Fuel Pressure 101 - Mini-Diary Fuel Pressure Regulator Test Fuel Pump Test | Ford Explorer Forums - Serious Explorations

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Fuel Pressure 101 - Mini-Diary Fuel Pressure Regulator Test Fuel Pump Test


EF Tranny Guru
Moderator Emeritus
February 8, 2003
Reaction score
City, State
Sacramento, CA 95827
Year, Model & Trim Level
1992 XLT
Recently I was working through a problem on a car of mine, and it dawned on me I had never seen a basic thread on fuel pressure testing in a fuel injected engine on this board. There are a lot of "old hands" on this board, and we forget we have a lot of younger folks in their learning phase. I thought maybe a thread on fuel delivery and pressure testing might be a good idea. This will be a short, basic thread on fuel from the pump to the rail. maybe in another thread we can talk about injectors.

All the pictures were taken on a 1st Gen Explorer.

In a fuel injected engine, we have a fuel pump which pressurizes a line running to the injectors... on the engine it's referred to as a fuel rail. All fuel rails have a pressure test port... a schrader (bicycle type) valve... it has a cap on it, and (keep it in place except to test) when you remove it, here is what you see...


In the fuel injected engine, the electric fuel pump pressurizes the rail, which is often controlled by a fuel pressure regulator. (Engine driven pumps are all but gone from the scene). Any excess fuel not needed or any overpressure is sent back to the tank. So now we have dual lines to a fuel tank. Some fuel pressure regulators are vacum controlled, others are just spring controlled. A bad FPR can cause you fits. And, they DO go bad.

Like other things, FORD has to be different. Their schrader is smaller, so if you are going to use a fuel pressure gauge, you need an adapter. On the 92, the schrader is in a bad spot... you have to finagle the hose under a wiring bundle to hook it up, can you see the adapter?


Maybe this is a good place to talk about a fuel pressure gauge. On fuel injected engines it is an essential tool in your toolbox. They cost about $30. Money well spent. Any old mechanic will tell you that in order, when an engine has trouble, check fuel delivery, then spark. (Course we have alot of other things to worry about now too, but the old saw is still good advice... Especially in a non-start.)

The fuel rail will remain pressurized even after the engine is off. So, to help prevent fires etc, gauges today have pressure relief valves in them so you can sfaely unhook them... here's ours hooked up to the rail, notice the second clear line...


It is the overflow.... here's a closer look at the pressure relief valve on the gauge... a button to push when you are done.


and here's where I run the overflow line.... lawnmower gas I have even held down the button to get lawnmower gas once!


When I hooked up the gauge on the Explorer, which had sat for over a week, I get pressure of:



I turned the ignition on until the electric fuel pump stopped (you can hear it run) then I did it again just to be fair to it.... now the pressure was


Good healthy pump. Pressures will vary and you should check spec, but 35-40 is pretty good. With the engine running I had 42.

Ok so we checked our pressure. We're done. [Note: not quite, read Mr. Shorty's post directly below!] All is well. Engine is shut down. We need to release the pressure, instead of spraying gas all over the engine compartment.... so we push the button, and the pressure....goes to....



And the lawnmower gets fed..... see the gas in the line? (frankly I still end up leaking gas all over the place once I remove the gauge set and pull it out of the engine bay, but at least I am not leaking it on a potentially hot engine...)


Unhooking the gauge is just the reverse of hooking it up. make sure you recap the valve... keep it clean.

I strongly recommend that you buy a fuel gauge. It is a beginning part of your serious toolbox on fuel injected engines. You need it rarely, but when you do, there's no substitute!

[I will add to this thread in terms of content about multi-port injection etc later, for now this is it]

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Can I add a few things to this one:

What Glacier describes for us here is one element of a basic fuel pressure test. I've found this can be a useful diagnostic tool. One disclaimer: I am only familiar with EEC-IV systems. I know EEC-V has similar functionality, but I'm not familiar with teh details. Some things I would add:

1) In addition to cycling the key to get the fuel pump to run, the EEC-IV self-test connector has a fuel pump test lead in it that you can ground to run the pump (key on). Short end of the connector if memory serves correctly. This makes it easier to see if the pump runs, if the FPR maintains the correct pressure, and to hear if excess fuel is returning to the tank.

2) The first part of fuel pressure test is as Glacier described: See if the fuel system brings the fuel pressure up to the proper value. If you like at this point, you can check to see if the regulator responds properly to vacuum by attaching a vacuum pump to the FPR and seeing what happens to the fuel pressure as you apply vacuum. Fuel pressure should drop roughly 5 psi as you apply full vacuum to the FPR.

3) Second part of a fuel pressure test involves shutting the pump off and seeing if the fuel rail remains pressurized. The fuel rail should hold pressure for several minutes after the pump is shut off.

4) Third part of a fuel pressure test involves starting the engine. If you've left the vacuum line hooked up to the FPR, the fuel pressure will drop 5-8 psi as the engine starts, but should hold steady there. If you disconnect engine vacuum, fuel pressure should not change from the engine off pressure. With the engine running, you can rev the engine and otherwise make sure fuel delievery is able to keep up with the engine's demand (you could theoretically check for a clogged filter this way).

5) Once, while tinkering on my '92, I unplugged the fuel pump fuse while the engine was running and watched to see when the engine stalled. From these few trials, it appears that the engine would stall when the fuel pressure dropped to ~25 psi and below. I submit that 25 psi is an unofficial, unscientific minimum pressure necessary for these engine's to run.

Useful information. Thanks Glacier.

Thank YOU Mr. Shorty! Great additional info.... and candidly I had forgotten about the lead on EEC-IV that keeps the pump running.... I'll find it and post it. I think it is the one on the far right hand side if the large flat surface is on the bottom....

[comment: Mr. Shorty is one of the best of the best "old hands" on this board. If you have an engine problem, he is a GO TO kinda guy!]

I invite others to also add their knowledge to this thread. It only serves to make it more useful.


Looking at the picture of the fuel port, I would say that this is a 4.0 engine. It's a good thing that you used an Explorer for these pictures. In the 93 Aerostar with the 4.0 engine, it's really buried, and the only way to take a picture of it is to use a scope like the type used for a colonoscopy! The 3.0 engine has the port closer to the front, and is easier to get to. There could be a few reasons for a fuel pump not starting, so the chances are that the pump might still be good. There are a few things which control the power going to a fuel pump. If the computer fuse, or relay is defective, it will prevent the fuel pump from turning on. On the 88 Aerostar, there is a ground wire near the battery that feeds the ground circuit to the computer. If that wire is disconnected, it will prevent the computer, and pump from turning on. One way to tell is if there is no check engine light. Some vehicles also have fusible links. They were used before high current fuse blocks were used under the hood. On all Fords that I've seen from the 80's and up use a part called an inertia switch, which is manufactured by a company called First Inertia. It has a ball inside of it that will hit a button in an event of a crash, or a hard bang, preventing the fuel pump from working. This is used as a safety precaution to prevent the fuel pump from working after a crash, and therefore feeding the flames with gasoline of a burning vehicle.

There is some great information in here. I just found this today, but believe I've seen it before through a link. Great information.


Let me add to this as I have experience with the Explorer returnless system. The explorer returnless system (99-??) has the presure regulator in the tank, as part of the fuel pump assembly. Then there is one fuel line up to the engine, and where the pressure regulator is on the earlier engines, there is a modulator that regulates the flow to avoid pulses of pressure. Note that the modulator looks much like the previous regulator with vacuum line and everything. The returnless system runs at a constant pressure (~62psi from memory) and there is no change in pressure when you remove the vacuum from the modulator.

There is also another variety of returnless system, where the fuel pump actually regulates and varies the fuel pressure depending on signal from the ECU. I am unsure if the 3rd gen Explorer went to this system or not.

I bought the fuel gauge tester and I was shocked at what it read pegged to 65psi, dropped to 0 in 5 sec, tried it again same reading, looks like my fuel pressure regulator has to be replaced, explains the rough idle, flashing ECL. How difficult is it to replace and price range. None of the local auto parts carry this part strickly a Ford part. Can somone tell me where is the sensor that's in behind the MAS and between the 2 large hoses on the throttle body I believe it's the IAT but not sure. My 99 xlt doesn't have it as shown in the Hayness manual. thanks

I'd like to help but am unfamiliar with the 99 setup...

On the 03 the spec calls for 62 PSI +or-8, SOHC V6

So I gotta spend 30 bucks on the gauge, huh? I can't just use the standard tire gauge? :D

Just read this post and I thought I'd add something. My '92 Sport has suffered from poor gas mileage for some time and I'd previously checked the fuel pressure with a guage. At that time it read fine at 35psi so I decided to just live with the problem for a while. No cels or other problems, just lousy mileage and a black tailpipe. Yesterday, I decided to try to track down the problem again. Connected my gauge, started the engine and again saw 35psi. Also, no sign of gas in the vacuum line. Went to get a flashlight for a better look and when I came back the pressure was now at 80psi!!!. Revved the engine and let it run for a while and the pressure stayed at 80. Installed a new, Napa fuel pressure regulator and the pressure is now in spec. Exhaust smells much cleaner as well.

Point of all this is that you should give your engine a few minutes of running with the gauge connected to see it your regulator is stable. Mine wasn't but I was too quick to read the gauge and shut things down. Looking forward to my first road trip to check the mileage now.


I just found this thread. Nice write up and very helpful :thumbsup:

Question: What should my pressure read at ~WOT? On a 92 EEV-4

Your pressure should be constant regardless if it is at idle, or at WOT. That is the purpose of the FPR. It is supposed to maintain a constant rate. Since it is not a perfect part, there is a little variance, so your pressure might fluctuate within a certain range.

Precision tune

I recently made the mistake of taking my ex to these people to have checked out. The check engine light came on and I have no code reader so I went to someone who did. I was prepared to spent $200-$300 to fix it and renew the safety inspection. My quess was the usual suspect the infamuos O2 sensor. After an hour of waiting, the manager came out with the prognosis. $1488.54 to fix the ole gal. I asked him what kind of crack he was smoking and demanded my keys. I then did what I should have to begin with, took it to Auto Zone and had them use their code reader. The same codes came up. I looked the codes up in the haynes repair manual. $175 and 10 min of my time in their parking lot.... no more check engine light. The culprit.... MAF sensor! Precision's remedy... Fuel pressure regulator/pump assembly. The icing on the cake is the manager offered me $1200 to buy my truck. I guess I should have mentioned to him that I happen to be a former master certified Ford sales consultant.:fire:

62 psi is correct for a 99 returnless whether it is WOT or idle.

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Old Post, Great Info

I just did a search and came across this. PERFECT

Thanks Glacier991