Hello all,

Truck:
1985 F250 manual transmission 7.5L v8 4wd

Problem:
Truck won't stay running over ~30mph (Mid 4th gear), sometimes 3rd.
It "bogs down" and acts like it's not getting fuel. You pump the gas and you get a low "whurring" sound. Then it dies. The truck will usually start right back up after letting it sit for a 5 sec count.
The truck needs to warm up to stay running at all, but doesn't like to get hot.
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More Triage:
When the engine is cool/warm the accelerator pump squirts fluid correctly.
When the engine is hot and I hold the throttle about an inch off idle it starts to "bog" and I hear a a very obvious "sucking" and I don't see very much fluid coming out of the accelerator pump vents.
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What I have done:
Rebuilt carb
Resurfaced Intake Manifold (and gasket)
Replaced EGR
Replaced Distributor cap
Replaced Ignition Module
Verified Fuel Pump

Where I'm stuck:
I cannot find the Self Test In (STI) plug, so that I can pull the codes.

What I am looking for:
Either another part to check and verify, or else where the self test plugs are.

Thanks guys,
Lodlaiden

 

You don't have any self test plugs, because your truck does not have a computer.

A guy I work with has this same truck, and had the same problem. Your truck probably has the complicated fuel system the 460 trucks had with electric fuel pumps in the tanks. These fuel pumps are fed by a fuel pump relay and the relay is powered from a oil pressure switch on the engine.

But his problem turned out to be simpler. This system has a large can looking thing where the fuel enters the carb. Inside this can it has a orifice where part of the fuel goes to the carb, but the rest goes back to the tank via a return line. He found this can thing to be stopped up with little plastic particles. Once he cleaned it out, his truck ran fine. He figures the plastic particles came from the pickup in the tank that had disintegrated.

 

Quote:

Originally Posted by Franklin2

He figures the plastic particles came from the pickup in the tank that had disintegrated.

Ah, yes, I've seen where those things disintegrate, the filter on the end of the
pickup tube in the tank:

 

The only little can that I could find was the in-line fuel filter, which is $60 at the Ford Dealership and not available elsewhere.

I replaced the fuel filter a couple months ago when this started, as that was what I figured first off. It's the only filter between the gas tank and the carb.


Any other suggestions?

 

Do you have a mechanical, or an electric fuel pump?

 

It's electric, I pulled it, applied some juice to it, and it threw fluid like a fountain.

Is there any way that something is getting confused and not sending the electrical signal once the truck has warmed up? Shutting the truck off seems to "reset" it and I can get going again.

Thanks for the help,
Lodlaiden

 

Yes.
The inertia switch and oil pressure safety switches are bypassed while cranking.
So it will fill the carb while you're trying to start it. Once you let off the key the pump will stop.
I'm on my phone now so I don't want to find old threads but you can use the advanced search function to find threads that 'subford' or I posted to for a schematic.

 

I read through a couple of your other fuel issues, and stumbled across one that had my same exact problem, almost word for word.

I checked the inertia switch works by disconnecting it, starting it and running it out of gas, then trying to start it again. Reconnecting the inertia switch worked.

I had a little bit of black flaking in the fuel filter, but not enough to warrant concern.

The Oil Pressure Switch was not connected all the way. I pushed it down and it seemed to run better, but I didn't have very far to go.

I think I may stab my carb mechanic...

I'll let you guys know on both accounts.

Thanks,
Lod

 

If your fuel system is all original, follow the fuel line up to the carb. Before it gets to the carb there should be a thing in the line with a return line coming off of it. Your system returned fuel to the tank all the time. This is the thing that gets clogged inside.

 

Are you calling this a filter?




I tend to agree with Dave Franklin.
Any debris in this vapor separator warrants replacement because the screen inside gets plugged.
They are available in three orifice sizes; .040" (E3TZ 9N176B, blue dot), .060" (E3TZ 9N176A, red dot), & .090" (E0TZ9N176-B, the earliest part that sometimes returned too much fuel to the tank, but solves vapor lock )

Put a cheap clear inline filter ahead of it to keep the crap out.
The carburetor itself should have a filter at the inlet (Wix 33098)


Hope this helps.

Edit: BTW, I've never had a fuel issue with my mechanical pump. Sometimes simpler is the "Better Idea"

 

The vapor seperator/orafice valve is not the fuel filter. The fuel filter is actually a little brass piece in the carb. If you look at where the fuel line screws into the carb, you'll see the piece it screws into is screwed into the carb. The filter is hiding in there.

 

I have an electric fuel pump. There was some black debris in the filter/vapor screen (Ford Dealership sold it to me as a fuel filter) It has a black mesh screen in it.

I will check the inlet on the carb, but I think I may be good after I throw some fresh gas in it.

Thanks again for the help, will let you know after I driver around for a bit today.

Lod

 

So, jiggling the Oil Pressure Switch didn't fix anything.

New Questions:
1) What is the other "switch"/guage/sensor directly beneath the Oil Pressure switch that also has a single red wire coming out of it, and is in the same wire run that goes into the dash?

2)Does anyone know the location of a "fuel pump relay", which may/may not feed power to the fuel pump? (Suggestion from neighbor). My truck does not have one where his was on his '86 460

3) Unrelated: Where on the manual 4x4 tranny do the reverse lights plug in? I can't find a plug and tried searching for a pic.


Thanks guys,
Lod

 

Are you jiggling the wire going to the oil pressure SENDER that controls the gauge on the dash? (can at the back of the intake manifold)
The oil pressure cutout switch is separate from this.

Here is the wiring diagram form subford:



Follow the yellow wire from your inertia switch or the pink/black wire back from the tank selector switch.

Should I assume you have the same T-19 4 speed Borg Warner gearbox as me?
Does it have a PTO "window" on both sides?



If so, you can see it high on the drivers side in the above photo.

Hope this helps.

 

The oil pressure sender & water temperature sender (both send to the gauges/idiot lights on
the cluster) have similar-looking wires... one is red/white stripe and the other is white/red
stripe (I forget which is which).