Hey guys. I have a '91 F-250 with a 7.5 and E4OD.
My problem is that my truck will run like a top for about 15-20 minutes and then act like it runs out of fuel. I have a buddy that says it sounds like a vapor lock problem, but the guy at the garage says a 91 won't do that. He told me I needed a new fuel tank and pump, then said, "Sorry, I can't help you." I seem to have plenty of pressure at the rail while running either pump.

I bought a new tank and pump to replace the rear tank. When I pulled the old one, I found a hose melted through. The guys at the auto parts store are calling this an evap hose. I bought the hose and am going to fix this as well.

I haven't found a diagram as to where this hose actually goes. It appears to run forward to the front tank but I haven't pulled the front tank to see what happens after that lol. I think the part on top of the tank is a pressure sensor or vapor pressure sensor of some sort?

My question is twofold. Could this hose/vacuum line explain vapor lock type symptoms? If this and the new pump and tank assembly don't fix my problem, what might be my next step?

Thanks in advance for any input.

 

Vapor lock isn't possible with the fuel system in your truck, the pump pushes a large volume of fuel to the engine constantly, the regulator maintains 32-40psi across the injectors at all times, and everything that isn't used by the engine goes back to the tank.
The vapor system collects fuel vapors from the tanks and passes it through a charcoal canister in the engine bay before it is metered into the engine and burned via the canister purge valve.

When your truck stalls will it start right back up or does it have to sit and "cool off" for 15-30 minutes before it will fire? If it has to cool off you could have a bad TFI(ignition) module.. these have a known heat failure problem, but it could have also been a weak fuel pump. If it starts right up again your problem could be with the ignition switch or wiring in the fuel pump circuit, I have had the wires corrode off the fuel pump relay and cause an intermittent stall like that.

 

What Conanski said, but a little more detail on why your truck can't vapor lock. Vapor lock is fuel vaporizing. When this happens the fuel system, which is designed to handle a liquid, can't handle it correctly, so the engine doesn't get enough fuel to run.

Boiling is (rapid) vaporization, so understanding what makes something boil helps you understand what makes fuel vaporize. High temperature and low pressure will both cause a liquid to boil (think of how water boils at a lower temperature when at a higher altitude = lower pressure).

Carbureted engines with mechanical fuel pumps gave the perfect scenario for this. The fuel pump sucked fuel from the tank, so the fuel was at a lower pressure before it got to the pump. If the fuel line got hot enough (and the mechanical fuel pump was usually pretty close to the exhaust manifold), it could vaporize the fuel (being at high altitude would make it even worse).

Even in a carbureted engine, vapor lock could be eliminated by putting an electric fuel pump near the tank and pushing fuel at ~5 psi pressure past the hot spots. Of course insulating the fuel from the heat also could help.

But with fuel injection you are pushing fuel at a much higher pressure, so it would have to get REALLY hot to vaporize. So vapor lock doesn't happen with fuel injected engines.

 

Vapor lock is rare in EFI , but still possible at higher altitudes or if you're not running fuel with ethanol........do you live in the mountains??

 

One other possibility is that the fuel pump is dying. For some reason, the main fuel pump on many Ford trucks from 7th through 9th generations is on the left frame rail near the exhaust manifold (on V8 trucks) and it doesn't have very good heat shielding. Not uncommon for the pump to get cooked after many years, especially in hot climates. My former 86 did this and it was emphatically the fuel pump. You can diagnose this by checking fuel rail pressure when the truck stalls and dies.

 

Ford stopped using the frame mounted high pressure fuel pump/in tank low pressure pump setup in 1989 on the F series trucks. The OP has a high pressure pump in the tank (s).

 

The fuel pressure across the fuel injector is at 39-40psi depending on the fuel pressure regulator under all driving conditions rather at sea level or at high altitudes. This pressure across the fuel injector stays the same at WOT or at idle.

Your fuel pressure in the fuel line to the injectors will change from 30-32psi at idle and to around 40psi at WOT when you check it at the test point on the fuel rail with a gauge. The changes in pressure at the test point is to maintain the 39-40psi a all times across the fuel injector under all driving conditions rather at sea level or at high altitudes.

Vapor locking was a problem on the fuel injected 4.9L engine so Ford added a fan to cool the fuel rail and also upped the gauge fuel pressure to around 60psi at WOT and 45psi at idle to stop the vapor locking. They later dropped the fan after a year or so model year.

The 460 in 1991 has the TFI (ICM) mounted remotely so it will not get as hot as when they are mounted on the distributor on other applications. So the TFI is not as big a problem with the 460 in 1991 as before.

My guess is you have a bad PIP sensor mounted on the stator inside the distributor but I have seen this also caused my the PCM Computer, TFI or the Ignition coil.

You have two fuel tanks and it sounds like it does this on both so I would not think it is a fuel issue. But to rule out the fuel as the cause you need to put a fuel pressure gauge on the fuel rail and located someplace that you can see it as you drive it. If you loose fuel pressure at anytime below 29psi then is the fuel. You should always get around 40psi at WOT as it dies, then you will know it is the ignition system.

The vapor lines coming off the roll-over valves would not cause this problem.

 

I have had the same symptoms with my 92 Bronco [vapor lock] type symptoms checked pressure at the rail was ok when checked. On hot days
the truck would act like I was running out of gas. If I could get to a gas station and was not already full I could put in cold gas and the problem would go away for a while. I replaced the pump in the tank and no more
[vapor lock ] like symptoms.

 

Thanks guys.
It does have to cool down before I can restart it. I do live in the mountains, usually around 2200-4000 ft. I am replacing tank and pump, probably finally get it finished around Tuesday.
Speaking of potential electrical/ignition issues, anybody have an opinion on HEI distributors?
I really appreciate the detailed explanations. I'm really enjoying the learning process.

 

Is a PCM the same thing as an ECM? I'm pretty sure I replaced the ignition coil, but it has been a little while.

Just ordered the PIP sensor and the ignition module. We'll see what happens with new tank/pump, PIP and TFI.

I'm still curious about the HEI distributor as I've heard good things about simplification as well as some improvement in mpg and power.

Thanks again.

 

I type corrected on that, but it's still a sign of a failing fuel pump no matter where it's mounted.

 

Yes and no, they are both a computer and they look alike. Ford called it the EEC: Electronic Engine Control when it just controlled the engine. It was a computer controlled system of engine control, later when the computer started to also control the transmission then Ford started calling it the PCM: Powertrain Control Module.

Note that the distributor has to be removed from the engine to change the PIP sensor.

If you have two fuel tanks and it will run on either fuel tank and it will not start on the other fuel tank until it cools down then it is not a problem with the fuel system.

The altitude has nothing to do with the fuel system as the fuel pressures stays the same at sea level or in the mountains. That is one of the advantages of the fuel injection fuel system.

 

Thanks subford.
The reason I asked about the ECM/PCM thing is because I can't seem to find a PCM when I shop online for a new one. They're all referred to as ECM. I don't want to purchase the wrong computer lol.

It can get a little confusing when every company seems to use different terminology and acronyms lol.

Thanks again. We'll see what happens when I get the new parts in place.

 

I think the fuel delivery work you have planned will fix the problem. Background info FYI:

Ford published a VAPOR LOCK TSB for the 7.5L (mainly for the E350 but it's the same engine and most publications carry it as pertaining to your F350 as well):

Ford Technical Service Bulletins (TSB) 91-13-8

Here's an informative discussion on vapor lock in EFI (rare but it does happen):

The RV Doctor: High Pressure Vapor Lock


And finally, be aware that Ford did produce ALTITUDE specific calibrations with a unique part number for the 7.5L EFI (some dealer discussion with part numbers here):

EECTuning.org • View topic - 89 Ford Truck ECU Definition?


Cheers..............

 

Note that TSB 91-13-8 is for a different fuel system than the OP has.
The TSB 91-13-8 is for the frame mounted fuel pump with boost fuel pump in the tank. These models only had the one fuel tank.
This fuel system is a low pressure fuel system from the tank pump to the HP pump on the frame, 8-12psi.
The TSB is talking about replacing the LP pump in the tank.
The vapor lock in these systems happened between the tank and the high pressure pump.



I am sorry but the RV doctor does not have clue of what he is talking about.

He quoted the TSB for the wrong application. The F53 has high pressure fuel pumps in the tanks. They are talking about a F53 and the fuel system and the exhaust system on an F53 are on the opposite sides of the F53.
If the fuel pressure on the F53 goes lower than about 28psi the engine will die.

Not sure what a high altitude PCM Computer has to do with vapor locking???
It is not going to change the fuel pressures.
It just has different tables for injector grounding time.
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