Fuel Pressure Sensor Mounting Location

[Tennpenn83]

I have some questions regarding the location for mounting the fuel pressure gauge.

In this thread, http://www.redlineforums.com/forums/...-location.html, Eric came up with a pretty smart idea about the fuel pressure sensor (AEM in particular) being mounted on the end of the fuel rail using 0TTP's fuel rail fitting, and an adapter from any home improvement store to mate the two together.

This works pretty well, and after a couple times of going back and fixing leaks, and applying teflon tape and whatnot, there are no issues with install.

The issue I am having is a consistent fuel pressure. This is baffling me, and I hope someone can give some insight.

When the car is first turned on, but the motor is still off, the fuel pump turns on like it is supposed to. It shuts off after a few seconds, as it is designed to do when the motor is not running. The fuel pressure gauge shows around 40 psi.

Upon initial startup, the fuel pressure reads 50-52, dancing around the numbers, not staying at any one number for more than a split second.

When driving, when the car warms up, the fuel pressure drops. When fully warmed up, the fuel pressure reads in the mid 30s, and when sitting and idling, the pressure drops in the 20s or below!

Obviously I have checked for leaks, and checked as many things as I could, but this is the same case every time I drive. There are no leaks at all, I am sure. I have never touched the fuel pump, and the only modifications I have made to the fuel system are a new fuel filter, the 79 # injectors, and this sensor at the end of the fuel rail.

Is it at all possible that the fuel pressure really does drop like this? It doesn't seem likely. Is it possible that when the fuel heats up, it messes with the pressure? Is it possible that when the motor heats up, it transfers heat to the fuel rail, which in turn transfers heat to the sensor itself, causing it to read incorrectly? I need to get this figured out before the installation of my ZZP BRFPS, as fuel pressure is directly affected with this system.

Thanks!

[Tennpenn83]

Also, I forgot to mention, that while I think this is heat related and not actually a loss in pressure, I went WOT while keeping a strict eye on IDC's, which did not have any difference than before the sensor install. They are around 80-85%, just like before. If there was a loss in pressure, it would definitely show on the IDC's, as they would go skyrocketing as soon as there was a drop in pressure.

Also, the wideband is showing a consistent 11.9 AFR, and is not varying any more than normal. If fuel pressure was dropping to half of what it is supposed to be, then the AFR would go extremely lean, and I would likely blow up.

[The_cobalt_kid]

interesting for sure.... LSJ right?

IIRC Rob said that most GM stuff runs ~60 PSI stock fuel pressure... that would make sense for it being a heat issue, as your reading close to that at INITIAL start up when its running but cold.... i by no means know if that is the issue but it sounds plausable to me.

[Tennpenn83]

Yep, LSJ. Stock fuel pressure is either 58psi or 60psi as you said. I've heard both ways. Another thing, when I first got this gauge, it did read 60-ish psi for a little while, then started dropping like stated above. Ever since then, it's been doing what I have described.

[The_cobalt_kid]

maybe the heat killed it.... did it have warnings about heat?

[Tennpenn83]

There is nothing written in the installation manual about heat. It is a fluid pressure gauge from AEM, which can read either fuel or oil pressure.

Here is a link to the installation manual. http://www.aemelectronics.com/Images...%2030-4401.pdf

Here is a link to the gauge itself. http://www.aemelectronics.com/fuel-p...to-100-psi-761

[Jn2]

as already posted,,,you should be seeing close to 60...my l61 was always @60psi with little variance between idle and WOT was like 59 or 60...

[The_cobalt_kid]

as already posted,,,you should be seeing close to 60...my l61 was always @60psi with little variance between idle and WOT was like 59 or 60...

where did u have it mounted?

[laser_racer]

Would you think a air pocket in that line running to the gauge would effect it? On my eclipse I run my fuel pressure sensor off the return side of the rail. You ever think of trying to use the test port on the rail instead? It is a an fitting there.

[Tennpenn83]

Would you think a air pocket in that line running to the gauge would effect it? On my eclipse I run my fuel pressure sensor off the return side of the rail. You ever think of trying to use the test port on the rail instead? It is a an fitting there.

As for an air pocket, here is what I am thinking, and please correct me if I am wrong.

As the pressure in the lines goes up, the air itself will compress. (Since fluids do not compress and air/gases do)... The fuel pressure should go up to 58psi, which will compress the air in the lines to a certain point, but the air should still be compressed to the same PSI. If the air was any less, the fluid would have more of an effect on it, and put more pressure on the air, compressing it further, and making it the same psi as the fluid, causing equilibrium. Am I wrong?

Besides, wouldn't air on the fuel rail eventually be absorbed into the fuel and run through the fuel injectors?

[laser_racer]

I am actually not sure if the air would just absorb or not in to the fuel. How long is the hose to your sensor? Also air does mess with how curtain gauges work. Let me go find me a gauge tonight and check my rail pressure on the Redline to see what I get at idle.

[Jn2]

where did u have it mounted?

fuel rail, i had a little valve there, screwed in a 90* adapter to be able to screw in the fuel psi sensor...than ran the gauge wires like normal, the LSJ should have one on the hard lines somewhere, this is was to the right, one of the 2 metal lines that come out the fuel rail has it

[Tennpenn83]

I am actually not sure if the air would just absorb or not in to the fuel. How long is the hose to your sensor? Also air does mess with how curtain gauges work. Let me go find me a gauge tonight and check my rail pressure on the Redline to see what I get at idle.

it is mounted on the end of the fuel rail, no hose or line running to the sensor as it is in direct contact with the rail

[laser_racer]

That's interesting maybe car is commanding less press once it's hot. Has anyone reported any problems like that with that gauge?

[Tennpenn83]

not that i have seen. i don't think too many have done a fuel pressure gauge anyways, but i believe most who have get an analog one. maybe the heat is messing up the electronics in the sensor, or something like that, but i haven't found anything like that on aem's forums

and i believe the fuel pressure is supposed to be a constant 58psi, no variation

[Darkmanx]

nice

[laser_racer]

I can't say I have ever ran in to a fuel pressure that moved around like that either but I am running a analog on my dsm. I wouldn't think the heat would mess a sensor up like that. Can you log it in hp tuner?

[Darkmanx]

go on the dyno and use a mechanical guage

[Tennpenn83]

i think that would be best. i can't log it in hpt yet since i haven't figured that PID out yet, but it should be reading the same since it gets the input voltage from what the gauge is displaying

[Tennpenn83]

update:

big thanks to josh (billabong9687) for making himself available tonight for some troubleshooting. he had a fuel tester that connected to the schrader valve, on the fuel line under the fuel rail cover. while my gauge was reading 15psi with the motor off, key forward, the schrader valve was reading 58psi, right where it is supposed to be. startup, idle and revving it was the same thing

it appears to be the gauge. i'm going to contact AEM tomorrow and see what they have to say.