Fuel pressure drops during WOT

[Luckybob2485]

I have been dealing with a fuel pressure issue for a while. I replaced the fuel pump 5 days ago with a wally 255 hp. I have a 3 year old aeromotive fpr.
My car started idling low recently, but at idle the fuel pressure is set to 43 psi. I am currently boosing about 21 psi, and while going wot, the fuel pressure goes up to about 56 psi, then starts dropping back to the mid 40's while still under wot. I do not notice any vacuum leaks, and checked my vacuum hose to the fpr, and did not see any fuel. I disconnected the vacuum hose from the fpr, and the fuel pressure went up about 4-5 psi at idle. I will take my laptop out tomorrow to log injector duty cycle to see what they are at. I am thinking that it could possibly be a fuel pressure regulator problem, but im not too sure. All help is appreciated.

 

[Demon33]

The fuel pressure should rise when you remove the vacuum/boost reference hose. This is how you set your base fuel pressure (with the vacuum/boost hose removed). So your true base pressure is 43+5=48 PSI.

The FPR is supposed to raise the fuel pressure 1:1 as boost increases. You're seeing a maximum fuel pressure of only 56 PSI @ a boost pressure of 21 PSI when fuel pressure should be 48+21=69 PSI. Something is definitely wrong. Are you running lean under WOT? If so, I'd suspect either the AFPR, vacuum hose at AFPR or fuel pump as potential causes. If your AF ratio at WOT is still OK, the fuel pressure gauge may not be reading the pressure correctly.

Did the problems start after you replaced the fuel pump 5 days ago? If so, start your troubleshooting there. Was anything else changed or replaced?

 

[hisandherturbo]

First like stated above........ ensure pressure is really falling and the guage just isn't lying on ya.

Check voltage at WOT
DSMLink? if so, turn on the fuel pump, and do a boost leak test.......... ie see if fuel pressure goes up like that.

Joe

 

[crazyquik22023]

Most likely causes of dropping fuel pressure:
1. Bad fuel pump
2. Fuel pump needs to be rewired b/c it is getting low voltage etc.. try hard wiring it with 8AWG wire
3. Clogged fuel filter
check all of this first and see if one of these issues is the problem if not then check back

 

[Luckybob2485]

This started a couple months ago, and I thought the fuel pump was the reason, so i replaced that along with the fuel filter.

 

[Demon33]

This started a couple months ago, and I thought the fuel pump was the reason, so i replaced that along with the fuel filter.

So changing the fuel pump and filter did not improve the situation? Regardless of the fuel pressure being low, are you running lean or have your fuel trims (LTFT) changed towards the positive (+) direction? Has your gas mileage suddenly gone up? Any of these would suggest that you are in fact running lower fuel pressure than expected and running lean as a result. Do you have a wideband O2 to verify this?

If no adverse running conditions have occurred as a result of the supposed low pressure, I would suspect that the fuel pressure gauge is not reading correctly. If you have access to another gauge you could easily verify this by swapping them.

 

[Luckybob2485]

Here are a couple logs i just pulled. we have the timing put down low at wot to be safe. I was watching my wideband, and stayed at about 11.1:1 through wot. I know my front 02 sensor seems to be on the fritz. This also shows that my idle is pretty low. I checked and my fpr is hooked up to the intake manifold and the boost gauge, it is not hooked up to the boost controller. It seems that my injector duty cycle is kind of high as well. I turned the car off, and turned my fuel pump on via dsmlink, and the fuel pressure was up to about 50 psi.

 

[92DSMErdrick]

when you activate the fuel pump with the car off and your seeing 50psi your base pressure is too high. Going by the logs your 02 looks like its working and doesn't show any sign that your car is going lean at WOT. You can see 1.0v at WOT which indicates it's reading as rich as it can be. I also adjusted the props for your A/F Ratio and that shows down around high 9's low 10's which is rich.

Your injector duty cycle is also very high b/c you don't have your Mass air calibrated correctly. In the log the boostest is up to 28psi when you say your only doing 21psi, this makes the computer think your flowing more so its pushing the injectors more.

 

[ramsack]

for starters your fuel pressure should be 43.5 psi, not 50. you also should get a better feed line. how do you view log files people post?

 

[Demon33]

when you activate the fuel pump with the car off and your seeing 50psi your base pressure is too high. Going by the logs your 02 looks like its working and doesn't show any sign that your car is going lean at WOT. You can see 1.0v at WOT which indicates it's reading as rich as it can be. I also adjusted the props for your A/F Ratio and that shows down around high 9's low 10's which is rich.

Your injector duty cycle is also very high b/c you don't have your Mass air calibrated correctly. In the log the boostest is up to 28psi when you say your only doing 21psi, this makes the computer think your flowing more so its pushing the injectors more.

+1

Based on this and the fact that you're seeing 50 PSI at idle and only 56 PSI with 21 PSI boost, I'm fairly certain that your fuel pressure gauge is on the fritz. If the problem was due to your AFPR, you would definitely be running extremely lean at WOT.

Get your base fuel pressure set correctly (using a working gauge) and adjust the injector settings in Link accordingly before calibrating your MAF Comp sliders to get BoostEst inline with actual boost (boost gauge). From there, you can lean it out to get more power, reduce injector duty cycle and save some gas in the process. I tune for 11.0:1 to 11.5:1 (on 91 octane) depending on the load range I'm at.

for starters your fuel pressure should be 43.5 psi, not 50. you also should get a better feed line. how do you view log files people post?

With ECMLink, you can run higher than normal fuel pressure. You just have to adjust the fuel settings (in software) accordingly to account for the higher fuel pressure and flow. Higher fuel pressure can help squeeze a little bit more out of your injectors if you don't want to upgrande to a larger size. Of course, there is a limit to how high you can go; as boost increases, so does fuel pressure. So fuel pressure under boost needs to stay within the working limits of your fuel pump and system as a whole.

Why should he get a "better feed line"? Is there something with the 2G's that limits fuel flow through the stock lines? I have a 1G, so I'm just trying to understand.

 

[jeffo]

Does your AFPR diaphragm hold vacuum if you suck on the vacuum nipple? A torn diaphragm will leak down. Some AFPR's also leak from the adjustment nut, and need some teflon tape to seal it.
Do you have anything else tee'd into the vacuum line to the AFPR? I prefer NOT to tee the boost gauge or anything else into it, so the AFPR gets an accurate 1:1 reference.
Is your vacuum line to the AFPR still routed through the FPR solenoid? If so, you should try temporarily connecting a NEW piece of vacuum line from the AFPT directly to the vacuum nipple on the intake manifold.

 

[hisandherturbo]

If he would do what I said a while ago............ turn on fuel pump, boost pressure test.......... all these other questions would be easily answered!!!! Set base pressure (that should have been done, before enen asking anything), then turn on the fuel pump, then add pressure, look at both fuel pressure guage and boost gauge and do the math............. that would answer sooooooooooooo many questions!!!

 

[Luckybob2485]

If he would do what I said a while ago............ turn on fuel pump, boost pressure test.......... all these other questions would be easily answered!!!! Set base pressure (that should have been done, before enen asking anything), then turn on the fuel pump, then add pressure, look at both fuel pressure guage and boost gauge and do the math............. that would answer sooooooooooooo many questions!!!

I do not have a boost leak tester, but i will get one this weekend and will do exactly what you have mentioned.

 

[hisandherturbo]

I just realized you were in colorado.............. come on down to the springs...........I'll help ya

 

[Luckybob2485]

I would come down there in a heartbeat, but I am currently in junction for school.

 

[hisandherturbo]

Just order a "intake leak tester" here................

Intake Leak Pressure Tester

Local good guy!!!

I use this, the nice thing about it, as your needs change, just change to another reducer coupler

 

[ramsack]

Why should he get a "better feed line"? Is there something with the 2G's that limits fuel flow through the stock lines? I have a 1G, so I'm just trying to understand.

it never hurts to upgrade to a -6 AN for running injectors that big. some people do, some don't. stock line is 5/16" OD pipe. i don't know the wall thickness, but if it's .049 which is thin, then ID is 0.215". 3/8" pipe (-6) with .049 wall is 0.277. you have a 23.5 mm² vs. 38.9 mm² areas of the ID of the pipe. this will reduce pressure drop and make the pump work easier, and flow a lot more gas.