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'91 Econoline 150, Fuel Pressure goes low

Engine starts, runs good for about a minute and then suddenly stalls. Have to wait a few minutes to start again otherwise it cranks with no start. I connected fuel pressure tester. The fuel pressure builds up but takes longer than it should - have to switch ignition on and off a few times to get pressure up. When engine is running, it settles about 30PSI. After about a minute, the pressure begins dropping. It takes about 2 or 3 seconds to get down to about 20PSI where it stalls. I can hear the high pressure fuel pump (the one on the frame) humming for a second or two after engine completely stalled. I pinched off the return line and pressure build up time doesn’t change and doesn’t go any higher. After building up pressure and turning engine off, I pinched off feed line and the pressure leaks out too fast (about 10PSI in 15 minutes). This is too much, but it’s not bad enough to stop the engine from running - probably some leaky fuel injectors. I had a helper switch the ignition on and off several times while I put my ear to the gas tank filler tubes. I could hear a single click each time time the ignition was turned on, but no buzzing sound as though the pumps are running. The fuel filter was already replaced by someone else. I believe that one of the pumps is faulty, but don’t know if it would be an in-tank pump or the high pressure pump on the frame. I’m guessing that the in-tank pump is bad. I’m interested in hearing the opinion from those who have experience with this and any recommendations on what I should do.

Remove the vacuum hose from the fuel pressure regulator.

https://www.rockauto.com/en/moreinfo.php?pk=950848&cc=1111846&jsn=510

If gas leaks from this connection, the regulator requires replacement.

Tester

Hi Tester, right after I made my post I noticed I forgot to mention that. During the short interval of engine running with good fuel pressure, I removed the vacuum hose and the pressure rose about 10PSI as it should. When putting the vacuum hose back on, the pressure dropped back down to idle pressure. I also revved the engine and saw about a 10PSI increase and then it fell back to what it was when I released the gas pedal, which also indicates a good pressure regulator. One other thing that indicates a good pressure regulator: I checked inside the vacuum hose for fuel and it was completely dry. I don’t see any obvious fuel leaks. The amount of leak down isn’t enough to cause the engine to stall. It took 15 minutes to drop 10PSI when the engine wasn’t running. The engine doesn’t stall until the pressure drops to about 20PSI. Thanks for your reply.

I’m guessing you checked the fuse for the rear fuel pump.

Remove the filter and check to verify it is not restricted or better yet just replace it and retest.

What is this high pressure fuel pump?

Rock Auto shows only one fuel pump for this vehicle. The one in the gas tank.

Tester

This van has two fuel tanks. Each fuel tank has a low pressure pump. Along the frame there is a reservoir that is filled up by either of the two lower pressure pumps and the high pressure fuel pump is attached to the frame along with the fuel filter, between the reservoir and the engine.

There are two fuel tanks, so two rear pumps. The schematic shows all three pumps connected together on the same power wire which is connected through the fuel pump relay to a fusible link. The external pump gets power and gives pressure to start the engine. The rear pumps make a clicking noise when ignition switch is turned on, but I think they aren’t running. One of the rear pumps may have been burned out for a long time. The owner of the van told me that the rear tank hasn’t been used for 20 years but doesn’t know the reason why. I’m suspecting that it’s because it stopped working long ago. So maybe both of them aren’t working now. It’s a very tight space at the top of the tank so no way I can get in there to check power to the tank pump, unless I at least lower the tank a little bit. At this point I’m thinking that I might do that as my next step.

The fuel filter is brand new. It was replaced by a “mechanic” the owner hired last year. The van has just been sitting since then. I used quotation marks because I don’t know if he’s a trained mechanic or not. The owner of the van told me that mechanic replaced the filter and it didn’t fix the problem and he then suspected the pump in the fuel tank must be bad.

The bottom of the tank may have a heavy accumulation of rust and dirt that is getting sucked up to the filter sock on the pump and stopping the flow.

That “reserovoir” your talking about, the one that has two inlets, one from each tank, also has a filter element in the bottom of it. That is if I remember correctly. I’ve seen those become completely obstructed on old trucks like yours.

But you will also need to verify the operation of each of the fuel pumps one way or another, either by measuring current flow at each pump or by using jumper wires and a fuel pressure gauge and operating each one independently.

Do you know if it’s possible to open up the reservoir or does it have to be replaced? Is there a way of checking it for clogging?

It could also be a problem with the tank selector valve as well… These are very well known trouble makers and often are the source of a whole host of running/not running problems.

This has been such a common trouble spot for decades that there is and has been a ton of information on the number of failure modes this unit can cause, out on the internet… Just look up “Ford F-150 Dual Tank selector valve problems”. Some selector valves switch electrically and others used fuel pressure and a diaphragm system to allow tank selection… each style presents its own problems and different failure modes. Definitely something to look into.

I was wondering about the filter in the reservoir. I read in a manual that there’s a filter in it but I didn’t see any mention of servicing it anywhere until your mentioning it. I was hoping to find more information about it and hopefully will be able to open it up and clean it out. I see that the unit sells on Amazon for almost $200 and I’m sure the owner won’t want to get a new one. I heard from one source so far that the filter can be purchased at a parts store, which would mean it should open up. I’ll have to check the parts store for that. Maybe the unit could be removed all together along with one of the tanks and just use one tank. I’ll spend some time on this and decide with the owner what we’ll do.

Thanks for your info on the tank selector valve. I see there’s info about it on the internet. I didn’t see any info in the past few days until I entered “tank selector valve” in my search. I’ll be checking this out along with the filter in the unit and confirming whether the pumps in the tanks are working or not in the next week and will update.

You’re going to end up dropping at least one tank, probably both. We’ve all been through this week of scheming to figure out what could be wrong before finally taking on the big dreaded task, and it almost never works.

@asemaster, have you ever pulled a filter from one of those plastic reservoirs? When an inline filter was mounted in the fuel line near the tank that reservoir/housing was always been empty when I checked it and somewhere long ago I quit opening them to look.

And I agree that the fuel tank selector system has a bad reputation on the late 80s and early 90s Fords.

You know, it’s been so long that I can’t remember the specifics of the model, etc., but I do recall unscrewing the bottom of the reservoir and finding a filter element in there, usually all but completely plugged. Then I also seem to recall that filter element being discontinued. So I’m not 100% sure of anything.

1991 is ancient history in automotive terms and when dealing with a fuel supply issue on a car this old you’re not likely to find one component failing and all the others in perfect order. To get to the bottom of this problem the OP will still have to isolate and check each and every component of the fuel supply system. Which means filters, all 3 pumps, switching valve, regulator, relays, wiring.

Yeah, but everyone wants to be told there’s a secret, easily corrected problem when they surf automotive sites. And how often does that work out?

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Maybe you could use a long screwdriver or wooden dowel as a stethoscope, place one end near the tank fuel pump, and listen to the other end. Attach a paper cup to the listening end to amplify the sound if it helps. If you hear a buzzing noise the tank pump is most likely spinning. To determine whether it is pumping enough gas or not you’d have to somehow figure out if the buffer tank before the high pressure pump is full or not. If that’s full then the high pressure pump has plenty of gas to work with, so the problem must be the high pressure pump. If you want to take a guess, probably the high pressure pump, as it is easier to pump low pressure than high pressures. Plus both low pressure pumps would have to fail for this problem to occur on both tanks.

One other idea, maybe the problem is the selector valve. There are a couple of recalls on the fuel tank selector valve gadget. 91S39, NHTSA91V146000. Googling those numbers might show the contents.