on the IP. Unfortunately raising my main fuel tank height is not an option. The generator is mounted inside a truck box and the fuel tank is mounted to the frame underneath it. The generator's tank is actually a stock side mount tank for the truck chassis that I added on from a truck junk yard. It's one of those cylindrical units and if it were raised, there would be no clearance for the filler neck.
I like the check valve idea if it were to be able to hold for a while, or would consider mounting a low pressure electric lift just out of the tank.