FUEL PSI questions

[scenario]

My 85.5 SVO will not start on the 1st cycle of the ignition. It will fire the 2nd time. I hooked a Fuel gauge up and when I turn ignition on (NOT RUNNING) the PSI barely moves. When I cycle a second time it goes to about 10 and then a third it gets into the 20-30psi range. If I start the engine the PSI will goto 38 PSI.

When I turn the engine off the PSI drops pretty quickly down to about 10psi and stays there a bit till it creeps down to 0.

When I am driving and boosting the PSI gets to 50PSI but did not get over that even on hard boosting acceleration. It steadily goes from 38psi to 50psi as boos increases but that is it.

Any thoughts???

I am thinking fuel pump

[SVOeric]

10% chance Fuel pressure regulator, 90% chance fuel pump is what my experience would tell me

[scenario]

I did not get any fuel leaking from the fuel pressure regulator when I removed the vacuum line while running.

[SVOeric]

they don't have to leak to be bad. but, again, -- 90% chance it's the pump.

[svono50]

I did not get any fuel leaking from the fuel pressure regulator when I removed the vacuum line while running.

Your FPR can fail in the pintle area vs the diaphragm too, so keep that in mind. Typically your symptoms point to a defective check valve in the pump itself, as others have noted. If the pump to bracket hose fails/splits, your fuel pressure will remain a little low at idle and never reach the full 1:1 rise in pressure with boost. A sending hose split will only get worse over time.

You can do a quick FPR pintle test by applying some pressure on the FPR (say 10psi) at idle. Your idle fuel pressure should show ~48 psi with 10 psi against the regulator (i.e.- 38+10). If it still falls off quickly when you shut the engine down, that will confirm the pump vs FPR.

[scenario]

I connected the vacuum line off the FPR to a vacuum tester and applied 10psi vacuum. I saw no noticeable increase in the fuel PSI. When I shut the engine down the fuel PSI dropped quickly and the vacuum psi did not move until I purged it from the tester.

[svono50]

You need to apply 'pressure', not vacuum to the FPR inlet to simulate boost from the turbo. My Mityvac hand pump can do both vacuum and pressure, but I know most models don't have that ability. If you have 10" vacuum on the FPR and shut the engine down, it will drop the pressure quicker than no vacuum at all.

[scenario]

You need to apply 'pressure', not vacuum to the FPR inlet to simulate boost from the turbo. My Mityvac hand pump can do both vacuum and pressure, but I know most models don't have that ability. If you have 10" vacuum on the FPR and shut the engine down, it will drop the pressure quicker than no vacuum at all.

OK I will see what I have. Never had to troubleshoot one before

[svono50]

Here is a link to a video I made when I was eliminating my fuel pump voltage switch. You can see how vacuum and boost pressure impact the fuel pressure output. You can jump out 2 positions on the EEC test connector to run the fuel pump(s) with the ignition in 'run' and the engine off, which is what I was doing in the video.

https://youtu.be/PpEu1zkEurE

[svono50]

Here is the EEC test connector jumper for running your pump w/o the engine running:

Fuel pump test jumper 86-93 Mustang 60%.gif

[scenario]

Thanks so much. The Tan/Gr and the Yellow wire are wires that already connected as shown? or do they need to be run? I am going out the door for a weekend trip and dont have time to look but I will look at this post on my phone. Thanks again it is hugely appreciated and I am gonna get a mityvac pump.

Is your mityvac a plastic housing? what model number?


Frank V

[svono50]

No, the solid wires shown in the graphic are the vehicle wires...don't get hung up on the color code either. The important detail is the location of the pins to jumper. I made a jumper from some wire with male spade connectors on each end to plug into the test connector.

I have the Mityvac MV8500 Silverline Elite Kit. The hand pump is a die cast aluminum unit, nicely made too. Prices have certainly gone up since I bought my kit, so look around to find the best price. It will last many moons based on my experiences. My main use for it is vacuum bleeding my brakes, along with various other diagnostics for fuel and wastegate issues that have happened.

[gbeaird]

I highly recommend that Mityvac kit. Previous iterations of that pump were 100% plastic. After I accidentally sucked brake fluid into my old unit (from the '70's, IIRC), I was expecting my search for a replacement to be nothing but disappointment. I was quite pleased to see this unit in all its cast aluminum glory. Works well, too.

[scenario]

UPDATE

I did try the "test" procedure but I did not have luck. No prob. Is that test to jump the fuel pump a KOEO procedure? I did not want to possibly short something out so I didnt troubleshoot too much. I guess your detailed explanation still isnt enough for my low IQ...Thanks you tho!

Either way, The car is up on the lift so I did not want to start it up. I was just doing this by leaving ignition on and just held the negative battery terminal in place a few times to get the fuel PSI up to about 30. It drops off quick so then I applied 10PSI of air pressure to the FPR from my new MityVac kit that you suggested for me to buy. The quick dropping of fuel PSI was just as bad with the 10PSI of air then it was without it.

I think that confirms the problem is in the fuel pump and I need to drop the tank

[svono50]

The jumper is to trigger constant power to the fuel pump in a KOEO state, correct. You won't short anything by jumping the test connector terminals shown in the diagram. Per a now retired Ford master tech, that is how you should set your 'base' fuel pressure per Ford's procedure (i.e.- without the engine running). Based on that advice, that is how I have always set it with my adjustable FPR FWIW. In my video, that is the technique I am using, the engine isn't running.

Based on your testing, sounds like your fuel pump's check valve is bad or maybe you have a split in your pump to outlet rubber tube. Only way to confirm will be to drop the tank and check. Let us know what you come up with.