Tracing Fuel Pressure Loss

[jas280z]

My '78 280z with stock EFI does not keep fuel pressure overnight, and I have been experiencing fuel vaporization problems upon hot starts. I believe these two issues are related, so I am working to trace where the fuel pressure is escaping.

 

Last night I ran the car for about 30 seconds to put it in its night time parking spot. I had my dad clamp off the FPR return line as soon as I shut it off. When I checked the fuel pressure this morning it had dropped to 0 overnight. This leads me to believe that either the fuel pump check valve or the injectors are leaking.

 

Today I repeated the test, only this time I clamped the line that leads from the pump right before the fuel filter. After 2.5 hours the pressure had dropped from 36 lbs to 28 lbs. This tells me either the FPR or the injectors are leaking. As soon as I removed the clamp the pressure dropped again to 20 lbs. It would seem that the pressure before the clamp dropped as well, and the two pressures equalized when I removed the clamp. So it seems that I have at least 2 culprits, the check valve, and either the FPR, the injectors, or both.

 

I am about to run a third test, this time clamping both the supply and return lines where I previously clamped for the last two test. I believe that if I still lose pressure that would mean that the injectors are leaking as well.

 

Pretty much I am wondering if my tests are rationale are correct.

 

I have a spare FPR I can put on if it comes back that it is indeed my FPR that is leaking, I was also considering putting an inline check valve in between the pump and filter to supersede the pump check valve which also seems to be leaking. I am thinking about this one from Summit Racing http://www.summitracing.com/parts/CRT-169-1002/ , after seeing that someone on Zcar.com is using it with no problems.

[jas280z]

I clamped off the supply line right before the fuel filter and the return line right after the FPR. Within an hour the pressure had dropped to 0. I guess that at least one injector or the csv is leaking as well. I was planning on having my injectors cleaned and tested, I was just hoping to wait until winter to do so, but I guess I may as well do it now.

 

Next I will try clamping off the supply line right before the filter, the FPR return line, and the csv, to isolate the injectors.I am assuming that the pressure will drop, but I may as well check to make sure.

 

I think I will also run another test in which I will clamp off the supply line after the fuel filter and pressure gauge. The purpose of this test will be to isolate the check valve, to see once and for all if that is leaking.

 

So far all I have been able to determine with some degree of certainty is that I have more than one component allowing pressure to escape. My main suspects are the check valve and the injectors/csv. Unfortunately I cannot think of any way to isolate the FPR and still get a reading from my pressure gauge so I cannot tell for sure if that is leaking, but with the amount of crud that was in the fuel lines when I bought the car I wouldn't be surprised if that is bad as well.

[2eighTZ4me]

Your fuel system should not hold pressure overnight. Think about it - it's a return style system. What fuel doesn't get used gets shipped back to the tank. This is perfectly normal. Try your second test again (clamp the FPR return and also right before the filter) and let it sit til you drop pressure. THEN pull your plugs and spin it over and see if you have any fuel squirting out of the plug holes. That'll show you which - if any injectors are leaking. I'd lean toward your check valve as being the culprit, as they have been known to go bad. Other than that - you've got a problem elsewhere. I thought I had a vapor lock issue - turns out it was a clogged filter.

 

With 36psi of fuel pressure, you should not ever vapor lock. That happens (mostly) on low pressure carb systems where the fuel goes through the system very slowly, under low pressure, and has time to percolate. Even if your check valve was bad, the stock pump builds enough pressure quick enough to fill that fuel system in a matter of a second or two. Don't believe me? Pull the fuel line off the fuel rail and start cranking the car. It'll shoot out of there like a fire hose. You have a problem elsewhere. What do your plugs look like?

 

Pretty car BTW!

[jas280z]

According to what I have read, "... the check valve and FPR should hold pressue ROCK STEADY OVERNIGHT AT LEAST -TonyD" Here is the original post, http://www.zcar.com/forums/883144/vapor-lock-heat-soak-voodoo-whatever-you-wanna-call-it .

 

If I understand the problem correctly the vaporization problem happens because some of the pressure is escaping, allowing the remaining fuel to vaporize. This remaining fuel remains at a somewhat steady pressure, making it appear as if 30 odd pounds of liquid pressure exist, when it is really vapor causing that pressure.

 

It is a return style system, but it appears as if the FPR is designed to bleed off only enough pressure to keep the rail at a designated pressure. Once the car is shut off it should not bleed off any more pressure under normal circumstances, as there should be no vacuum going to it. Again this is my understanding of the problem, which may or may not be correct. Also it seems that if there is not a priming of the pump before the injectors fire the sudden loss of additional pressure can cause a flash vaporization of the entire rail screwing you even more. I am fairly certain that the pump priming was not instituted until the 280zx came out, and may be compounding my problems.

 

Thank you for your help, this problem seems to be relatively common, but unfortunately commonly misunderstood as well. I have been digging through quite a few old posts to see what I can learn about it and its many names. Thank you for the compliment on my car as well, it has taken a LOT of work to get to this point, hopefully only a few more issues to iron out until it is "finished" (it never really will be though ).