Fuel pressure problems

Chris_H · 06-04-2002, 07:05 PM

Ok, I have noticed lately that the car is spitting out a lot of smoke at higher RPM's, which is normal. But it has been REAL bad here lately. I put a fuel pressure gauge inside the car (autometer mechanical, with an isolator going through the fire wall) and have been keeping an eye on it. This morning going to work I got on it pretty good, took the revs up to about 5800rpm or so, and watched the fuel pressure gauge. I went from running about 42psi, where its set, up to almost 80psi or so. I don't think it should be running that high. Not sure why its doing it, but I am not happy about it. Could this be my FMU trying to compensate for the higher RPM's? I am afraid with that kind of pressure that I can damage the injectors somehow. I could really use some help on this one. I am not sure what to do.

Chris

Chris_H · 06-04-2002, 07:41 PM

One thing I forgot to add that may make a difference. Since the supercharger install, I have added a Walbro 255lph fuel pump, and an Aeromotive adjustable fuel pressure regulator. Just something I left out that I realized is not in my signature.

Chris

Chris_H · 06-06-2002, 03:05 PM

Anybody have any ideas??

Chris

jimberg · 06-06-2002, 03:24 PM

How much boost are you getting?

Chris_H · 06-07-2002, 09:03 AM

At about 4500rpm, where the fuel reallystarts to take off, I am probably seeing about 2.5-3 pounds of boost. I know that at 6000rpm I am getting 4psi. I took it for a ride the other night with a buddy and of course it didn;t smoke at all, but the fuel pressure did go up. I don't like it that high because I feel like I can crack an injector or something. Any ideas?

Chris

jimberg · 06-07-2002, 10:03 AM

Chris,

Okay, I'm gonna stick my neck out and offer and idea as to what might be happening. This is just a guess since I don't have any expertise with superchargers. You may want to re-title your message so that SC guys will be drawn to it.

The pressure at which the fuel flows through the injector is the combination of the pressure inside the cylinder and the fuel rail. Think of someone pulling on a big desk and someone pushing at the same time. If you want to move the desk at a constant rate by using a specific amount of force, say 100 lbs, you can have someone push with 80 lbs of force and someone else pull with 20 lbs of force. If one of the people stops pulling, the other can push with the full 100 lbs of force. The fuel pressure regulator is what works to maintain that constant force. This is why the vacuum line is hooked to it. When vacuum increases the pressure in the rails has to drop to maintain the fuel pressure that was set with no vacuum applied. I hope this makes sense, and if you already knew it, other people may hopefully get something out of it.

A supercharged engine will not only have vacuum, it will have a positive pressure inside the cylinder which will be like having someone pushing on the desk in the opposite direction of where you want to go with 20 lbs of force which would mean that the guy pushing in the right direction would have to push with 120 lbs of force. You didn't mention what injectors you have, so I will guess that they are still 19#ers. Since boost isn't constant and increases with RPMS, your injectors may not be able to deliver the proper amount of fuel for the increased air flow so the FMU allows the pressure in the fuel rails to increase at a greater rate than the pressure inside the cylinder so that the injectors will provide more fuel when needed. Going back to the desk moving example, the FMU needs more work to be done so the total force applied to the desk is increased to 130 lbs. This may explain why your fuel pressure is going so high.

My guess is that, if the car seems to be running fine, everything is probably working as it should.

I hope this helps.

89 Cobra LX · 06-07-2002, 10:41 AM

Jimberg, as always, good stuff.

Chris, sorry I don't have any solutions to offer, but are you concerned that you are only seeing 4psi at 6k? I thought the Novi 1000 was a 6psi kit.

Chris_H · 06-08-2002, 06:06 AM

Jimberg, thank you for the reply. I wasn't sure if my buddy and I were thinking right. He had the same idea for the most part, but we are both new to the supercharging world, and were not sure what was going on. The car seems like its running great. Just need to get my mass air meter in the fender and away from the head unit.
89, I am not too worried about only getting 4psi at 3900ft. The kit if I am not mistaken is only a 5psi kit, and so its about right for my elevation. Being up here with the birds doesn't help much of anything with the car. My only concern is rapping it up to 6000 everytime I go to the races. I have enjoyed the kit so far though. Would love to have an aftercooler for it.

Once again, thanks guys.

Chris

exgmguy · 06-09-2002, 08:47 PM

With 19 pound injectors you should probably see around 70 psi at 6000 rpm. I have the Novi 1000 with 30# injectors and my fuel pressure hits about 60 psi @ redline.

I now have 36# injectors and I am eliminating the FMU.

06-04-2002, 07:05 PM	#1
Chris_H Registered Member Join Date: Oct 1999 Location: New Mexico Posts: 488	Fuel pressure problems Ok, I have noticed lately that the car is spitting out a lot of smoke at higher RPM's, which is normal. But it has been REAL bad here lately. I put a fuel pressure gauge inside the car (autometer mechanical, with an isolator going through the fire wall) and have been keeping an eye on it. This morning going to work I got on it pretty good, took the revs up to about 5800rpm or so, and watched the fuel pressure gauge. I went from running about 42psi, where its set, up to almost 80psi or so. I don't think it should be running that high. Not sure why its doing it, but I am not happy about it. Could this be my FMU trying to compensate for the higher RPM's? I am afraid with that kind of pressure that I can damage the injectors somehow. I could really use some help on this one. I am not sure what to do. Chris __________________ 1987 LX, Best times: 13.47 @ 102.49, 1.80 60ft 6900' DA

06-06-2002, 03:05 PM	#3
Chris_H Registered Member Join Date: Oct 1999 Location: New Mexico Posts: 488	Anybody have any ideas?? Chris __________________ 1987 LX, Best times: 13.47 @ 102.49, 1.80 60ft 6900' DA

06-06-2002, 03:24 PM	#4
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	How much boost are you getting? __________________ 351W 89 Mustang GT Convertible

06-07-2002, 10:03 AM	#6
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	Chris, Okay, I'm gonna stick my neck out and offer and idea as to what might be happening. This is just a guess since I don't have any expertise with superchargers. You may want to re-title your message so that SC guys will be drawn to it. The pressure at which the fuel flows through the injector is the combination of the pressure inside the cylinder and the fuel rail. Think of someone pulling on a big desk and someone pushing at the same time. If you want to move the desk at a constant rate by using a specific amount of force, say 100 lbs, you can have someone push with 80 lbs of force and someone else pull with 20 lbs of force. If one of the people stops pulling, the other can push with the full 100 lbs of force. The fuel pressure regulator is what works to maintain that constant force. This is why the vacuum line is hooked to it. When vacuum increases the pressure in the rails has to drop to maintain the fuel pressure that was set with no vacuum applied. I hope this makes sense, and if you already knew it, other people may hopefully get something out of it. A supercharged engine will not only have vacuum, it will have a positive pressure inside the cylinder which will be like having someone pushing on the desk in the opposite direction of where you want to go with 20 lbs of force which would mean that the guy pushing in the right direction would have to push with 120 lbs of force. You didn't mention what injectors you have, so I will guess that they are still 19#ers. Since boost isn't constant and increases with RPMS, your injectors may not be able to deliver the proper amount of fuel for the increased air flow so the FMU allows the pressure in the fuel rails to increase at a greater rate than the pressure inside the cylinder so that the injectors will provide more fuel when needed. Going back to the desk moving example, the FMU needs more work to be done so the total force applied to the desk is increased to 130 lbs. This may explain why your fuel pressure is going so high. My guess is that, if the car seems to be running fine, everything is probably working as it should. I hope this helps. __________________ 351W 89 Mustang GT Convertible

06-07-2002, 10:41 AM	#7
89 Cobra LX Don Corlione Join Date: Jun 2001 Location: Omaha NE USA Posts: 491	Jimberg, as always, good stuff. Chris, sorry I don't have any solutions to offer, but are you concerned that you are only seeing 4psi at 6k? I thought the Novi 1000 was a 6psi kit. __________________ Eric - 89 Mustang LX coupe 5 speed 302-Cobra Intake-GT40p heads-TFS stage 1 cam-FRPP 1.6 roller rockers-3:73 gears-KYB shocks and struts-Eibach Pro-kit-HPM Mega-bite upper and lower control arms-FRPP aluminum driveshaft-16x8 Chrome Cobra Rs R.H.C. #14

06-04-2002, 07:41 PM	#2
Chris_H Registered Member Join Date: Oct 1999 Location: New Mexico Posts: 488	One thing I forgot to add that may make a difference. Since the supercharger install, I have added a Walbro 255lph fuel pump, and an Aeromotive adjustable fuel pressure regulator. Just something I left out that I realized is not in my signature. Chris

06-07-2002, 09:03 AM	#5
Chris_H Registered Member Join Date: Oct 1999 Location: New Mexico Posts: 488	At about 4500rpm, where the fuel reallystarts to take off, I am probably seeing about 2.5-3 pounds of boost. I know that at 6000rpm I am getting 4psi. I took it for a ride the other night with a buddy and of course it didn;t smoke at all, but the fuel pressure did go up. I don't like it that high because I feel like I can crack an injector or something. Any ideas? Chris

06-08-2002, 06:06 AM	#8
Chris_H Registered Member Join Date: Oct 1999 Location: New Mexico Posts: 488	Jimberg, thank you for the reply. I wasn't sure if my buddy and I were thinking right. He had the same idea for the most part, but we are both new to the supercharging world, and were not sure what was going on. The car seems like its running great. Just need to get my mass air meter in the fender and away from the head unit. 89, I am not too worried about only getting 4psi at 3900ft. The kit if I am not mistaken is only a 5psi kit, and so its about right for my elevation. Being up here with the birds doesn't help much of anything with the car. My only concern is rapping it up to 6000 everytime I go to the races. I have enjoyed the kit so far though. Would love to have an aftercooler for it. Once again, thanks guys. Chris

06-09-2002, 08:47 PM	#9
exgmguy Registered Member Join Date: Feb 1999 Location: Utica, Michigan Posts: 2,631	With 19 pound injectors you should probably see around 70 psi at 6000 rpm. I have the Novi 1000 with 30# injectors and my fuel pressure hits about 60 psi @ redline. I now have 36# injectors and I am eliminating the FMU.

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