idle fuel pressure inconsistant

[MyBeatGSX]

I have 2 questions... first.. I have an Aeromotive FPR and 255HP pump, the stock FP solenoid is disconnected. Base pressure is set at 40psi.

When starting the car cold (even on a warm day), base fuel pressure is 45psi and 40psi with vacuum applied.WTF

When it warms up, base fuel pressure drops to 40psi (where I have it set) and with vacuum applied about 35-36psi (20in/hg). This is what should be normal.

After a long run down the highway or driving for awhile until everything is really hot, base fuel pressure is still 40psi but with vacuum applied (still 20in/hg) pressure is down to 32-33psi. WTF

Is ANY of this normal?? I was under the impression that with the FP solenoid disconnected base pressure should ALWAYS be what its set at (40psi for me). And pressure under vacuum should be consistant, if its 35psi at 20in/hg it should be that WHENEVER its at 20in/hg.

Also, and this question goes along with the first... When you disconnect/pinch the vacuum referance line, how long should it take to hit base pressure? Mine slowly climbs up for 2-3 seconds, I was under the impression that especially with a 255 it should hit it almost instantly. And FYI, on the dyno fuel pressures are consistant and pressure increases in a timely manner (it doesn't lag behind the boost). I have NO fuel leaks, the system holds 40psi for at least 5-10 minutes after the engine is shut off and holds some level of pressure for a half hour - 45 minutes. My fuel filter is brand new, the injectors are brand new (like that would matter), and pump is about 3-4 years old but has less than 25k on it.

 

[DSM90AWD]

When starting the car cold (even on a warm day), base fuel pressure is 45psi and 40psi with vacuum applied.WTF

Just to be clear. Base Fuel Pressure is what the FPR is set at with no vacuume/boost reference. So in the above you're saying with no vacuume/boost reference you'd set the base FP to 40psi, but now (only on cold starts) it shoots to 45psi with no vacuume/boost reference?

Another clairification, did you completely remove the FP Solunoid from the vacuume routing to the Aeromotive vacuume/boost reference or did you simply unplug it?

If the answer is yes to both, you may want to pressure test the line to the Aeromotive AFPR vaccume/boost reference. Mine had a pretty boost leak on both the reference nipple and the top adjustment screw which was fixed with a bit of thread sealant

 

[Goobdog]

Another question is, do you have a liquid filled gauge on your FPR ?

 

[MyBeatGSX]

Yes to all of those. "Base" means no vacuum referance. The FP solenoid is hanging off in its own world, plugged in electrically but has NO vacuum lines going to it.

Both fittings are thread sealed, all lines are secured with zip ties. Yes I have a liquid filled gauge.

 

[HIGHPSI4]

Get rid of the liquid filled gauge.
If you do not have any vaccum/boost leaks then check for constant power to the pump. Also check that the pump is good and not starting to go bad.

 

[MyBeatGSX]

WHY would I want to get rid of the liquid filled gauge? You're supposed to have those for under the hood stuff, it keeps it safe from the vibration and accurate.

How can I check the pump? I don't have a multi meter or anything like that so I can't check power to the pump. Does how quickly it gets up to pressure show anything? That's why I asked before.

 

[GVR4592]

Is your fuel pump rewired? It may seem like a stupid question but I had a similar issue a few years ago, I rewired the pump and it went away. Why are you setting your base pressure at 40 psi?

 

[mirkoelek]

It sounds you've got the prob with your fuel pump. Rewire your pump (properly), get some good cables, solder....

 

[901ggsx]

Get rid of the liquid filled gauge then check your hoses for lecks

 

[MyBeatGSX]

Is your fuel pump rewired? It may seem like a stupid question but I had a similar issue a few years ago, I rewired the pump and it went away. Why are you setting your base pressure at 40 psi?

Honestly I don't even know if its rewired. The previous owner had a DSM shop install it, so I'm assuming it would be... who doesn't rewire a 255?? Its also relatively quiet, which I've heard most people with correctly rewired 255's say theirs are (or I've heard people say rewiring it cured their whine). Do you have a link to a fuel pump rewire how to? I don't even know how to get at it.

2G base fuel pressure is supposed to be something like ~42.5psi correct? My AFC is already set as high as -32% for the 6.5k and 7k points. I don't want to bump up pressure and have to go lower, that's more timing advance than I'd like. The A/F is fine how it is and the car runs well, why mess with it?

 

[GVR4592]

43.5 psi is stock for a 2g, I was just curious, it's probably not hurting anything. I would verify that the fuel pump is rewired, most people don't do it.

 

[GVR4592]

Why is everybody saying to get rid of the liquid filled gauge? If you are going to say something like that at least back it up with some facts. Is there a specific reason why liquid filled gauges shouldn't be used?

 

[steve]

Why is everybody saying to get rid of the liquid filled gauge?

Because we've seen problems with them heating up, the fluid expanding, creating a pressure change inside and throwing off the reading. The gauge reads lower as it heats up.

Steve

 

[DSM90AWD]

Have you removed your Emissions Vapor canister and/or plugged the vapor line that connects from the fuel tank to the canister?

Excess pressure in the tank can cause a restriction to the fuel return and artificially bump pressure upstream.

 

[daren_p]

Because we've seen problems with them heating up, the fluid expanding, creating a pressure change inside and throwing off the reading. The gauge reads lower as it heats up.

Steve

The gauge doesn't have to be liquid filled for this to happen, it can happen to a dry gauge as well. A pressure or vacuum can be created inside the case because of changes in temperature between outside & inside the gauge. With both dry & liquid filled you should pull the plug on the top before you adjust the fp to vent the gauge to atmosphere, to releave any pressure or vacuum built up inside the case. The advantage of the dry gauge is you can poke a hole in the plug so it is always vented, while if you do this with a filled gauge you will usually end up with a mess.

 

[MyBeatGSX]

That's some very interesting stuff. I think that heat thing probably explains it. Because it does read slightly lower when warm.

And the lack of EVAP might explain the higher pressure cold starts, the tank could have lots of pressure built up after the fuel evaporating all night. But would that bump it up 5psi?? Oddly enough, a few months ago out of no where I just stopped getting the rush of gas vapor when opening the cap... I'm sure that's a whole nother issue.

So how do those theories sound? I'm still going to check the pump as soon as someone post a link to a fuel pump rewire or at least how to get to it.

 

[oldman]

Have you removed your Emissions Vapor canister and/or plugged the vapor line that connects from the fuel tank to the canister?

Excess pressure in the tank can cause a restriction to the fuel return and artificially bump pressure upstream.

Yes, I remember that thread as well.

 

[mirkoelek]

Yes, I remember that thread as well.

Yep, a have a lot of air and pressure coming out every time i open my gas tank cap....

i removed my EGR completely and Emissions Vapor canister ... is this dangerous

 

[oldman]

Yep, a have a lot of air and pressure coming out every time i open my gas tank cap....

Is your afpr able to pull pressure down to where it should be at idle?

 

[snox135]

http://www.raktron.com/fuel/rewire1.html

That is for 2g fwd fuel pump rewire, should be basically the same for awd.

 

[mirkoelek]

Is your afpr able to pull pressure down to where it should be at idle?

I didnt look at it... I'll check it, and I'll back to you...
thanks

 

[MyBeatGSX]

^Yea let me know how that turns out.

I'm going to check/do the rewire this weekend, we'll see how that goes.