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Ajustable fuel pressure regulator

3061 Views 11 Replies 4 Participants Last post by  cr29george
I have a question for the tech savy rubicon owners. I'm working on a 2006 rubicon. Currently it has a throttle body spacer and cold air intake. Of course, the fuel pump, regulator, filter, and sending unit are in the tank. I'm planning on replacing those items with a fuel pump, filters, and adjustable fuel pressure regulator with a vacuum port outside the tank. Currently, the jeep fuel system runs at 59 to 60 psi but it is static despite throttle position and load. The factory ECM does not read fuel pressure and does not have any influence on adjusting the fuel pressure. I'm am moving the fuel system outside the tank for easier servicability, and adjustability (I'm also hoping for improved driveability, and maybe a little bit more power). A friend of mine has done the same and found substantial improvements. My question is, using a small range of adjustment of approx 5 psi (60-65psi) and the vaccum port on the pressure regulator, what kind of ECM/programming issues might I experience if any? I understand that I can only increase my fuel pressure so much before I loose power, decrease my effeciency, and/or have no improvements at all if the ECM tries to de-tune if it experiences a rich mixture. My thoughts are that I will likely have a small window of adjustment where I'm hoping to see improvements before increasing the fuel pressure becomes detrimental. The fuel injection shop I bought the parts at said that I should see improvements but if I leave the fuel pressure at the factory psi, and change nothing else except connect the vacuum port on the regulator, I should see improvements. Any thoughts? Has anyone tried this?

Thanks, Chad
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i am having a hart time understanding how you expect to see imporvements by adjusting fuel pressure (across the board). Without the abiulity to reprgram the ECU, any increase fuel pressure will (unless I am way off base) lead to less power due to over fueling. With as much fuel as out motor drinks, i would imagine that the WOT fuel tables are borderline rich. Plus, have you modified the enigne that is making it inhale more air? if you are't bringing in more on the intake, what are your thoughts about wanting to bring in more fuel?

Also, is our system a return-less? I have very limited experience with returnless systems, but i do know they can be tempermental.

Its possible that I am way off base here, but in my experience with PCM programming on the wife's Grand Prix, these were my observations....
It sounds like you are wanting to use a mechanical FMU like on a boosted motor?? Yes bumping pressure would result in more fuel but you would be better off re-flashing the computer to better timing and fuel maps.

The 2005+ 4.0s can be reprogrammed now. Diablo Sport Predators can do this. I'm not just talking about the program in the unit it's self. You will have to find someone with their CMR software that can make you custom fuel and timing maps.

Our systems are return-less from the rail. They run 49.2 PSI at the injectors. The regulator is on the pump in the tank, that is also where the pressure return port is. Fuel is pushed from the pump into the regulator, which is also where our fuel filter is, the return port in the regulator is how pressure is maintained. It is controlled by a spring. Spring holds shut until 49.2 psi then opens relieving and maintaining proper pressure.
The YJs used to run a vacuum regulator on the rail. It ran at 31 psi until no vacuum, wide open throttle, then it would run at 41 psi. This was old OBD1 systems though. Thats a different ballgame than our OBD2 setups.
Actually our TJ's are set lean from the factory for emissions. I have a CAI which I think actually made my power problem worse. If anything, my jeep is running too lean. With that being the case, I should be able to slowly incr the fuel pressure (minute amounts), hopefully incr power a little however, I know if I add too much I will eventually start to lose power; you're right. This system will have a return line. I didn't know I could find a reflashed PCM. I thought I may actually have to do that anyways so thank you for the information. I was looking around for a programmer that would work w/out causing my jeep to ping. I previously tried a superchips flashpaq but although it incr my power a little bit it also caused my jeep to ping badly even on the 87 octane mode. Thanks, Chad.
You are right how lean our tjs run. Bumping the fuel pressure would help but you still have the timing advance to worry about. Our tjs are over 40 degrees.
This discussion applies only to the open loop modes such as warmup anyway. When running, the OBD2 TJs spend most of the time in closed loop operation where mixtures are managed with the oxygen sensors using the fuel trim values established earlier. Fuel pressure will have no effect whatsoever in a closed loop operating mode.
KaiserJeep said:
This discussion applies only to the open loop modes such as warmup anyway. When running, the OBD2 TJs spend most of the time in closed loop operation where mixtures are managed with the oxygen sensors using the fuel trim values established earlier. Fuel pressure will have no effect whatsoever in a closed loop operating mode.
Yes and no. Its correct to say that in closed loop that the PCM will constantly adjust to maintain 14.7:1 AFR. But, the computer is programmed to recognize the the injector size and fuel pressure. The fuel maps are built off that info. If you add fuel without addressing the PCM, the PCM will take longer to "find" 14.7. The longer the PCM searches, the less efficient it is. As it adds and pulls fuel, you are loosing performance.

40 degrees....DAMN! I am assuming that is at cruising speed? What is the WOT timing?

Also, my tuning experience taught me that a WB O2 is your best friend! I would not attempt to tune an engine for perf. without one.
I won't be changing fuel pressure initially, I will set it at 60 psi as it sits now. The only change that will be present initially is having vacuum hooked up to the pressure regulator. Then, it will only see more fuel pressure w/ change in vacuum or when I give it the beans. WB o2 sensor? Wider range o2 sensor? The other option I have considered is using something like the Diablo tuner afterwards to see if it helps further. I'm a bit leary though after the bad experience w/ Superchips. What about Unichip? Please keep in mind I'm not expecting a race car or huge gains in horsepower but I'd like to hopefully get some benefit in driveability and power. My thoughts were that I likely made my jeep run even leaner after adding the CAI and throttle body spacer.

JimmyC, I was born in Mtn Home, ID, some of my family still lives there. I hear it's getting pretty cold there.

a narrow band O2 sensor uses a 1 volt scale. They are only good for a VERY narrow range of readings which are just above and just below 14.7:1. This is because while in closed loop, that is the AFR (air/fuel ratio) the computer wants to maintain. Once you go into open loop, the computer uses preset maps to add fuel as they are programmed. There is no adjusting done my the computer. Whatever the computer has programmed, that is what it commands thru the injector.

A wide band O2 uses a 5 volt scale. The Innovative sensor that I had would read between ~8.5:1 and ~17:1. As you can imagine, the WB allows you to see what your AFR is while in open loop, that which the NB cannot do.

Chad, im actually currently serving in Iraq. The wife tells me its pretty cold. They were below zero a day or so ago and just last night got several inches of snow....
Ok, so I finished installing the new pump and regulator this afternoon. My first impressions are good. I have the fuel pressure currently at 60 psi w/ an approx 2-5 psi increase w/ increased throttle; basically the factory fuel pressure. I will play with the pressure in the future but I'm going to drive it for a week at this pressure and see how it drives. So far, it appears to start better, idle better, and accelerate better. I can only attribute this to the vacuum being hooked up thus far. When I get the chance, I'll take a picture and attach it. I ended up putting the return on the filler neck.

Jimmy C, where can you get a wide band O2 sensor?

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