I have an in car FP guage. It usually reads about 41PSI. Sometims after driving a while, 30-45 min, the pressure drops to around 35PSI at idle or cruising. If I go into boost the presssure does rise up according to how much boost I am running. Should the pressure drop happen? I have a fuel filter on there that is only about 5k miles and two fuel pumps with about the same mileage.
FP drops after a while
- Thread starter jpratt
- Start date
Bobs rare 6
Member
- Joined
- Apr 11, 2004
Mine does the same. I have tried 2 different gages, one on the rail and the other on a 6ft. line. Drops 3-4 psi. when hot. I think it is typical. Later, Bob.
- Joined
- Apr 4, 2002
Mine does the same thing. 
Makes tuning more difficult. My thought is that it could be that the spring pressure of a cold fuel presure regulator is different than a hot one. NOTE: This may or may not the problem. Also, could be the heat affecting the FP gage under the hood. I've got a billet KB adj fpr, hot wired Walbro 340, and a fuel pressure gage mounted on the radiator support (near the charcol canister). Does anyone have, or not have, or have solved this anomaly .
Makes tuning more difficult. My thought is that it could be that the spring pressure of a cold fuel presure regulator is different than a hot one. NOTE: This may or may not the problem. Also, could be the heat affecting the FP gage under the hood. I've got a billet KB adj fpr, hot wired Walbro 340, and a fuel pressure gage mounted on the radiator support (near the charcol canister). Does anyone have, or not have, or have solved this anomaly .Bobs rare 6
Member
- Joined
- Apr 11, 2004
I was told to use cold psi. readings for adjustments. Bob.
- Joined
- Apr 4, 2002
TTT
I guess this is normal?????????
I guess this is normal?????????
- Joined
- Apr 4, 2002
Fontana
I start the car and check fp (43). I drive it for ten min or so and check fp(39). I let car sit for a hour or two then check it and it's back to 43. All checks are done with line off. Fuel pressure is 3 to 4 lbs. lower with line on.
I start the car and check fp (43). I drive it for ten min or so and check fp(39). I let car sit for a hour or two then check it and it's back to 43. All checks are done with line off. Fuel pressure is 3 to 4 lbs. lower with line on.
The purpose of the FPR is to maintain constant fuel pressure across the injectors, which means it senses the manifold vacuum/pressure and through the vacuum line that attaches to the FPR it then compensates for any variation. Everybody checks to make sure that fuel pressure tracks with boost, which is important. But equally important is that the fuel pressure also track during vacuum or non boost conditions. Most people miss this important measurement, so you're not alone. So what you have described is that your FPR is not regulating during the vacuum part of the regulation. As an example, if the base fuel pressure is 40 psi with the hose unhooked, and if the motor idles at 18 inches of vacuum, then when you hook up the vacuum hose to the FPR, the fuel pressure should drop to 31 psi.
So here is what you do....remove the vacuum hose from the FPR, then adjust your FPR all the way down in fuel pressure and note this value. I'm guessing that this value will be about 38 psi. Then, whatever the minimum fuel pressure you can get, add 10 psi to it. So you get 38psi + 10psi = 48psi. Adjust your base fuel pressure to this new higher value and see how well the FPR regulates the fuel pressure. I personally have to run 55psi base because the fuel pump I'm using is overwhelming the stock fuel system. Most people run the 340 and it also can overwhelm the fuel system.
Wayne
So here is what you do....remove the vacuum hose from the FPR, then adjust your FPR all the way down in fuel pressure and note this value. I'm guessing that this value will be about 38 psi. Then, whatever the minimum fuel pressure you can get, add 10 psi to it. So you get 38psi + 10psi = 48psi. Adjust your base fuel pressure to this new higher value and see how well the FPR regulates the fuel pressure. I personally have to run 55psi base because the fuel pump I'm using is overwhelming the stock fuel system. Most people run the 340 and it also can overwhelm the fuel system.
Wayne
- Joined
- Apr 4, 2002
Yup, min. fp I can get out of this thing is 38 psi. Line off pressure tonight is 50 psi cold and 43 psi hot. So, I guess its time for a new fp reg (not a KB)?
