I have a fuel issue with my 89 f-250 I6,5 spd 4x4, and am stumped.


so far This is what I've done:

Check codes, no codes
new lift fuel pump in both tanks
new high pressure pump in the rail
all lines are good, no holes, or worn spots
new fuel pump relay
new ecm relay
new fuel pump relay wiring harness kit
new ignition module on the distributor
new fuel filter
fuel return transfer can replaced several years ago

I also have been from front to back in the harness, and all looks good.
All grounds are cleaned and good

what it does:
fuel pressure gauge is hooked up
cold start, 50 lbs, all is good.
removing vacuum source from the pressure regulator raises the pressure to 65 lbs, back to 45-50when vacuum is re-applied
as soon as the thermostat opens, the pressure starts to drop
the pressure continues to drop until the truck quits.
I can use a test light to energize the fuel pumps, but they don't build pressure


once the truck cools back down, it starts real hard, but it does start, then repeats the cycle.
any suggestions you can come up with would be greatly appreciated, I am pulling my hair out here.

 

With the cold engine.
Ground the fuel pump relay wire at the self-test plug.
Hook up the fuel gauge.
You may need a battery charger hooked to the battery.
Turn on the key.
See if the fuel gauge will hold the 65 lbs (seems a little high, should be about 60 lbs) for as long as the engine takes to warm up without starting the engine. May take 15-30 minutes.
If the fuel pressure starts to drop you have a bad pump either in the selected tank or on the frame.
Select the other tank and if the fuel pressure does not come back up the frame pump is bad. Take it back for a free replacement. If the pressure did come back up then the in tank pump is bad.

Let us know what you find.


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Thanks for the reply

Went out this morning, and did the test, first on the front tank for 15 minutes, it held 60 lbs.Second on the rear tank, for 15 minutes, it held 60 lbs.


I then started the truck, the gauge went to 50 lbs, and stayed there, until the motor came up to temperature, then the fuel pressure dropped, and the truck quit. It would not re-start until it cooled off. I ran the fuel pumps as in the prior test, but the pressure would not go above 10 pounds, until the truck cooled back off. It is not overheating, and the injector fan is not coming on. I jumped the injector fan through the test port, and it came on, so I believe it to be in working order.

 

Sounds like the pumps are working OK but the regulated head is going away when the engine warms up.

The pumps will pump about 100 lbs into a closed head and the fuel pressure regulator opens the head to hold the pressure at 60 lbs with no vacuum.

With 10 pounds you do not have any head left so we have one of two things that can cause this. The Fuel Pressure Regulator (FPR) spring losing all tension or the fuel injectors are all open. I am not sure if all injectors being all open would drop the fuel that much or any at all below 45-50 lbs. Does it smoke when you start it after it cools back down like the engine was flooded?

I do not like telling someone to do this but I do not know what else to tell you to do. Get a pair of locking fuel line squeeze pliers and when you only get 10 lbs squeeze the return line to the DFR at the FPR and see if the pressure goes back up. If the pressure goes back up you have a bad fuel pressure regulator.

 

There is no smoke at all out the tail pipe.
It is definitely starving for fuel, you can hear the cylinders start to drop off, until it is only running on one, or two.

It's a 6 cylinder, so the fuel line at the regulator is metal, I have a new regulator I bought, but didn't want to take the plenum off. Maybe I can sneak it in there. I will let you know how that comes out.

Is it possible for the fuel return transfer can to block off all the lines, and overheat the high pressure pump to the point of the fuel is vapor locking? My reason for that question, when the pressure dropped, and the pump was still running, I touched it with my hand to verify it was indeed running, and it was very hot. not burning hot, but hot enough to feel abnormal. The pump I replaced two days ago was doing the same thing.

 

The pump uses the fuel going through it for cooling.
There should always be fuel going through the pump and returning to the tank through the FPR and DFR. Sounds like there is no fuel going through the pump when this happens. Make sure the selected boost pump is still running and pumping when this happens also.
If fuel can not get back to the tank the fuel pressure would go to about 95 to 100 pounds.
I am not sure I understand what you asked but I hope this was the answer you were looking for.
I dough the FPR being changed will make any difference but who knows.



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By your diagram,I see your point about the fpr, it is not in a place to effect fuel from the engine side of the pump. I wonder how the fuel selector valve works, if it would be possible for something to fail inside, and cut off pressure from the lift pumps, and leave the high pressure pump dry? You can put your ear to the fill, and hear the rear tank pump running, and the front tank pump is so noisy, you can hear it, and the high pressure pump standing beside the truck, even when the pressure is steady at 10 lbs.

Just to make sure the switch in the dash works to switch tanks, I borrowed one from another truck I own, and the result is still the same, the truck runs out of gas, as soon as it gets to operating temperature.

 

Did you not say you changed the fuel selector valve (DFR)?

 

Yes I did, but it's been a while, and the truck sat for two years while I took a long vacation.

 

If I were to guess I would say your in tank boost pumps are bad.
When the problem happens un-hook the pressure line from the selected tank and point it in a bucket with pin #6 shorted to ground and turn on the key. See if you get a good flow out of the tank. It should have 7-12 pounds pressure on the tank pressure line.
If it does and you do not have the same coming out of the DFR then the DFR is bad.

 

Ok, Thanks. I will try that tomorrow as soon as I get home from work. Can I just unhook the lines from the back of the DFR for this test?

 

Yes, just pry out the hair pin clip and pull the hose off.
Then just push the hose back on and put the clip back in.

 

Tested the transfer can in between rain drops today. Both in tank pumps are good, so I spent the $240 and bought a new one. The old one weighed a whole lot more than the new one, so I took it apart. Here's what I found:







I'm guessing the sediment would get into the reed valves, and not allow the fuel to go in, or out.


My thanks go out to you for helping me figure out this problem, and get me back on the road!

 

Glad you got it going.