Did this as you described and it is correct on my truck, so now how do I test the tank selector and wiring to the tanks as well as the sending units. I'm trying not to remove the tanks because they are full of fuel.

 

???? You just tested the wiring per the previous post and said it was good?

 

I'm kinda confused. Post #11 describes in detail how to test the wiring, both from the dash to valve, and valve to tanks including the sending units. By "tank selector" are you talking about the valve or the dash switch? Post #11 has already tested the dash switch, and you said all those steps were good. Not trying to bust your behind, but in post #10 you also said the valve was good. Hence my confusion...

If you want to test the valve itself again: Ignition on, but engine off for quiet. Crawl underneath and listen while a helper operates the switch. This is a motor-driven valve, so you should hear a faint whir when in transit. If you can't hear anything, put your hand on it to feel for vibration.

If the valve responds electrically, the next step is to determine if the plumbing portion is responding correctly. It's possible the shaft is sheared and the plumbing is not switching over, even though the motor is running. Probably the easiest method to test is to remove the valve for better access and blow compressed air through it. Reconnect the plug to switch over the valve and then test again with compressed air.

 

Ok, right, I got it. Tanks are full so I will continue that part next. It runs great on the front tank, just need to figure out why it won't switch and for sure how much because the gage will wander from full to 1/4 and in between while driving.

 

@rich1785

Bumpity bump..Was this ever resolved? We've got another forum member with a similar issue. An update might be a big help.

 

No resolution yet.
I will have to drop the tanks.
the more I drive it the stranger it gets. the tanks will switch fine I believe. I can run on the front tank just fine or the rear as well, drive all I want and it is just fine starting if I go somewhere and it does not strand me. If it sits for a few days it may or may not start, it will fire right up then 3 seconds later die. If I don't flip the switch to the other tank I will run the batteries down. If it starts, dies and I flip the switch it will fire up again.
I am going to order two sending units, drop the tanks and investigate the pick up tubes.

 

Hey, you finally described the problem you are having. It's a very common problem with the diesels. Starting right up and then stalling after it sits means you have a air leak in the fuel system on top of the engine.

Inspect all the fuel lines from the fuel filter all on top of the engine, all around the fuel injectors, around the injection pump, and down the return line going down the top rear of the engine. Check for dampness were you might have dirt sticking.

Common places for leaks are on top of the fuel filter around the fuel heater. The rubber line that leaves the filter and goes down to the injectors. Very common places for a air leak are the o-rings around the injectors. There is also a rubber return line on front of the injection pump. Any dampness means a tiny bit of fuel is getting out, but a lot of air can get in.

Have you ever taken a straw at a restaurant when it's in your drink, put your thumb over the top and pull the straw out of the drink and watch how the drink stays in the straw as long as you hold your thumb tight over the top? That is how the fuel system works on these old diesels. When you turn the truck off, the fuel system is totally sealed with a return line running back to the tank. As long as there are no air leaks, the lines stay full on top of the engine. If there is any tiny air leak, air will come into the system ,and that will let the fuel run back to the fuel tank. Most of the time the air leaks are very small, and it will take a day or two for it to empty the fuel system enough to cause a problem. It will run right away because there is still fuel in the steel lines to the injectors. But once the injection pump sucks in the air from the rest of the system, the engine quits. Like you said, takes about 3 seconds for the air to hit.

If you want to experiment, sometimes if you park the truck with the nose down hill, the fuel tanks up hill, it will take longer for the fuel to run out of the system if it has to run up hill to the tanks.

 

I know this is is a common problem. I had to replace the injector o-rings 2 times and finally bought a better system to stop the problem of injectors leaking all over. Usually when a diesel has that problem it is not intermittent. This truck can sit a week or more and not have the problem, then it will after sitting a day but switching the tank starts it up. The fuel gage has been checked and is fine but it dances all over while I'm driving. shows full when the tank is full then drops down after a few minutes. I can do a pressure check and test it, but I still think the sending units or pick up tubes are troublesome.

 

Another test some people do, they temporarily swap some of the fuel lines on top of the engine with the clear type of hose. So they can then see the lines and little bubbles forming as air enters and the fuel leaves, when the truck is sitting.

Some people also add a electric fuel pump. It will not make the problem go away, but it reduces the amount of cranking to fill the system back up with fuel.

 

I know on the GM 6.2 the injection pump where the throttle cable hooks to the throttle shaft seals would start to weep and have a long crank time before it would start.
It was enough to let air in to the system so you cranked till full again.
Dose the Ford 6.9 do the same thing?
Dave ----

 

No, I have never heard anyone complain about shaft seals on the international engines.