Weird Fuel pressure problem

[Kim]

Hi all. My Ranger sometimes has difficulty starting. About 90% of the time I can hit the key and it will crank as long as I hold it without firing off. When it does that, it takes a second or third attempt before it lights off. I won't stay in Start too long though, because I know how bad it is for the starter but it seems like I could just crank it until the battery dies or the starter gives up. I did a little digging around here about fuel pumps and/or fuel pressure and decided I need to get a good fuel pressure gauge tool and check it.

So here's what happened first Test. Key On/Engine Off gave a whopping ~5 lbs. While cranking, it climbed up to about 20 before popping off and once it was actually running by itself there was ~40 lbs. Key Off/Engine Off that pressure immediately dropped to zero. Because of the somewhat random starting issue, I'll repeat the Test a few more times but for now this is what's going on.

I've read here that the fuel pump assembly has some kind of internal check valve to hold the fuel pressure at the rails when the engine isn't running, so if I understand correctly I will need to replace the whole sending unit/assembly. What puzzles me more is how the pump and pressure behave depending on the relation of Key Position and Engine Start/Run status. Of course the pump should charge the rails when Key On. But why would it NOT do that until the starter is engaged? Am I possibly looking at some miswired B.S. as well as a bad fuel pump assembly? Thanks.

 

[Bgunner]

The fuel pump turns on as soon as you turn the key to the run position. You should hear the pump run for 3 seconds I believe before it turns off again. This is a prime function to boost fuel pressure back up to what it is supposed to have.

Do you hear the pump run every time you turn the key to Key on/engine off?

It sounds like the check valve is bad leaking the pressure back into the tank. The reason you only get 5lbs is the pump is not staying on long enough to charge the system up to what is needed. This is normal when the check valve is bad. As a work around turn your key to the on position a few times before trying to start the engine till you can change the pump. This will help prime the fuel rail.

 

[Kim]

Thanks, Bgunner. I don't hear the pump like I normally should when Key On/Engine Off but it does at least make the gauge needle move, so it's doing "something".
But like I said, I'm actually more concerned now with the reason why it doesn't try to build any real pressure until Key Start. Pressure is also fine once engine is running. What does the check valve care where the Key Position is, and why "bad pump/good pump" depending on Key? That sure seems to me like there's something else going on as well as a bad Fuel Send unit. I'm still replacing the Fuel Send unit, but with the task involved to get to that I'm going to be 100% if my truck still starts like it does now.

 

[Bgunner]

The reason it is not building the proper pressure is the pump is not running long enough in its prime cycle to overcome the loss in pressure due to siting. If the check valve was good it would run long enough as the pressure wouldn't drop as much so the short time is enough. Everything you are experiencing is normal when the check valve goes bad.

When you let off the key and put it in the off/ACC position then turn it back to the On position again the pump will cycle again building more pressure. This is why cycling the key for now is a work around till you can change the pump/sending unit. I can't remember if the pump runs in the start position or if the ECU waits to see the engine start before turning on the pump again. If it runs during cranking this would explain why it builds pressure while cranking.

As a tip, pull the bed bolts and slide the bed back enough to get to the sending unit. You will need to remove the wires from the tail lights to prevent wiring damage and gain the room needed to get to the sending unit. This is the easiest way to change it. Before tightening them back down just line the gap between the bed and cab back up to keep the bed from being twisted looking when done. A bump from your hips should help slide the bed side to side to line them back up as the bed will slide side to side pretty easy along with front to back once it is close. Use a ratchet or breaker bar to pull the bolts and not an impact gun. Some beds use clips as nuts and others use nuts welded inside the cross rails, breaking these loose will cause you to have a very bad day.

 

[RonD]

Well in a 2001 there are two things in the fuel pump assembly that can cause your issue
First off 2001 Ranger should have 55-65psi fuel pressure when running

With key off pressure should hold above 30psi, for months and months
Key on gets 2 seconds of run time from fuel pump, which = about 10psi
You can repeat key off and on as much as needed to build up lost pressure, doesn't hurt anything, but if its not holding, even for a few seconds, then its not helping either, lol
Computer won't start fuel pump full time until RPMs are above 400
Cranking RPMs, starter motor, is 200rpms

In 2001 the fuel pump assembly has the fuel pump, sender(float) AND the fuel pressure regulator(FPR)
The Fuel Filter(which you should also change) will have 3 ports
The regular IN and OUT like any filter, and a 3rd port that runs back to the gas tank and the FPR

So your issue could be the fuel pump OR the FPR, but both have to be changed with the assembly
A working fuel pump can do 80+PSI
The FPR is set for about 60psi, +/- 5psi

 

[Kim]

Thanks, guys! Great info, much appreciated.