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I'm about to replace an injector and wanted baseline info on fuel pressure before doing the work. Using a gauge tapped into the top of an aluminum alloy fuel filter cap (from bulletproof diesel), I turned the switch key on but did not start the engine.
Static pressure jumped to 60psi, but as soon as the pump cut off the pressure dropped off to zero. Again, engine not started. I cycled the key off and back on and the pressure hit 60psi and held that until the pump cycled off, then dropped back to zero.
I'd like to get a few confirmations that fuel pressure should or should not fall off when the pump kicks off.
Side note, during the crank process the pressure jumps and holds at 60psi and stays at 60psi while idling. I have not conducted a dynamic road test yet. Didn't have the adapter to connect my long hose...ugh.
No question is dumb!! If you don't know then it needs to be asked!!
I did the same thing the other day. It is normal and 60 PSI is good. What you will need to know is your PSI at WOT (Wide Open Throttle) under a load once you are done with your repairs. If it dips too low at WOT then there's another issue and that will need to be addressed.
I have my fuel pressure gauge tapped into the fitting at the front/bottom of the fuel filter housing. Mine jumps to 70 and stays there. Cant say I've watched it for a long time but as long as the key is on mine stays up. I dont know if the same thing happens when taken from the top of the fuel filter housing so hopefully some can add to this.
Since your idle is staying constant I would be more concerned about getting your pressure at full throttle. I think thats where they tend to drop off most often
That's normal. As other's have said pressure under load is what counts. Also pressure from the cap is not pressure after the valve, so at the least it needs to be measured at the test port at the bottom of the filter housing. The best place to test is at the back of the cylinder heads.
I finally ran the WOT test with fuel gauge coming off the top of the fuel filter housing. I taped it to the windshield like most other did on you tube videos.
At idle it maintained 58-59psi. From about 30mph I held WOT until about 90mph with a very small increase in grade for some of it. fuel pressure dropped slowly to about 54 Psi and held there until I let off. Then as soon as throttle was off it rose back to 58-59psi. One other note, the needle never waivered or vibrated. It was steady and never jumped or bounced.
This sounds like good results to me. Any other feedback?
Yes. There is one, but I didn't use it. Instead I purchased an alloy replacement fuel filter cap from BulletProof Diesel (and there are others as well) that came with 1/8" hole in the top and a 1/8" plug that is set in with a hex wrench. I just bought adapters at Lowe's and connected a hose to a fuel pressure gauge. There are numerous videos on You tube about this.
There is an expensive special port adapter if you want to test the fuel off the front port. I'm told if you buy that special adapter then you can do the test at idle, but I used the cap option and just drove it. When I was done I just put the plug in the top of the cap, tightened it down and I was done.
The difference is, when testing from the filter cap, you're seeing fuel pressure before the valve that's under the filter. The injectors are on the other side of the filter and the valve.
If the filter gets gummed up, or the valve doesn't open properly which I have personally seen, you will show normal pressure at the cap while pressure at the injector is zero (again, I've personally seen this).
The front port tests after the valve, so it will respond to filter and valve problems.
my apologies. is this the adapter that will allow for dynamic fuel pressure test without having to drive the vehicle?
No, this is not an orifice. You're referring to the adapter kit Ford uses for testing fuel pressure at a simulated wide open throttle under load. I believe that adapter tester has a .040" orifice that when in use it gets bled off and returned to the tank. If the pressure maintains 45psi or more it passes.
The adapter I posted is for adding a gauge or using a pressure tester that I'd 1/8 npt fitting.
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