Fuel Pressure Consistency

[nufoundglry5]

Hey Guys,

I've searched and haven't came up with much on this topic. My problem is that my fuel pressure is never consistent. I have an Aeromotive 13105 (the compact one) and a Marshall Liquid filled guage along with a rewired 255 Walbro fuel pump. It seems like just about every day i check my fuel pressure it is different than what i set it at. I set my base pressure at 37 psi (with the vacuum source removed) and i will drive somewhere and get out to check, remove the vacuum source and it will be at 39 or 40. Then if i let the car sit overnight, I will start it up and check the pressure (again with the vacuum source removed) and it will be at something like 43. It has no consistency in how it does it besides that it is always higher when i first start the car. Somewhere i heard that some of the liquid filled guages change readings at different temps (which sounds like a total fluke) but anyway if they do at what temp would i set the pressure at. Also i have checked my fuel pump O-ring and it is good. I have a new OEM fuel filter and a -6 AN fuel feed line. If someone could help I would greatly appreciate it.

Thanks in advance.

 

[DSMunknown]

I believe the fuel pressure is supposed to be higher at cold start up. Try adjusting the AFPR after you've driven the car around for a while (so the engine is nicely warmed up). Also do this. Make sure you don't have any boost leaks.

 

[nufoundglry5]

I guess i dont understand how the engine being heated up would change the pressure. If maybe it has something to do with the fuel pressure solenoid... i removed it when i put in my new regulator. But anyway, I do set the pressure when it is hot and it still doesn't stay consistent at all. Thanks for the link though, that is a good idea.

 

[DSMunknown]

If maybe it has something to do with the fuel pressure solenoid... i removed it when i put in my new regulator.

Why would you do that?

 

[thekellbeast]

Why would you do that?

With higher flowing/pressure fuel pumps, it's not needed to bump up pressure on startup. The better pumps don't need the help.

But there really is no reason to remove it, except to clean up your firewall.

 

[nufoundglry5]

According to what I read across this forum and DSMtalk, the fuel pressure solenoid is only used on hot starts and there are usually no side-effects of removal. And by removing it, i didn't have to look at the ugly thing (and the 2 other solenoids that were on the same bracket) anymore .

 

[viprez586]

According to what I read across this forum and DSMtalk, the fuel pressure solenoid is only used on hot starts and there are usually no side-effects of removal. And by removing it, i didn't have to look at the ugly thing (and the 2 other solenoids that were on the same bracket) anymore .

Don't complain when your car won't start in the winter.

 

[nufoundglry5]

I don't drive my car in the winter.

 

[FORMONTOYA]

According to what I read across this forum and DSMtalk, the fuel pressure solenoid is only used on hot starts and there are usually no side-effects of removal. And by removing it, i didn't have to look at the ugly thing (and the 2 other solenoids that were on the same bracket) anymore .

Correct. The FPS is for "hot-starts", similar to driving around in hot-a$$ Houston summers all afternoon then pulling into the store for something to drink, come out and try to re-start the car. The FPS opens to atmosphere to allow the FPR to bump up the fuel pressure in order to combat vapor lock. Cold weather starts are not effected by this solenoid. And yes, an upgraded pump will also help to combat this issue, however, since it is very hot in the summer here in Texas, I still run the FPS on my car for extra insurance on those super hot days (although down here we usually store our cars in August and September since the days usually are over 100*F).

 

[nufoundglry5]

Thanks for the info on the FPS... but does anybody have any ideas about the constant pressure change? Has anybody experienced this problem previously? Thanks for the help.

 

[FORMONTOYA]

The only time I've ever had the FPR NOT act as I expected was when I accidentally kinked the vapor line coming off the tank. My symptom was that with the hose off I would be set at 44PSI, but with the hose on at idle I would only drop down to 38-39PSI, not the 33-34PSI I would expect.

Maybe something has been kinked or otherwise restricted in the return line or in the vent system of the tank.

 

[nufoundglry5]

Thanks, I guess I'll have to check the vent hose and return line for damage or kinks. Is the vent line the line that runs into the charcoal canister? If it is, I have removed the canister from the car and that hard line is left open.

 

[thekellbeast]

I just came across this post, and I was wondering about my problem, and if it's related. I recently removed all of my emissions stuff, and I capped the charcoal cannister line where it comes out of the firewall. I'm getting inconsistent idle - it keeps raising as the car warms up, on hot days. Could this be a fuel pressure issue, related to the vent line being capped? I'm just thinking that I have a similar problem to the OP, but I don't have an AFPR (yet) and I don't have a fuel pressure gauge either.

 

[Talesin]

You mean that you capped the EVAP line from the fuel tank to the charcoal cannister? The one that relieves excess pressure in the fuel tank? The one that most people when removing the charcoal cannister leave open under the car, and suffer with the fuel vapor loss (and associated reduction in mpg), so the tank doesn't over-pressurize?

 

[Zenja]

When I had the same problems and thought that the FPR I just bought was broken, I found out after some time that it's just the gauge. Fuel pressure was pretty random and hard to adjust. But then I tried tapping the gauge a few times and it moved a whole bunch. I don't know if this can happen to liquid filled gauges, but try it anyway. Hit the gauge lightly a few times with your finger to try to get the needle to unstick and go to the true position.

 

[nufoundglry5]

Thanks for tip about the guage. I'm gonna go try that actually. If that doesnt do anything, I'm just going to repalce my feed line from the tank and the return line and also get a different guage.

 

[dsm-onster]

I have that same Aeromotive FPR, I *believe*. And the same liquid filled gauge.

I have another port I designed and tapped at another point in the fuel lines and checked a known acurate fuel pressure agains the aeromotive gauge this past week (vacuume disconnected). The gauge would drop after I set it at 37 psi. So I would reset the pressure w/ the regulator. Then it would drop again. I looked at the known accurate gauge and my pressure was not actually dropping. But instead it showed over 50 psi base! I had been driving her around like this fore some time before I thought to check the accuracy of the kit's gauge. I have to see 30psi on the liquid filled kit gauge for the good gauge to show 37.5 psi.

I havn't had a chance to check the tap source on the regulator. I will swap gauge locations to see if there is a "feed" problem at the regulator. . .