L300 fuel pump failure?

Does your Mitsubishi L300 make a strange noise? Need wheel alignment specs?
Post Reply
Posts: 3
Joined: Sun Jun 12, 2016 9:42 pm
Member's Photo Album: http://www.delica.ca/Photos/
Vehicle: L300 Exceed
Location: Vancouver

L300 fuel pump failure?

Post by blackbrent » Sun Jun 21, 2020 7:51 am

Hi folks, this is my first post so please forgive any error in format.

I'm having trouble on starting my '92 L300 (4D56) turbo diesel automatic. The vehicle is always on block heater when parked and up until recently would turn over with virtually no cranking / minimal cranking. A few months ago I noticed some diesel dripping from the front fuel tank but didn't deal with the problem immediately. Eventually the fuel leak got worse until this past week I patched the tank with steel reinforced epoxy - no more leak. Because of the leak (which was on the top side of the tank) I had been keeping minimal fuel on board and letting the tanks get quite low. I'm also someone who for years has been driving till empty / till fuel light is on, which I know can be hard on fuel pumps. Just before I did the tank repair last week the motor was idling low then dying at stop lights (electrical still working) and becoming increasingly hard to get started - having to crank way more than usual.

Now here's my question / problem : Since patching my tank I've noticed I have to crank quite a bit to get the motor to turn over. Even if the van has been sitting for a few minutes or hours it doesn't seem to be holding fuel pressure or feeding the motor as needed on startup. I have to crank it much more than previously even when on block heater (it's summer now, plugs replaced less than a year ago). I'm thinking the extra abuse I've likely put on the fuel delivery system because of the leaky tank with low pressure and low fuel has strained the diesel fuel injector pump or some other part and I'm trying to narrow it down. In looking at the pump /schematic of the pump itself I'm thinking it could be the issue. I've taken the pump out a few times previously to fit new filters and I can definitely replace the pump myself. I guess what I want to know is : what is it exactly that 'pulls' fuel from the tank to the injector? And : can the fuel pump fail? In looking at it I have a hard time understanding what could cause it to fail, maybe I just don't understand how it draws fuel from the tank. I don't want to spend the money / time on a replacement if I should be looking elsewhere. Is there a way to test for fuel pressure before/at/after the diesel fuel pump? Is there something I should be troubleshooting at the injector? The van runs perfectly once started, no longer dies on me or idles low, patching the tank solved that.

For now I'm going to keep the fuel tanks as full as I can to see if having tank pressure helps on starting. I've always noticed the van is sluggish on hill climbing and am thinking maybe the pump has been dying for some time and is just done. Given how simple the design is - would that be likely / possible? It looks original, so that's almost 30 years.

Any assistance or experience with diagnosing L300 fuel pressure / fuel pump issues is hugely appreciated.

Posts: 182
Joined: Thu Oct 06, 2016 3:24 pm
Member's Photo Album: http://www.delica.ca/Photos/
Vehicle: '91 Delica L300
Location: Pennsylvania, USA

L300 fuel pump failure?

Post by sk66 » Mon Jun 22, 2020 5:56 am

I think there's an error in there somewhere... no-one in their right mind would pull the injector pump in order to change the fuel filter. I think what you are meaning is the priming pump on the fuel filter mount.
The injector pump is next to the engine and connected to it by a pulley/belt and hard pipes to the injectors. It is what pulls fuel from the tank and sends it to the injectors under high pressure. And I highly doubt that you have strained/hurt it unless you've run it dry.

IMO, the most probable issue is a restriction, especially if the tank was open to gathering additional crap. I would start with the easier things. Replace the fuel filter, and check the injection pump inlet/outlet screens in the banjo bolts (may have one or both). If you don't find/fix the issue there, then I would probably guess the pickup screen inside the tank; which can be pulled/cleaned.

Actually, before doing any of those things you might want to crack an injector and see just what kind of fuel flow/pressure you are actually getting. You can also run the IP inlet line directly to a fuel can; that will isolate the fuel filter and pickup screen/plumbing and give you an idea if the issue is before/after the IP inlet.
1991 L300
Harrisburg, PA

Posts: 117
Joined: Tue Jan 09, 2007 2:46 pm
Member's Photo Album: http://www.delica.ca/Photos/

L300 fuel pump failure?

Post by helibrian » Tue Jun 23, 2020 9:46 am

Could be your rubber fuel lines. Sounds like your loosing prime.

Post Reply

Return to “L300 Technical”