h2kfrosty and I were talking about his fuel setup and how in 4th gear he will run lean on top end and start to loose fuel pressure when low on fuel. I am not sure why that is or what exactly causes it but I have been trying to search and figure out what we can do about it or if why this happens. I searched and was only able to find one post specifically talking about this. http://forums.maxima.org/showthread....lbro+fuel+pump

here is what I can hypothesize: (sorry I had to throw that word in)

1) The wiring is not giving sufficient voltage to the pump and thus not enough fuel. Some have actually hard wired their pumps and used resistors and other wiring schemes to increase voltage to get additional fuel. Write up of how to do this for a 3000gt/steath http://www.team3s.com/~egross/3S/Mods/TT/FPRewire/

2) Something wrong with the OEM FPR, or FMU. I wouldn't rule it out, but others seem more likely. BTW he had the OEM FPR and 270cc 5th gen injectors with a Vortech FMU.

3) They are simply more sensitive to low fuel levels.

3) Bad batch of or some inherent problem with the Walbro GSS342. This is frosty's assumption. This is a very possible cause.

 

I'm not a big fan of the FMU's but if there's a vaccume line going to it pull that off and do a leak down test with a vaccume pump to see if it's bleeding down. My brand new Aeromotive did!!!! And most do if there's not a small dab of RTV or a nylon washer under the steel one provided by the maker.

That would be my first test.
Secondly, you could "hot wire" the pump by putting a relay back close to the tank. It's a comon thing to do to provide more amps to the pump. GN guy's do this all the time. There is also a product made that increases pump voltage under boost. (it's new) I think aeromotive makes it?

Change the fuel filter if that has not been done yet.

Doubt if the pump is good your going to max out it's capibilitys with 240cc injectors. Check to make sure the pump is properly installed at the factory height and it's not too low sucking the bottom of the tank.
It's just got to be somthing basic! Start there.
Scott~

 

I have seen the same behavior with two different Walbro GSS342 fuel pumps. They will hold pressure if the fp doesn't rise above about 65-70 psi, but anything higher than that the fp will bleed downward after hitting a peak. I haven't determined if the problem is with the fmu and not the pump.

 

Wow.. WTF happens about 4G's? How low did it go? Yikes! Double check like I stated above with a vaccume pump to see if it holds 30in of vaccume. If it does then it's not the FMU. That whole FPR and FMU all combined seems like it'd fight them selves to death trying to work together. But I guess others have had it work for them. Proper Injector sizeing and Fuel pressure along with a program to edit fuel/timing curves is manditory. FMU in my openion is a crutch that i'd rather not rely on.
~Scott

 

What SM said, what kind of fuel pressures is he seeing in 4th gear when it starts to go lean?

 

Well I don't have a fuel pressure gauge, and I've put less than 500 miles on the car since February. :x

 

I also have the same issue and my only suspicion is the pump.
Another problem I have is when I shut off the engine (even if its a trip to the store) and I dont let it sit for atleast 3 hours, itll have a hard time starting. Atleast a 2-3 sec delay in fuel. Does anyone else have this problem?

 

I had the same problem on stock injectors when i was hitting 90+ fuel press. It was fine in the first couple of gears as far as maintaining fuel press., but when I got into higher gears and sustained this high fuel pressure for longer times, the pump output definately decreased. For me, it was at the top of 3rd gear. My solution to this was to lower fuel press to 45 psi peak with 555cc injectors. BTW, I had the same problem with my old walbro 255lph as well. Looks like flow rate is crap at higher pressures.

 

He is supposed to be getting much bigger injectors soon and have lower fuel pressure. I would also hope he will get rid of the FMU. I think this will help out. He could have the non hp 255lhp. I don't know whether he has one or the other but I am not sure if he would want to go into his tank unless he was going to change it. He also needs a good fuel pressure gauge. Somehow he never installed one, even with this problem. However he would always try run at least 1/2 a tank whenever he was boosting just to be safe.

I was just wondering if it is just running it with high fuel pressure and stock injectors or if those with bigger injectors and lower pressure had the same problem. My best guess is that the fuel is moving toward the back of the tank and the pump can't keep up at such a high pressure when the tank is low.

 

The issue with the Walbro pumps has been discussed several times in the past. As I stated earlier the Walbro really needs to be slightly modified to correctly seal itself. There are two tiny plastic towers on the top of the pump that need to be trimmed to clear the fuel pump/filter housing (in 5th gens at least). If this isn't done the outlet spout wont go far enough into the filter and won't seal correctly. The pump is also physically taller than the OE unit which adds even more difficulty to the install.


You also have to be very careful not to trim too much off the towers, if you do the brush tension springs will pop out. That error is a PITA to fix.

But, that is for the 5th gen setup. I don't know exactly what the 4th gen cartridge looks like but it could be a similar problem.

 

I just recently replaced my Walbro pump, and I took a good look at what you are talking about. Apparently this problem is limited to 5th gens, there is no interference and no trimming needed with the Walbro pump in a 4th gen.

Also, I agree that the fuel pressure drop off appears to be a problem in higher gears only, and I also never saw it until I adjusted the Cartech fmu to give me more than 70 psi. Fortunately I don't need that much fuel pressure to have an afr in the 11's, but I agree that larger fuel injectors and lower fuel pressure is the answer to the problem.

 

I found on my Walbro that simply the pump was installed incorrectly and the mesh filter wasn't pressed all the way down to the bottom of the tank.

When low on gas, FP would drop because simply the pump didn't reach that far down.

I got in there and fixed it then it was fine.