Hey guys,

Having problems with the fuel pressure on a 1997 explorer 4.0 (VIN X).
How can I tell if the pump is to blame or the regulator is faulty?
The fuel pressure is at 18 psi. When the vehicle is shut off the pressure drops like a rock. It wont hold for even a second. Also, it takes almost 30 seconds to get to 18 psi.

Any ideas guys?

 

It could be either. First of change your fuel filter, a plugged filter can cause problems. Does the pressure go higher when you disconnect the vacuum line? To test the pump, clamp off the fuel return line with a brake line clamp (yes, they do have those) Be careful since you don't want to damage the hose. Then check the pressure. t should rise to at least 60-65 psi. If it doesn't, your pump is weak. If it does the problem is the regulator.

While this forum is an invaluable resource, you should still do yourself a favor and purchase a good service manual. Haynes and Chiltons are both great. They contain the technical specs and troubleshooting procedures. When you work on your own vehicles you should have one. I have one for every vehicle I own.

 

Yeah I thought of that.(clamping off the fuel return line) However...the fuel lines, both input and return are steel lines. No where from the reg out to the pump are the lines "clamp-able" That is why I am searching for a different diagnostic mehtod.
What can I do to simulate clamping off the fuel return line?

As far as the FSM goes...We have Alldata & Mitchell Ondemand5 At our shop. I agree that every one should have a decent service manual for the vehicle they are working on.

 

There should be a soft part of the return line. I guess try to do some tests that will see if the regulator is working at all. Look for signs of internal leaks, see if removing the vacuum line makes a difference. Otherwise, you may have to measure the pump pressure by dropping the tank and measuring the pump directly. At which point, you probably will have isolated the problem to the pump anyway, and the test would only verify.

 

When the vacuum line is removed from the fuel regulator...the engine rpm should...? rise or fall?
If my memory serves me correct, I would say that there was no change at all when the vac line was removed from the regulator.
However, during the fuel pressure test, the reading was/is only 18-20psi. Would such a low reading cause the Fuel/Reg to not respond properly?

 

im having a similar problem on my F150 and about the same spot in time that you are.. I have removed the regulator and dont see anything wrong. blew thru it, appears o.k. I have put it back together... it should regulate at somewhere above 40 psi, but if it fails in the open position, it could not regulate at all. I was considering taking the return steel line apart at the clamp and putting a short hose with plug and clamp on to shut off all the return... i have not lookd down under the seat area to see if there is a short rubber line at any point.... you might be able to take the supply line off at the fuel filter, take the old filter end off and somehow connect the pressure gague to it, reinstall in the line and energize the pump to see if it builds up any pressure.??

 

if this was my truck this is what i would do. Replace the fuel filter unless you know that it is good. I would run a few bottles of a good injector cleaner in the tank. some of the fuels screw up the pumps "techron" claims to be able to fix it. then i would test for fuel pressure. you want about 40 koeo, 30 koer, 40 koer with the vacuum line disconnected. You also should check the vacuum line on the regulator for fuel. if you have fuel in the vacuum line the regulator is bad. if the fuel pressure does not holld at all. I would suspect a faulty check valve.. repeating myself again....replace the fuel filter and add the techron first. easy and cheap.

 

The fuel filter has been replaced all ready. It is known to be good. When checking fuel pressure, it take about 30 seconds +/- to rise to psi(KOER). When the ignition is switched off, the pressure falls like a rock to 0psi. It won't hold for even a second. How much vacuum should the fuel pressure regulator be able to hold and for how long?
My original intent of the thread was to try to find out if any one had any sure fire ways to check/diagnose if the regulator and/or the fuel pump are working properly.
What I wanted to do was to clamp off the fuel pump return line and test the fuel pressure. this would tell me if there was some kind of fault "north" of the clamp. If the pressure dropped while clamped, the problem lies somewhere up front; i.e; regulator, injectors.
Since I know that the pressure will drop from 18 to 0 psi in a few short seconds when the gage is hooked up to the fuel rail, I wanted to be able to rule out the forward components rail, reg, inj.
I could disconnect the return line at the "chrome" connector,(where it changes from a steel fuel line to a braided plastic fuel line).
However it is very corroded/rusted and I am afraid it will open a can of worms if it breaks or can not re-connect properly.
Hence the reason for question of: Are there any alternate ways to test the regulator? I have removed the vac line while KOER. No fuel spurted out. But that does not mean that it is working properly.

 

i pulled the fuel filter hose off on my F150 yesterday. I took a 5/16 steel tube and connected with a ferrel to a 0-60 psi gauge. Plugged it into the hose from the tank/ pump, and turned the key on. Built up to 44 psi in 2-3 cycles... mine will drop to 20 psi in 30 minutes.

 

When you pull the vacum line of the regulator pressue sgould go up.check that there is vacum on the line. You can apply 10 inches hg of vacuum. If the pressure goes down the reg is OK if nothing happens the reg is bad. My guess after studying this thread is that your pump/checkvalve is Fubared

 

FPR is a simple backpressure regulator. The methods for testing whether a low pressure condition is due to a bad FPR or not is to determine if the FPR is open at the low pressure condition. One technique has already been suggested: clamping off the return line. If this causes the pressure to increase, then the FPR is opening too early. There are other techniques, but each of them basically play off of the same idea -- determine if the FPR is open when the pressure is low. On my '92, I disconnected the return line at the FPR, attached a hose so the gas didn't spray all over, then manually ran the pump. Nothing ever came out, which told me that the regulator was staying closed as it should, which meant that the fault was upstream of the regulator. On my BII, you could hear the gas being returned to the tank, so it would be possible, in that case, to listen for the return flow at the tank.

 

I am not sure if this thread is becoming a "theory" thread or if it is meant to get your truck fixed. Your truck holds ZERO PRESSURE. Your PRESSURE is low. You don't have fuel in the vacuum line. Sure sounds like a fuel pump setup issue to me

 

Speaking of "theory". Let's pretend the Regulater is toast. In "theory" should the sytem still be able to hold some pressre after it is turned off. In "theory" if the system will hold ZERO pressure can this be caused by a falty FPR or is it from a massive leak. I.E. The check valve. In "theory" if the massive leak was somewhere besides in the tank wouldn't fuel be everywhere. I am not positive of this ? Can a bad FPR cause the system to hold ZERO PRESSURE

 

Yes, if the regulator were stuck open. If I were "rolling the dice" to decide which was at fault, I'd say the pump, as well. The pump seems to fail a lot more frequently than the regulator.

 

so if the regulator was stuck open, meaning the spring failed so much that it put no pressure on the diaphragm then the fuel could "theoretically bypass and go straight to the return line. In "theory" if this happened then by applying extra vacuum with the vacuum pump should correct the issue. right? so if you apply extra vacuum to the regulator and it corrects the issue then that is your issue?