BJ40 - air in the dieselsystem (1 Viewer)

[Erwin]

I have this 1976 BJ40 that have a problem with air in the diesel system. While I was restoring it it was fine, but on first trip after putting the plates on it started to run uneven and then stopped. If I prime it, it runs for 5 minutes, then stop again. I thought I had found the problem in the return hose which I had overseen while restoring it. It was old and stiff, and I changed it. After that it was OK on one trip, then the problem started again. Now all hoses are new, all clamps have been checked, but still same thing. No trace of leakage.
I have read that somebody apply a pressure to the tank to provoke a leakage. I am afraid to do that. It is not much of a pressure that is needed to blow up the tank. So I plan to apply the pressure to the outlet pipe from the tank. Either with air, or just a hose to the ceiling filled with diesel. But how much pressure? I have been thinking 2 bar/ 30 PSI.
Any thoughts , anybody?
Other ideas?

 

[MyCruiserisaHogBeast]

I just went through this on my BJ74, ended up being a simple solution but went through the entire fuel system to find it.

I originally had a bad primer pump. I would just replace it on yours, it's a 5 minute, $30 fix. They are a common failure. On mine I replaced it and still had leakage in the system. Changed all the hoses, isolated everything one at a time only to find out that the original primer pump had a bad seat and some sort of defect, the leak was TINY and really difficult to find. It was hugely frustrating but I learned a lot. Not saying yours is a bad seat, but just an example of how small the leak can be. Mine would run for a couple of miles then start to stumble, eventually stalling out and requiring a prime again. It sounds like yours is similar, whatever leak you have is tiny, eventually accumulating enough air at the highest point in the system (probably the fuel filter) to disrupt fuel flow.

Here's what I would do if I were you:

1. If your primer seems good, prime the system as much as you can and search for leaks after the lift pump, check every fitting for wet fuel or bubbles at your hose connections, swages and hard fittings. If you notice anything tighten or replace and try again. This is low hanging fruit. You can also crack each injector line while idling just to make sure it's bled.

2. Check/empty sediment trap. Check the connections. I guess they will also rust through. Try again.

3. To eliminate leaks under the hood, I would run a fresh fuel hose from the lift pump to a gas can with a couple of gallons strapped to your front bumper and go for a drive. Yeah, it's a cluster **** but it'll answer a lot of questions. If it eliminates the problem then you know the problem is under the hood and it's time to take a closer look at your lift pump, fuel filter and injection pump. I replaced all of the plastic compressor washers between all of the soft line connections between the lift pump and injection pump. There is also a small filter on the banjo fitting going into the lift pump, I'd check that and make sure it's clear. *worth nothing that after I went through the whole process and got tons of false positives, the only way I caught my leak was by drying the whole system out and priming with a dry fuel system all the way from the tank. After priming for a long time I got tired and heard a tiny popping sound and thought it was weird, so I stopped and looked for it. Turns out it was bubbled foaming out from around the base of the primer like soap from diesel I had spashed on it while ripping the fuel system apart to start the whole isolation process over again. So what I would do is dry the system out and running blowing air, maybe like 10 lbs or something, into the fuel system from the lift pump and look for bubbles, wetness, ect and go from there. There are so many little things it just become process of elimination. It could also be a bad lift pump, with the lift pump not pulling enough, and the injection pump demanding more than it can provide. This would create a vacuum between the two, and suck air in from any less than rock solid connection.

4. If everything runs right after your last test, I would bypass the sediment trap. If the results are all the same, it's down to the fuel lines or the fuel tank.

5. I would then run the gas can test again, the first start by running a line from just after the sediment trap to eliminate from use after the sediment trap. This eliminates leaks in the hard lines on the firewall between the trap and the lift pump. If that is the same, move to pulling fuel from the sediment trap, eliminating the sediment trap and connections in between.

6. Repeat the gas can test, this time I would pull fuel from just after the fuel tank at the back of the truck at the first hard line connection. If the results are the same chances are good it's your fuel lines and you should just replace them. If it runs well, its either the soft line between the fuel tank and the hard fuel line or its the fuel pick up.

It's all process of elimination. I wrote this out for you so it's all in one place, hope it helps. Good luck!

 

[Erwin]

@MyCruiserisaHogBeast
Thanks a lot for writing this up. I will certainly try this out. I have changed all gaskets on pump already.

 

[MyCruiserisaHogBeast]

@MyCruiserisaHogBeast
Thanks a lot for writing this up. I will certainly try this out. I have changed all gaskets on pump already.

You're welcome. Everyone got super crusty in the 70 section and was super unhelpful saying the information was out there, but I couldn't find it all in the same place. I ended up reaching out to a friend who helped me learn the system. I hoped writing this out helped you, it would have helped me.

 

[fjc-man]

If the early Toyota diesels are anything like 70s and 80s Benz diesels I used to work on there is a possibility your fuel tank vent could be clogged. They didn’t have a closed evaporative system back then. So the point I’m trying to make is your prime pump could be starving itself because of a self created vacuum in the tank.

 

[Erwin]

If the early Toyota diesels are anything like 70s and 80s Benz diesels I used to work on there is a possibility your fuel tank vent could be clogged. They didn’t have a closed evaporative system back then. So the point I’m trying to make is your prime pump could be starving itself because of a self created vacuum in the tank.

Thanks, I will check it out.

 

[roscoFJ73]

I have been thinking 2 bar/ 30 PSI.

I would not do that, you will probably introduce new leaks and end up more confused. Its most likely your primer pump.

 

[Scubadoc]

found this on EBay... wrong number on the description, but correct on the actual device. I have not installed yet, but this is the only genuine Bosch I could find on eBay. Sold by B2B automotive. Pretty fast shipping.

Good luck!

 

[Erwin]

@Scubadoc
Thanks for the tip. Easy to find here in Europe. Ordered now.

 

[roscoFJ73]

There is 76 listed on ebay if you search under mercedes.. There are some quality german brands for $14 USD