Engine/Transmission (1994 - 1998) Fuel Pressure

[MMcCallie]

Industrial Injection told me that my truck needs to have a minimum fuel pressure of 45 psi at all times and that it would be best if I can get around 70 psi. I had ordered an AirDog2. I want to say it was a 165 GPH, came set at about 14 psi and only goes up to maybe 25 psi. Turns out I was setup with the wrong one for a 24 valve and the 12 valve was supposed to be set at 30 psi. So I just got another AirDog2 either 200 or 250 GPH. AirDog said they were cranking it up to either 60 or 65 psi so I should have healthy fuel pressure.



I'm on my fourth mechanical lift pump in 2 years. First was stock, second was from Cummins, next two from Piers. This one leaks and I have a gauge with a sensor going into the P7100 that shows a digital gauge on my BD X-Monitor. At idle it bounces around 20-25 psi. Light throttle driving I see teens up to over 40 psi. On a hard run I'll get as low as 0 psi. Without this gauge on all the time I never would have known the fuel pressure was swinging so radically. I'm curious if the mechanical pumps are always so sporadic and if the range is supposed to be so great.



I've also been told the stock lift pump should put out over 50 psi if its healthy. Is that true? I've been told because my engine makes strong power that it needs more fuel pressure, at least 55 psi at all times. Anyone running an AirDog or Fass who can verify improvements from higher, more consistent fuel pressure?



I'm told low fuel pressure can still be present when having black smoke out the exhaust, of which I have plenty. Black smoke comes out not just because of over fueling, but because of hot cylinder combustion. I'm also told the timing can be affected by low fuel pressure, that not enough fuel is being delivered in time, and that injector's spray patterns can also be altered from inadequate fuel pressure.



I'm wondering if someone can straighten out some of this information I've been given. How much pressure should a stock 12 valve have and how much will performance be affected if not at that amount? My truck has never seen over 19 miles per gallon though many tell me it should get 20 mpg. Should I expect that this AirDog will provide constant pressure or will it drop under load like my mechanical lift pump?



Will I have any issues with the overflow valve I bought from Piers a couple years ago? This new model supposedly doesn't require replacing the fuel pickup in the tank due to modifications in being able to simply connect to the return line, only feeding the injector pump what is being demanded. Can I expect better mileage, more power, less smoke, lower EGTs, etc. from constant 65 psi?



Thanks,

Matt

 

[Missouri Mule]

Stock fuel pressure from 96 service manual 20 psi minimum at idle. I have read in threads/magazine 25 psi normal at idle, revving engine to 2000 rpm in neutral will cause pressure go up to 30 and up.



What plate do you have in the p7100?

What horse power level are you at?



19 mpg seems reasonable to me if you got a lot of horses.

 

[12V Ford]

The mechanical pump does fluctuate because it's rpm driven. The way yours is right now sounds right. Mine would drop close to 0 psi on a wot run. With the overflow valve shimmed, it idles at 32 psi, goes to 38 psi driving around, and can get to 25 psi on a wot run. I don't think the stock pump can make 55 psi, unless the ofv is practically closed off.



I don't think you need 65-70 psi all the time. 40 to 45 psi should be plenty. I believe it was Jim Fulmer who tested his truck on a dyno, increasing the fuel pressure to see if it made more power. IIRC, it quite increasing in power after 45 psi.



An airdog should make your truck run smoother and it may gain a little power. I'm not sure if the pressure will drop on a wot run, but I've heard that it may drop a little, but not enough to matter. It just depends on the fueling.

 

[Joe G.]

There are pressure spikes from the mechanical piston type pump. In order to get correct FP you need to use a needle valve adjusted so the gauge barely works. That dampens the pressure spikes.

 

[LVJUNIOR]

Joe,

I am getting 20-22psi at idle and around 37psi at WOT 2200rpm. Is this enough for 350-400hp or do I need to find more pressure.

 

[Joe G.]

37 should be plenty.

 

[MMcCallie]

I put almost 600 Hp and 1,200 TQ to the ground. Perhaps the guys at Industrial Injection are wrong. They claim to see stock 12 valves making 55 psi from the mechanical lift pump at an idle and that the pressure shouldn't drop any lower than 45 psi for a stock 12 valve, and especially not for one with injectors and injector pump modifications. They told me they crank FASS systems up as high as 80 psi if they can get that much out of the pumps. Pureflow Technologies gave me a box that says 200 GPH but inside the manual and the plate on the pump read 165 GPH so I may once again have the wrong pump. Have to call and find out tomorrow.

 

[PRohr]

My FASS stays rock solid at 42psi..... no matter if I am idleing, cruising or WOT. I have the 195/90 so it's a 90gph pump. I went to the FASS on my 12V because of the same issues you had witht he lift pumps..... 4 in two years... . plus it's a lot easier to change fuel filters on the FASS than the stock one. As for the FP... ... I had 30 psi with a shimmed OFV on the stock pump and that was the most I could get..... it would idle around 17-20 and then go up with RPM's like it should. No matter which pump, FASS-AirDog-Mechanical you will need the needle valve on the pressure gauge to stop the pulses. The P-Pump will cause the pulses do to the mechanical design with the cam and the plunger set-up as the fuel fills the empty plungers and is blocked off from that one and must go down the line to the next and so on..... IIRC Air Dog works off a higher volume pump vs the FASS pumps higher pressure..... they will both work but they have a different take on which is better.