Hello everyone,

My first post, long time lurker . I apologize if this was covered in another thread.

Anyway, before i describe my issue, a little bit about my my truck:

1990 f250XL. 460, zf-42, 4x4 133"WB 188,000 miles. PO says motor was swapped with 1992 460 with 120,000 miles. Dual tanks, but the midship tank is not installed for whatever reason.

i am slowly fixing things on the truck. one problem i have encountered is hard starting when hot. I have been through the starting circuit so i feel confident that the electrical is all good. it is not bad when it is cold outside or the truck is cold but the other day i was at operating temp and hauling wood and it was 80+degrees. the truck would turn over plenty but not start. one trick i figured out is to quickly turn key and restart after a failed start attempt. it fired right up. it runs good enough, i might expect a little more power but it accelerates and climbs ok.

I have the haynes manual and followed the fuel procedures, starting with KOEO. when i turn the key, the fuel pump primes normally, but no pressure shows. then, when I ground pin#6 on the DLC the fuel pressure climbs slowly and holds at about 30-32 psi. manual says KOEO pressure should be between 35-45 psi. KOER was the same, about 30, which is barely spec according to haynes. (30-45psi). no real change when i pulled the vacuum line from the regulator although haynes says should see an increase of 5 to 10 PSI? throttle up shows a slight decrease in pressure, which i think is to be expected.

after removing the jumper wire the pressure dropped quickly down to about 10 and then more slowly down to zero. I attempted to do some of the tests to isolate the regulator but i never saw any dramatic increase in fuel pressure when i pinched off the return line downstream of the FPR. it was difficult to do with just one person. i never tried the one where you pinch off the send line.

I had a coupon, gift card, etc. to the parts store so changed the FPR for grins, remembering how my fuel filter looked after I bought the truck , but after changing i still had the pressure leakdown issue and so i dont believe that will completely cure the problem. also when vacuum line pulled on new FPR no increase in PSI.

I did not see a pinch in the lines and the fuel filter is new. My guess is that the check valve in the FDM is stuck open and i have read that that was a common problem? (leaky injectors not out of the realm of possibility)

something else that i am dealing with is: 1) there are 2 or three splices in the hot wire feeding the rear tank fuel pump (its a miracle the thing runs much less accelerates) and 2) since the midship tank is missing, the fuel return line is "looped" into the midship send line and then there is an open line which I am assuming is for fuel vapors. (the line that runs along the outside of the passengers side frame rail and has all sorts of bends in it.)

So, I have a few questions:

1) is the pressure leakdown the reason for the hard start?
2) if the leakdown is caused by the FDM check valve (and not leaking injectors) it sounds like the only way to cure that is to replace the entire fuel pump?
3) what should be the proper voltage delivery to the pump? is there a way to check without removing the pump/tank?
4) am i wrong to completely rule out electrical?

I dont really want to drop the rear tank on my own since i am largely working on this alone. so what i thought i would do is hookup the midship tank i got from the junkyard and basically repeat the test with that. re: question #3 i had to solder a connector to my trucks wiring in order to install this midship tank. when i probed the newly installed connector i got about 8-9 volts going to the power wire. i did hook up the tank just long enough to see if the pump motor would come on and it does. the fuel pump relay shows battery voltage (12.6) so, should the voltage at the fuel pump connector also be 12.6v?

Also, my truck has (had) an aftermarket high pressure fuel pump on the rail but has since been disconnected. at some point the midship tank hot wire was used to power this pump. i havent tried to see if it works. basically i would like to restore the truck to the stock fuel system so i can use both tanks (i need them both with the 460). also, I do not see any sort of reservoir on the frame rail anything. so i just want to confirm that with my current setup of a 1990 with a motor swap to 1992 that i do not need a reservoir or rail mounted high pressure fuel pump as those items were not used after 1989?

thank you all for providing such a great support network for the DIY FTE

-Thomas

 

Your main problem is the loop of the fuel lines that go to the missing tank.
This will not work at all.
You need to either replace the missing tank and pump or undo the loop and plug each fuel line. Then most of your problem will be solved.

Second get ride of the haynes manual.

The idle fuel pressure should be 30-32psi with normal vacuum. Higher the vacuum the lower the fuel pressure will be.
When you punch the throttle the fuel pressure should go up not down.
With pin #6 grounded and the key on you should have about 40psi fuel pressure and not 45psi. The spec is 39.13psi fuel pressure across the fuel injectors at all times under all conditions.
The only way you would get 45psi is if you have a blower on the engine and you had positive pressure in the intake system and not a vacuum.

With only one pump module (FPM) a bad check valve may cause long cranking time but it will not cause any pressure problems while the engine is running.

 

The 8-9 volts you read was the either the fuel gauge wire or the the fuel pump relay was open at the time of the test.
You may have a little voltage drop at the tank pump with the fuel pump running but no drop of the voltage if there is no load on the circuit with just the meter.

You do not need a reservoir or rail mounted high pressure fuel pump as those items were not used after 1989 in the F-series trucks. Only the E-series used them up to 1992.



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Thanks for the excellent information and attention to detail. Once I have a chance to work on this I will post my results.

 

Sorry it's been a while since i posted. but I installed the front (2nd) tank and it worked except for the shuttle valve which was sending fuel to the back tank. The back tank at that time had a functioning shuttle valve and fuel would return back to the back tank.

on the day that I installed the tank I mistakenly ran the vehicle until the rear tank overflowed. i stopped using the front tank for a while. A few weeks while later I was working on the clutch and i noticed that one of the lines had come loose on the front tank. We were able to resecure the line and ever since then the shuttle valve works great on the front tank but now the back tank is filling up the front tank this time around. Also the front tank pump sounds good and strong while the back tank pump sounds weak when it primes the system for startup. it makes a lot more noise. IT will still run the truck. Any ideas? should i probe the wiring for the rear tank and check voltage? Thanks to all of you for your help.