Fuel pressure drop off after shutdown

[teletekman]

So in addition to my cooling issue I am working on, I wanted to throw out another issue I am having with my Ex.

I have been seeing quite a decrease in fuel mileage lately (10-12mpg) with normal driving down from the 16 to 18mpg I was getting before and while it may have something to do with my cooling temp fluctuating high & low and possibly dumping more fuel when it reads low, I decided to throw my buddies HF fuel pressure gauge on it. During a KOEO test, I noticed about 36psi on the gauge. However I started to notice a drop off to about 22psi over about 15 to 20 seconds. It will eventually drop to the 0 over a while. During KOER it sits at about 32psi and holds steady.

I know from past experience that a typical bleed-off should be nowhere near this quick and from reading some other forum posts on here some have attributed it to a possible leaky check valve, leaky injector or FPR fault. Before I drop the tank to check the check valve, I checked the FPR and its holding 15 inch pounds for a good amount of time, so I would assume that it's working correctly.

Fuel filter is new and I do not smell gas anywhere around the engine nor around the quick-disconnect fittings of the filter so no leak I would think. If it was a leaky injector, wouldn't it run rich? No codes are thrown either.

The Ex does not sputter nor run rough cold or warmed up but I do notice a slight hesitation during up hill driving.

I understand pretty much how an FPR operates and got the system to 36psi, put my finger over the vacuum line connection of the FPR and while I did notice the bleed-off slowed a bit, it still eventually dropped as I stated before. All vacuum lines appear to be fine.

Have I ruled out enough to determine my FPR is ok and should I be thinking about replacing the fuel pump assembly? I ordered an OTC fuel pressure tester to rule out that cheap HF gauge since alot of people have complained about it leaking.

Thanks,

Sean

 

[teletekman]

up

 

[natenkiki2004]

I'm quite interested in this as well. I tested my fuel pressure just today in my 1991, here's the results I got:



Arrived at auto parts store, sat in parking lot for 5 minutes and also lost fuel while fumbling with getting the adapter and hose screwed on:
https://dl.dropboxusercontent.com/u...lorer/Pictures/Fuel Pressure/Sat at store.jpg

Key on, engine off:
https://dl.dropboxusercontent.com/u...Pictures/Fuel Pressure/Key On, Engine Off.jpg

Engine running at idle:
https://dl.dropboxusercontent.com/u...er/Pictures/Fuel Pressure/Engine On, Idle.jpg

Shortly after shutting off the engine:
https://dl.dropboxusercontent.com/u...tures/Fuel Pressure/After Engine Shutdown.jpg



The reason I tested mine is that I've been getting regular cold starts that seem to catch a couple cylinders and then die. Cranking the engine again and it runs fine. Only happens when cold, like if I leave the Explorer overnight. Never when warm. My issue might be vacuum or intake gasket or injectors or....

I'll be watching this thread

 

[teletekman]

What kind of a bleed-off do you get, if any, after you shut the engine off?

 

[natenkiki2004]

Since it was a loaned gauge, I wasn't able to drive home and let it sit for a few hours. Check the last image, that was after a couple minutes. Beyond that, I couldn't tell you. I might have bleed-off issues which could be the cause (or one of them) of my cold-start issues.

I guess one thing I could do without a gauge is go poke the schrader valve tomorrow since my Ex has sat for over 24 hours and see if I get a strong spray or a piddle.

 

[arco777]

Typically the leak is from the fuel pressure regulator. It may hold vacuum but not necessarily hold pressure. Likely it is returning fuel to the tank through the return line when it bleeds off.

 

[natenkiki2004]

Don't they just have 1 line straight from the pump? Then the line going to the rail and the line for the vacuum?

Perhaps I don't understand how it works. Is there another line that goes directly to the fuel rail which would mean the FPR regulates fuel LEAVING the rail to go back to the tank? I'm quite curious now.


teletekman, I found some information for you:
http://www.explorerforum.com/forums/showthread.php?t=142801
Read the 1st and 2nd posts, it's said that the pressure will hold for several minutes after shutting off the engine and drops from there.

 

[arco777]

There's a supply line and a return line. The pump pushes out way more fuel than the fuel system needs. The regulator maintains proper fuel pressure in the rail, which the injectors feed from. Excess fuel supplied beyond what is needed is returned by the regulator. The return line goes back to the tank. If you ever do a fuel pump job or view the top of the tank, the supply and return lines are right next to each other.

A properly functioning fuel pressure regulator should hold pressure for a solid 10-20 minutes after shutdown. A little bleed down is normal but not much.

 

[natenkiki2004]

arco, in a 1991, what would cause the fuel rail to empty? I'd guess the FPR? Seems like in newer Explorers, there's just 1 line to and from the gas tank and there's a check valve that goes bad in the pump. But that's not the case with the 1st gens?

I ask because, more consistently, when I go to cold start the Ex the first time that day, it won't start the first time I crank it unless I turn the key on/off a couple times during which I hear a woosh of liquid to the front. I would guess my fuel rail is almost empty if not entirely empty. By the 2nd or 3rd time of turning the key on/off, the woosh is no longer there, just the sound of the pump.

 

[arco777]

They switched to a returnless fuel system during the second gen run. I believe it also required running a higher pressure but not sure.

Ideally when you turn the key to ON the fuel pump should run momentarily and "prime" the system by pressurizing the fuel rail. If it has to be primed multiple times you certainly have a problem. I would hook up a fuel pressure test gauge to the rail and see what exactly is going on.

 

[corkey]

on the returnless the regulator is in the tank , and bleeds off there,, no second line required,,

 

[natenkiki2004]

Lovely, I had a whole post typed up and... expired token, all lost -_-

Anyway, Arco, my fuel pressure readings are the 3rd post in this thread. Tell me what you think.

My thoughts are that the FPR is slowly failing. From what I understand, there's a spring and diaphragm that seal the outlet for fuel to return to the tank. The vacuum is meant to overcome that spring and let more fuel back. Since you have the highest vacuum when you're not using throttle, you'd want less fuel/pressure. If that system is sticking or not sealing, then it's always letting excess fuel back to the tank. For me, it's not a problem (yet) because my acceleration is unaffected, it's just cold starts after sitting a while (empty fuel rail).

That's just my theory.

 

[arco777]

I can't view the pictures in your post (blocked site where I am now). Sorry!

I would guess if your fuel pressure is falling off, there may be a pinhole in the diaphragm or it may not be sealing completely, allowing a slow leak.

 

[natenkiki2004]

Look when you can, no rush

Also, I just thought, maybe it's a stuck injector as well? Though, I would imagine that would have more consequences than just an empty fuel rail. Correct me if I'm wrong, don't you have to remove the upper intake to remove the FPR or is it held down by something else?

 

[Maniak]

Look when you can, no rush

Also, I just thought, maybe it's a stuck injector as well? Though, I would imagine that would have more consequences than just an empty fuel rail. Correct me if I'm wrong, don't you have to remove the upper intake to remove the FPR or is it held down by something else?

If it is an injector you should be able to see which it is by pulling the plugs on a cool motor. In other words, turn the key on/off/on a few times to get pressure, wait for it to drop then pull the plugs and see which one is wet... This is assuming it is leaking fast enough, and enough to wet the plug.

As to how to take them off, Yes, the upper plenum comes off which is easy to do, then then there is a fuel rail spacer looking thing that uses an e10 torx bit to remove (I'm pretty sure 10 is right)..

Since you have a '92 you don't have to worry about EGR which is what usually makes pulling the plenum a pain. In your case it is just one bolt for the coil pack that can be a pain, the rest should be easy..

On our '92, when we also were loosing fuel pressure and eventually we replace the fuel pump.. Partially because of the issue, but also because the pump had many miles on it (> 200k). There is a check valve, sort of, on the pump. It is more like a piece of hose that looks like a duck bill. We didn't replace the FPR until somewhere around 355k miles.

~Mark

 

[teletekman]

Typically the leak is from the fuel pressure regulator. It may hold vacuum but not necessarily hold pressure. Likely it is returning fuel to the tank through the return line when it bleeds off.

So if I am understanding this correctly, if the FPR holds vacuum for a while with no bleed off then is it safe to assume its functioning normally? My concern with the issue I am having is could the diaphragm of the FPR not completely close causing my pressure gauge to register a pretty decent bleed off when vacuum is removed? No fuel seems to be escaping the vacuum nipple either.

I tried manually putting a vacuum gauge on and like I said, can pull 15 inches and hold it there with no problem.

I am trying to rule that out first before pointing to a possible check valve on my fuel pump but don't want to replace the FPR if it's not an issue since they aren't cheap either.


Sean