fuel pressure erratic

[BBaker]

Does everyones's fuel pressure readings jump around, or is it just me? I have SPA gauges and my fuel pressure readings jump from 6 to 14 lbs. and everywhere in between. Even at idle it jumps 4 lbs back and forth. I am on my fourth lift pump setup now and think these wierd readings have something to do with it. I've had the high output Carter, a Summit/Carter two pump setup, and now I am running a Aeromotive single pump at the tank with a high volume filter and a bypass regulator and my pressure readings have always bounced around like crazy. Could it be the return valve at the VP44? Any help would be tremendous.

 

[JJMiller]

I had what sounds like a very similar problem. Fuel pressures bouncing all over the place under steady throttle, but mine returned to "normal" at idle. I tested the sender, it was fine. In the process I re-plumbed the setup, and I havn't had the problem since. The only thing I can think of that might have caused the problem was either air in the line or teflon tape. (I pulled some of that out from places where it shouldn't have been when I took it apart).



Since you have the problem at idle, put a guage on the factory schrader valves to see if it's a problem with your guage setup or with the fuel system. If it's your gauge setup, check your wiring, then check the sender. I suspect you'll find the problem somewhere in there, since you've had the problem with all your fuel pump setups.



I would be suprised if it is the return valve on the VP44.



--J

 

[Bigsaint]

Change the SPA settings

If it has been doing this since you put in the Aeromotive pump, at least you know it's probably not the pump. You can change the avg. sample rate displayed on the SPA gauge from it's factory setting to a somewhat longer delay. The SPA always samples the psi's at a pre-determined interval, but you can delay the rate that the screen refreshes to suit your needs. I don't have my SPA manual home tonight (it's in the truck, which I left at work,DAMN!!!), but I changed my display rate from the factory setting (several times per second if I recall correctly) to a display refresh rate of once every 2 seconds. All you are doing is then averaging the sample rate for every 2 seconds and then displaying same, which worked for me. I didn't have a problem with the SPA factory setting when my sender was pre-filter, but when I moved the sender to post filter, it would constantly change +/- 2 psi's. Speaking of sender location, the SPA doesn't like the direct approach on the inlet of the VP44 too much, you will see much more stable readings from the bottom of the fuel filter canister, according to my SPA mentor, Brandon. Hope this helps. If you don't have your SPA manual handy, let me know and I'll hook you up with the directions on Tuesday night, when I get my truck home. Which Aeromotive Pump are you running? I was thinking of trying the 'Street rod' 150 GPH pump next if my second Mallory dies.



Oh yeah, by the way, WELCOME ABOARD



Scott W.

 

[Todd T]

you da man Big Saint

I was going to suggest the same thing.



and also ask where his sending unit was located. if its ON the engine (not on a bracket) it will behave this way too.

 

[BBaker]

Thanks for your replies. I will adjust the SPA gauge tomorrow, but the fluctuation still bothers me. I am running the 150GPH Hot Rod unit. The fuel pressure sending unit is now located right at the VP44 with a modified banjo bolt. Prior to this it was located on an adaptor right off the Carter before the filter. I do not have the original schraders or the original filter any more. I may relocate the sending unit, possibly attached to a seperate line so the motor vibration will not affect it. My bypass regulator is also attached to the motor where the stock filter was bolted on. I'm wondering if that was such a good idea. Should I put the sending unit pre or post regulator? I ran the return line to the unused nipple on the fuel tank (the one with the rubber cover, not the breather valve) does anyone know what this is for, and is returning fuel through it a good idea?



P. S. Anyone know where to get stronger intercooler hoses? I split the one at the intake manifold.

 

[Bigsaint]

Thanks for the info on the pump. I have my baby home now and was just reading the SPA manual. It takes the pressure measurements every 0. 10 Sec. and then refreshes the display at a rate of every 0. 4 Sec. That, coupled with the turbulence at the inlet to the VP44 is probably why you are seeing the extreme fluctuations. If I'm reading your post right, you don't have the OEM fuel filter anymore? If so, try relocating the sender post regulator, should help.



I have the Aeroquip elbows and Weber adapters. Using the OEM fuel filter I placed the sender at the bottom of the filter housing, works great after I adjusted the SPA.



I'm not running a regulator yet, though I will if/when I replace the Mallory with something else. I'm going to tap into the overflow hose at the tank for my return, if that helps.



Scott W.