My 1994 Ranger with the 4.0 engine loses fuel pressure only when it sits in the rain. It could rain but if it’s in the garage and it’s fine. It can sit indefinitely and not lose pressure under any other circumstance. When it loses pressure, I have to wait up to 5 minutes for the fuel pump to re-pressurize for it to start. Nothing gets wet under the hood. The only thing I can think of is that when it rains, the atmospheric pressure drops lower than the pressure in the fuel system causing a seal of something to allow it to depressurize. The question what?
on a side note, I do notice under heavy acceleration, the engine does surge a bit. So something is not quite right in the fuel system.
Also of note, it has a new fuel pump. It made no difference in the depressurization issue or surging. I’ve had this truck for 10 years and it has done this from day one.

 

Welcome to the forum

Interesting symptom

Yes, I would say it has more to do with humidity than anything else, there are no pressure sensors for the fuel system

There is a safety mechanism that prevents fuel pump from running when engine RPMs are under 400
So when you turn on the key the computer only powers up the fuel pump for 2 second
You can hear it in the cab, a HUMMMM from behind the cab, it is not quiet
So cycle the key on and then off as much as needed until you can HEAR what the fuel pump should sound like, it runs for 2 seconds with each key on
And no you can't over pressurize the system

Then when there is a no start, rainy day, see if you can HEAR the pump with key on

Possible issue could be corroded connectors at the gas tank, at the fuel pump relay in engine bay fuse box, or fuel pump fuse in engine bay fuse box
You can pull out the fuel pump relay and have a look at its connectors/pins, same for the fuse

1994 runs 30psi fuel pressure, but will start up with 10psi
Fuel pump running for that 2 seconds will increase pressure 10psi
So even if pressure was 0psi one key on should get a start up

So I think your system is losing pressure BUT...................thats not the issue, issue is fuel pump is not running when humid
'If you have a fuel pressure gauge hook it up
Running engine should be 30-40psi, if lower then either Pump is weak or Fuel Pressure Regulator is leaking too much pressure
Shut key off
Pressure should drop to 15-20psi or so and then stay there for months
If it drops to 0psi then check valve inside fuel pump is bad, which is not a big deal since engine should re-start with one or maybe 2 key on/offs

If you get a chance I would pull out the computer and replace the 3 blue capacitors inside it, under $5 fix
These WILL leak and fail after 20 years and cause running issues, could even cause your current symptom, and the surging

General idea of that repair seen here: Capacitor Repair: Ford A9L ECM « Moates Support

I had to do my 1994 4.0l computer last year

 

When I turn the ignition, I can hear the pump running. It runs non-stop until it pressurizes the system. That means it has ran for as long as 10 minutes straight to get the pressure up. Only when the pump shuts off, the truck will start. And when it starts, it instantly fires. Any time before the pump shuts off, it will crank until the battery is dead and never fire. Not only is this annoying waiting, I’ve already burned up several fuel pumps. This one is brand new. And the third I’ve put on it. Everyone has the same symptoms, thus it has nothing to do with the fuel pump. All the connections to the fuel pump are good. No corrosion.
humidity has no effect. In the summertime the humidity gets over 95% almost everyday and has gone up to 100% and still not rain. Even when it’s 105 degrees with 100% humidity, the truck fires instantly. And yes, under those circumstances. I heard the pump for a brief second.
I don’t understand how bad caps can cause the fuel system to lose pressure only when the truck is left out in the rain.
I’m old school, I know carburetors. I don’t know fuel injection. Thus there must be something in between the fuel pump and injectors that is leaking. I would think that if it was a rubber hose, it would have gotten worse over 10 years, so I think I could rule out a dry rotted hose. It has to be some component that I never heard of and/or some O ring somewhere. But I have no clue where to look.

 

Time to pull out the computer and replace the capacitors, for sure

 

I had the EXACT problem with my 1992 2.3 2wd. The fuel pump would continuously prime whenever it rained. This caused a no start condition. Usually, once the weather cleared, and the truck dried, the fuel pump would prime properly and the engine would start. But that stopped happening. The pump would continuously prime even in dry weather, when the key was on. I
pulled the ecu, and noticed a watermark from leaking spot in windsheild/ dash area. And when I opened it. My 47uf capacitor was fried!!! And I mean burnt to a crisp. I pulled it and and the 3.3uf cap (which looked okay, but was fried on bottom

) and cleaned the board with isopropyl alcohol. Then I soldered in new caps. BINGO! Truck not only starts properly, and primes properly, but it also has way more power and doesnt smell so fuel rich!!

 

Almost but not quite, mine will start in the rain. I just have to wait for the fuel pressure to build up. I don't have to wait for it to dry out. I turn the key and sit and wait for the fuel pump to stop. Once it stops, the truck will start wet or dry. If I try to start it before the fuel pumps stops, it either will not start at all or if I try when the fuel pressure is close to the max, it will start and run rough as hell until the fuel pump catches up. Something is causing the fuel system to depressurize only when it rains.

 

You have some very sound advice from two sources about the capacitors. With your very little knowledge of fuel injection, you appear to have determined they must be wrong. I don't know if your capacitors are bad but I would at least do a quick inspection.

Your fuel pump is not shutting off like it should and that sounds to me like it is related to those capacitors as Mrbosschild experienced. If you are going to dis some good advice, what is the point of asking for it.

 

The symptom of "waiting for the fuel pump to prime" was how it all started for me. First it would rain, I would go try to start truck, it wouldn't start. I would let the ignition switch stay on until I heard the pump stop, then the truck would start. This lasted over a year, so I didn't really care, until one day I was riding home in rain, and my truck began sputtering and smelling rich. I got home and when I turned it off, and attempted to turn back on, the pump never stopped priming. And from that day forward it would no longer start in the rain period.
Then it never stopped priming regardless of weather, and I went on an electrical demon hunt. The caps are your problem, and are super easy to fix with minimal skills

 

Pull the computer and have a look..........its FREE to do that

Many fuel injected systems lose pressure when engine is off just a matter of age, it doesn't hurt anything as long as it can hold pressure when engine is running

With key on fuel pump should only run for 2 seconds, if it runs longer then there is a problem, usually Capacitors in the computer
Each 2 second run time adds 10psi pressure
So cycling key on and off 3 times would get any fuel injected engine to start, unless there are other problems

Rain = humidity
Humidity effects electrical systems not mechanical systems
Pull the computer and have a look


i.e. if you have a flat tire you can swear its not a nail in the tire, but its FREE to just take a good look at the tire for a nail hole, lol