Have done a resto on 1989 f150 4x4 dual tanks with 4.9 6cyl. original owner had removed and bypassed the factory fuel switching valve on the frame rail and had disconnected the rear tank completely. We installed all new parts back to OEM with the exception I used Spectrum from Orielly's in-tank fuel pumps / sending units. Both units went out in within 2 months, I contributed this to driving around the shop / property with very little fuel in the tanks, less than 1/4. they were both warranty replaced. now 2 months later both appear to out again. We have kept 1/2 tank of fuel in them will moving around. Now I am thinking I have missed something somewhere. truck will run for 5-7 minutes on either tank then you can hear the external pump change tones and start running dry. few seconds later dies. wait 15 minutes it will do the same again.
any ideas?????
here is the link to my first thread when we originally replaced pumps
https://www.ford-trucks.com/forums/1...ery-issue.html

 

update: i found some badly repaired hot wires on the starter solenoid from PO. fixed all that. Also PO had cut off the engine block to frame rail chassis ground. Fixed that. Pulled codes, found PIP sensor failure code. fixed that. Still same issue, starts runs 5-7 minutes dies. Pretty sure in tank fuel pumps are bad, I just need to figure out why now? this is the second time both pumps have gone out?? Anyone have any thoughts??

 

Have you verified the high pressure fuel pump is good?
When I bought my 87 I had a similar issue with the truck running intermittently. I could drive it down the road and back, but it would die when I got home or out on the road. I could hear the inline pump running. Tested pressure at the rail and it was low. Put in a new inline pump and problem was solved.

 

thanks for the reply. Yes today I verified the external at 40psi holds good at idle. and after engine is off. When truck dies it does drop pressure down to 20psi , but the external pump has a notable tone change as if it no longer pumping fuel.

 

40psi is too low for your engine.

 

Could it be a mostly plugged fuel filter?

 

Is it supposed to be 60psi for a 300?

 

If you ground pin #6 of the self-test connector it would be about 60psi with the key on and the engine not running but at idle 45-50psi.
Also if at idle with the vacuum line to the Fuel Pressure Regulator (FPR) off it would be about 60psi.

 

Thanks guys for the response.

88n94: Fuel filter was changed when replaced both fuel pumps two months ago. I have another on the shelf. I will change tonight and recheck fuel pressure.

Subford: Will try this tonight

If you ground pin #6 of the self-test connector it would be about 60psi with the key on and the engine not running but at idle 45-50psi.
Also if at idle with the vacuum line to the Fuel Pressure Regulator (FPR) off it would be about 60psi.

New question: If the key was left in run position, for an extended period of time will the , for example listening to the radio while working on the truck. Would this cause the in tank fuel pump to run the whole time? or is there a switch somewhere that kills power to them after a time period or w/o oil pressure? I am looking for a reason the in tank pumps have failed twice.

 

On my 87 w/dual tanks - When I turn key to run the inline fuel pump and whichever intank fuel pump is selected will hum for about 3 secs then turn off.
If someone has changed wiring to the fuel pumps in your truck maybe they are continuously running. If they are running you should be able to hear them.

 

With the key in the run position and the engine not running or cranking the pumps will run for one second and quit and not run again until the engine cranks or starts.

 

Grounded pin # 6 on test plug, key on engine off. 50psi is all it puts out, then drops quickly after key is turned off. 3 times same outcome. Do I need to replace external pump?

 

I would take a look at the Fuel Pressure Regulator (FPR) first.
True the HP pump has a check valve to hold the pressure but would not think the pump and the check valve inside of it would both go bad.
There are three items that keep the pressure up after the pump stops running. They are the check valve in the pump, fuel injectors and the FPR.

 

Subford, thanks for the continued input. I will replace the FPR tonight. and let you know the findings. Would any of these issues have caused the low pressure / in tank pumps to fail?

 

The in tank pump is just a lift or boost pump and it is the same pump that was used on the Carburetor engines that had electric fuel pumps in the tanks.
Maybe if the head for the in-tank pump was blocked it might go bad. All the boost pump does is select its self to let its fuel go to the high pressure pump and fill the reservoir for the high pressure pump on the frame. It puts out between 7-12psi fuel pressure