My 88 f250 with a 460 and c6 will randomly stumble over itself. It usually lasts 5-10 seconds. It sort of acts as if it is running out of fuel but it continues to happen if I switch tanks. No check engine light comes on when it happens. I have to pump the pedal to keep it from stalling. The truck has new plugs, wires, cap, rotor, fuel filter and coil. Timing is dead on 10 degrees. Any idea what may cause this?

EDIT: So after further investigation, I can not pull codes for some reason. I jump the connectors and the CEL does not flash. Secondly I'm starting to think I actually WAS running out of fuel. I was using the rear tank because I thought it to be leaking so I was trying to run it dry. After driving over 100 miles the tank still showed 3/4 of tank which it had been showing for the last 50 miles, yet the front was near empty even though I had not used it. When I bought the truck and drove it home I only filled up the front tank and there was no leaks, a few days later I filled up both tanks and when I parked on a hill there was fuel dumping on the ground, so I assumed it was the rear tank. Now I'm thinking the rear tank is not fueling the vehicle and it is overflowing the front tank, that would be caused by the switching valve/filter assembly thing on the frame correct? When I flip my fuel switch and cycle the key I hear a fuel pump come on in both positions so I believe both pumps are working.

 

fuel pressure regulator, vacuum issues, is the exhaust original?

What about O2 sensors or catalytic converter?

Never had an 88 idk if they have o2 or not

 

I updated the main post with more info. I believe it to not be switching to the rear tank, yet I hear a fuel pump come on in both positions, what would cause this? The switching valve assembly? I also believe the rear tank is overflowing the front tank when they are both full

 

Sounds like you may have multiple issues...

"I have to pump the pedal to keep it from stalling"

I believe you are EFI this year, so pumping the pedal gets you more air, not fuel. Possibility your Air Bypass Valve is acting up.

How is your idle? Notice any particular RPM when this is happening?

 

well the pumping the pedal is just to keep moving since it surges. It's not so much like it keeps it running. I believe it is fuel related. I went and pulled the fuel switch out of the dash and verified it functions correctly with a multimeter. Had someone cycle the key for me and the rear pump doesn't come on in either position, so I'm getting the reading of the rear tank while using the front. The front to was low on fuel so I'm thinking on hills and turns it was starving for fuel. On top of that I think the switching valve is bad causing the rear to flow into the front. I filled up my front tank and I'll see if it happens again tomorrowith. Why would my front pump come on in both positions if the switch is operating correctly? I even tried jumping the terminals on the connector to power just the rear pump and the front stayed on but the rear didn't. Is there something that defaults to the front tank if the rear doesn't come on?

 

"Why would my front pump come on in both positions if the switch is operating correctly?"
I'm guessing the switch or wiring have an issue.

"Is there something that defaults to the front tank if the rear doesn't come on?"
No, what ever pump is running switches the valve, unless you have a Relay Powered Valve. The valve maybe 29 years old and sluggish or hanging up.

Check the wiring to the switch; Is the switch labeled 1-6? (For Non Relay or Motorized Powered Valve)
1. Red
2. Red/White
3. Brown/White
4. Dark Blue/Yellow
5. Yellow/White
6. Yellow/Light Blue

When your switch is in the Front Position, you should have power on the #1 terminal, Red.

When your switch is in the Rear Position, you should have power on the #3 terminal, Brown/White.

 

Have you checked your Fuel Pressure?

Pull the vacuum hose off the Regulator, plug it, and note if there is a change in Idle RPM @ Idle.

Pinch off the return hose, any change in RPM.

Check the hose for a smell of or residue of fuel.

 

You should buy yourself a $25 Code Reader.........

 

He should get a code reader.

I believe he has one O2, Heated, I don't know the location for the 7.5L, but maybe in the back of one of the exhaust manifolds. They were putting them in the passenger side manifold in the earlier EFI.

 

After looking into the diagrams more I'm thinking what is happening is because the valve is not switching to the rear tank, the frame mounted pump is pulling fuel from the front tank even with the pump in tank off. I'm going to unplug the frame mounted pump and test both tank pumps. The switch is working fine, already ohmed it out. Seeing as the switch works and the connector looks fine, there is no way for the front pump to be staying on. I would pull codes but someone at some point must have messed it up because using a paperclip wont flash the CEL. I'm going to buy the adapter for my OBD2 code reader to plug into the connector and see if that works, otherwise I'm going to have to replaced the computer sometime in the future

 

I tried the paperclip and it wouldn't flash out codes so I'm assuming that someone messed up the computer at some point. I'm going to order the adapter for my code reader to connect to obd1 and check

 

So went and tested the fuel pumps using the OBD connector and the frame fuel pump and front pump both work but the rear does not come on, so I was correct on the frame mounted pump pulling fuel from the front tank.

 

So if the rear pump is not coming on, the valve never switches to the rear tank. I have heard that the HP pump will pull fuel from the tanks.

If you are switched to the rear tank, and the HP pump is struggling to get fuel, I can see where the fuel pressure may be low, and cause the stumble.

But if you are switched and running off the front tank, the pressure should be normal and you would not feel a stumble due to fuel pressure, provided the filter is passable and the regulator is not sticking open.

But in the beginning you say the stumble occurs no mater what tank you are switched to. So you may have another issue.

You should check the pressure difference between the two tank selections.

Check the OBD-I, STI connector for 5vdc.

 

So I have an update as of tonight. It did fine with a full tank for a few days until today. I noticed my voltmeter start to dip down below where it normally sits and it had a harder than normal start at the store. I drove home and it did the hesitation again. I parked at home and tried to start it again and found it to be even harder than before. By the time I had access to my voltmeter the truck cooled down and it was charging fine. The voltmeter was showing what it would normally again so I came to the conclusion that when it gets hot, the alternator would stop charging. I decided that it was time to do the 3g alt swap and went to the parts store and found an alt that would work and swapped it out ( which by the way, the write up that everyone points to on here says not to remove the old connectors from the truck which makes no sense considering you only use one wire front the circuit.) Drove the truck and my lights were much brighter, hesitation off the line was gone and it had a lot more power and didn't notice and more jerking hesitation at random times. I'll update again if I have the issue

 

How old is your Battery ?