12HT Dies Intermittantly

[60Works]

'89 JDM HJ-61 with 12HT and 5 speed, 130k miles, mostly stock with suspension mods:

At random intervals, sometimes at idle, sometimes on the highway, the engine starts running rough, loses power and usually dies. Sometimes it can be coaxed to shudder, huck and buck for 2-3 minutes then it will come back and run fine for days. If it dies, sometimes it will restart if cranked multiple times. It has always restarted after sitting for a couple of days.

My guess is air in the fuel line. But how? There are no fuel leaks. It has been getting more frequent and severe over time.

Are the 12HT fuel pumps fussy? Where could I send it to get it overhauled in the US?

Thank you in advance for any help steering me in the right diagnostic direction.

 

[duncanrm]

Its very unlikely to be the injection pump.. and almost certainly a fuel flow restriction or air getting into the lines. Its a process of elimination.

• Fuel pickup in tank (unfortunately requires the tank to be dropped)
• Sedimenter blocked (attached to inside chassis rail on right hand side, I would start here!) - remove, disassemble, clean - or bypass temporarily
• Gauze screen fitted on the inlet the to Lift Pump (remove, clean, replace with new crush washers)
• Crush washers on the 5 banjo fittings (one per side)
• Primer pump (screwed into the top of the lift pump)
• Injector return line (soft line)

Its remotely possible that the Lift Pump (not the Injection Pump) is problematic - these are easily replaced.

Next time it happens it would be worth cracking the bleeder on the fuel filter housing and confirming that repeated pumps on the Primer Pump dont result in immediate good spurts of diesel out of the bleeder - this would indicate air (noting that fuel restriction will also find a source of air more readily - nature abhors a vacuum and a fuel restriction will aggressively seek out a source of air - often a source that wouldnt normally cause issues).

 

[Paco]

My 12HT had an aftermarket fuel filter that I did not know about until a similar problem surfaced. Running fine and would lose power and then sometime die or just run real rough. Found the filter and it was clogged. Replaced it and problem was fixed.

 

[mudgudgeon]

My guess is air in the fuel line. But how? There are no fuel leaks. It has been getting more frequent and severe over time.

The fuel pump pulls fuel from the tank under vacuum.
You can have enough of a leak that air gets sucked into the fuel system with no visible fuel leak.

I'm not super familiar with the 12ht, but you should be able to temporarily replace the last rubber hose before the injection pump with a piece of clear PVC hose.
Watch for air bubbles with the truck running.
Air bubbles mean you have air being sucked in through a split or loose hose, a bad filter o-ring, or primer pump diaphragm.

 

[60Works]

Its very unlikely to be the injection pump.. and almost certainly a fuel flow restriction or air getting into the lines. Its a process of elimination.

• Fuel pickup in tank (unfortunately requires the tank to be dropped)
• Sedimenter blocked (attached to inside chassis rail on right hand side, I would start here!) - remove, disassemble, clean - or bypass temporarily
• Gauze screen fitted on the inlet the to Lift Pump (remove, clean, replace with new crush washers)
• Crush washers on the 5 banjo fittings (one per side)
• Primer pump (screwed into the top of the lift pump)
• Injector return line (soft line)

Its remotely possible that the Lift Pump (not the Injection Pump) is problematic - these are easily replaced.

Next time it happens it would be worth cracking the bleeder on the fuel filter housing and confirming that repeated pumps on the Primer Pump dont result in immediate good spurts of diesel out of the bleeder - this would indicate air (noting that fuel restriction will also find a source of air more readily - nature abhors a vacuum and a fuel restriction will aggressively seek out a source of air - often a source that wouldnt normally cause issues).

Looks like you made a revision just for me, Duncan. Thank you! Wealth of information on your page. Really appreciate you aggregating so much good data.

2H - Sucking Air/Fuel Restriction

2H - Sucking Air Version: 1.0 Author: Duncan Margetts Source URL: https://docs.google.com/document/d/1qXKR1Vwj5sX6R6tVLRebIYl4J-5uoU4Xd6II2Rfu_9Y/edit?usp=sharing Symptoms of a 2H sucking air into its fuel system: Engine completely lacks it’s normal power Engine intermittently chugs to a s...

docs.google.com

This 61 doesn't have a sedimenter. The fuel lines run directly to the lift pumpt. I don't think the engine/fuel system has been modified in any way. Truck was all OEM with just the typical Japanese lighting mods, aluminum wheels, and a modest rear shackle lift.. Very much an on road truck. The PTO winch cable has never been unwound.

I'll check the gauze screen and probably change the fuel filter. Dealer said it was changed and there's only 2k miles on it but I didn't change it.

Next will be changing the priming pump. When I unscrewed the cap and cycled it, fuel immediately came out. Figured that was normal.

A couple of years ago I added a long range fuel tank. The pick up looked clear. The problem occured prior and was unchanged with the new tank and fuel cap.

This will be a slow thread. It may take weeks or even months between steps as this vehicle is a hobby project and I don't get much time to play.

Thank you to everyone who has replied. All very sound advice.

 

[duncanrm]

Looks like you made a revision just for me, Duncan. Thank you! Wealth of information on your page. Really appreciate you aggregating so much good data.

Well spotted.. I had been meaning to write that for a while, was going to link it here later but you beat me to it.

Good luck with the diagnosis, I'm very keen to hear back!

 

[hdj8081]

Similar happening to me on hdj81.