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Thread: GMTech - LNF Low Side Fuel Pressure Adjustment

  1. #1
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    GMTech - LNF Low Side Fuel Pressure Adjustment

    I have seen it posted about from you multiple times and I for the life of me can not figure it out. Kind of makes me feel car dumb. I directed it to GMTech because he is the only one to post about it, but if anyone else know please comment. I have even called GM and Bosch with no luck. Both places have some pretty rude people as well.

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    Quote Originally Posted by MikeM173 View Post
    I have seen it posted about from you multiple times and I for the life of me can not figure it out. Kind of makes me feel car dumb. I directed it to GMTech because he is the only one to post about it, but if anyone else know please comment. I have even called GM and Bosch with no luck. Both places have some pretty rude people as well.
    Wow, my internet ears must have been burning! I've been staying away from all the forums but something told me to check in here.

    So yes, the fuel pressure can be "adjusted" on the LNF in tank pump. It's a trick I figured out literally like 30 years ago when the first fuel injected GM stuff was coming out. Those older port and throttle body injected setups really liked it when you bumped up the fuel pressure a bit. 90% of GM cars have used the same basic style regulators for years so it's something that works on a ton of cars. Basically all you have to do is increase the spring pressure.

    (I'm working on a house remodel today so I'm going by memory since I don't have a pump or regulator in front of me.) So obviously you have to pull the pump since the regulator is on the pump module in the tank. The regulator is the little round metal thing, usually held in with a snap ring. Take the regulator off and you should be able to figure out how it works by looking at it. There is a metal "hat" on top, under that is the spring. (none of this comes apart, it's a sealed unit on that setup) Under the spring is a diaphragm with a needle that closes off the flow when the spring pressure is stronger than the fuel pressure. When the fuel pressure gets higher than the spring pressure, it moves the diaphragm and needle up and the fuel pressure is bled off. In the case of the LNF, it just dumps back into the tank.

    So you can do it the scientific way or the quick and dirty way...
    Scientific is like this... Find something that will fit in the opening where the needle is. If you push on the needle, you can move it off it's seat. That's the spring pressure. Now you need some way of measuring that force. If you have a drill press, it's pretty easy. Get a scale and set it on the drill press. Mount the regulator body somehow (lol) in the chuck of the drill press so you can move it up and down. Put something in the hole against the needle so that it will push on the scale. If you slowly push the regulator against the scale, you will be able to feel when the needle moves off it's seat. That's your spring pressure. It's been years since I did the LNF regulator, but I believe I went up 10% or so on the spring pressure by comparing the scale readings before and after. I'm thinking my low side pressure was around 70-75psi after I bumped it up, not positive though. BTW bumping the pressure helps the 5th injector setups work WAY better. So to adjust it, you literally have to "bend" down the top of the regulator hat to put more pressure on the spring. You can use a socket and hammer, that usually works best. The spring is about 3/4 of the diameter of the hat so try to put the bend pressure about 2/3's out from the center. (not on the edge or in the center of the top of the hat) Just give it a tap or two, put it back in the drill press and see if you increased the weight reading on your scale.

    Quick and dirty... Take the regulator off, smack it with a socket and hammer, put it back in and see what the fuel pressure is. lol. That's actually the way I usually do it. The thing you have to remember though is it's pretty much impossible to go back and make the pressure lower if you go too far with the socket and hammer. So it takes a little bit of experience to know how much to smack that regulator hat.

    Hope that helps Mike! I'll try to check in when I get a chance in case you have questions.


    edit- I just remembered that I put an upside down drill bit in the drill press and put the regulator upside down on the scale. That way you just push the round part of the drill bit into the hole on the regulator and feel for when the needle moves.
    Last edited by gmtech16450yz; 04-09-2015 at 12:59 PM.

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    Thank you! I can't tell you how many threads I've read, or how many damn phone calls I made trying to figure this out! The only answer I could come across is why would you want to. I definitely got sick of explaining. If you don't have an issue with it John I'm going to do this (not sure when since I am moving soon and I work like crazy) but I would like to do a write-up and possibly do a tune write-up encompassed into one about this. I will definitely not take credit for any of it since everything came from you and you only. The sad thing is no one ever bothered asking you about this except for a few small questions but then they dropped it. Again thank you.

    For everyone else, I know this doesn't seem tuning related but indirectly it is in my opinion. Sorry if people feel I should have posted this elsewhere.

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    I'd say this is the place for it Mike and as John mentioned, it helps the 5th injector setups work better so I would say it could be tune related.

    I remember doing this same thing on an older Lincoln I had but luckily the regulator was on the fuel rail - not in the tank so it was like a 5 minute job. I also remember doing this on the DSM wastegate acutators to increase boost.

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    I was definitely thinking of something else and had my mind wondering why I couldn't figure it out. Definitely looking at the write-up makes me realize I should have been able to figure that out, but hey we all stumble haha. Just glad that part is over and that John still helps us all out. I couldn't tell you how many posts I read trying to figure this out and all the hate he gets for being on his "High Horse". I post a question that no one cares to talk about and he answers it as soon as he saw the post. No hate, just pure help, and this coming from a guy who people would still his tunes (it's most of the reason I will not post any of my stuff). Not trying to build you up to much John but I figure you deserve some credit for what you have brought to the community when everyone else seems to bring you down.

  6. #6
    Quote Originally Posted by MikeM173 View Post
    Thank you! I can't tell you how many threads I've read, or how many damn phone calls I made trying to figure this out! The only answer I could come across is why would you want to. I definitely got sick of explaining. If you don't have an issue with it John I'm going to do this (not sure when since I am moving soon and I work like crazy) but I would like to do a write-up and possibly do a tune write-up encompassed into one about this. I will definitely not take credit for any of it since everything came from you and you only. The sad thing is no one ever bothered asking you about this except for a few small questions but then they dropped it. Again thank you.

    For everyone else, I know this doesn't seem tuning related but indirectly it is in my opinion. Sorry if people feel I should have posted this elsewhere.
    No This helps a lot!

    Crazy. No joke I was going to pose this question tonight. I was thinking the adjustment was power related. Just finished my 5th injector set up and I am thinking I need a little more lowside pressure behind it. I was still struggling to keep up on e85 when its cool out unless I ramped boost in slower. Especially 3500-4500rpm. This should do the trick...Maybe this weekend even...but I am getting static from my fiance about being too intimate with my Solstice lately.
    Solstice GXP
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  7. #7
    John if you had to guess, how far down did you push the top of the regulator in? What is spec for LNF fuel pressure low side stock?

    Please LMK if this is correct. I checked the fuel pressure at service port key on ENG off 59-60psi then I hear a slight tick after about 2 seconds and it drops and holds at 57-58(not sure what number to reference). Something to do with the HPFP? Tick is audible from under the hood pressure drops slightly. Pulled out the pump and got the regulator off. Pulled all the o rings off the regulator and put it into a 22mm socket. Then I used a 12mm socket on top and gave it a couple cracks and pushed the top down about 1.5-2mm. Thinking this would increase my pressure about 10% and it did, but I am still only at 65psi then like before a click and pressure drops to 62-63. Regulator could have been low from the beginning and that could be why my car had a little trouble with e47 when cool without the 5th inj. I haven't driven the car yet because I did that last night so unsure on how this will affect the tune. Does not take long. 90min total start to finish. PITA is the lock ring...must be a proper tool for that which I don't have. I will put a little more pressure on the spring if recommended. Took some pictures, but can't upload them till I am home. If I do it again I'll just put it in the vice.
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    Quote Originally Posted by hhrfreek View Post
    John if you had to guess, how far down did you push the top of the regulator in? What is spec for LNF fuel pressure low side stock?

    Please LMK if this is correct. I checked the fuel pressure at service port key on ENG off 59-60psi then I hear a slight tick after about 2 seconds and it drops and holds at 57-58(not sure what number to reference). Something to do with the HPFP? Tick is audible from under the hood pressure drops slightly. Pulled out the pump and got the regulator off. Pulled all the o rings off the regulator and put it into a 22mm socket. Then I used a 12mm socket on top and gave it a couple cracks and pushed the top down about 1.5-2mm. Thinking this would increase my pressure about 10% and it did, but I am still only at 65psi then like before a click and pressure drops to 62-63. Regulator could have been low from the beginning and that could be why my car had a little trouble with e47 when cool without the 5th inj. I haven't driven the car yet because I did that last night so unsure on how this will affect the tune. Does not take long. 90min total start to finish. PITA is the lock ring...must be a proper tool for that which I don't have. I will put a little more pressure on the spring if recommended. Took some pictures, but can't upload them till I am home. If I do it again I'll just put it in the vice.
    Yaayyy! You did it! lol.

    Good job. You did exactly what I would recommend, trying not to go too far the first time. You got an increase, even if it was only 5psi, that's fine. So the click you hear is the LPFP relay. It turns on for 2 seconds when the key is turned on. It's perfectly normal for the pressure to drop a little bit after the pump turns off. 60psi sounds right for stock pressure also. I just looked at a couple of old logs, stock pressure looks like 60-65psi and my last LNF logs looked like I was running around 80psi. I know 80psi worked fine FOR ME for a couple years of daily driving, but I wouldn't recommend going that high unless you really have the need for it. I was over 500whp, running E47 and a 5th injector when I was running 80psi. That was without the ZZP larger fuel pump lobe cam, and I didn't have any fueling issues. Bumping the low side pressure and making the 5th injector work better is why I could do that level of power without losing high side pressure.

    On another side note, yes there is a J tool for the fuel module clamp. You can get it off by using a hammer and a punch but I would STRONGLY recommend using a brass punch. There is a chance a steel punch could cause a spark and if you have spilled fuel around you might have a problem. If I use a punch and hammer, I use a brass punch and make sure I get all the spilled fuel dried up before hammering the clamp on. If any of you want to buy the J tool, I could look up the part number for it and post it here.

    Congrats on the successful mod and the new trick you mastered!

    edit- since I know somebody was gonna ask...

    Kent Moore tool number J 45722

    http://www.amazon.com/dp/B000F5LJ7S/...l_8w28q8w56v_e
    Last edited by gmtech16450yz; 04-21-2015 at 01:17 PM.

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    Great info hhrfreek and John!

  10. #10
    2 photos showing the regulator in the pump(after adjustment). Retaining clip holding it on. Easy to remove with a very small flat blade. Couple pictures showing the regulator before adjustment and 1 after easy to tell. Trims off by ~-3-5% throughout the range(-5ish at idle, -3ish higher loads/wot) so it is easy to see the pressure increase while logging. I think I will bump it up a little more. I put a fuel pump in my 2000 Blazer this winter and adjusted the regulator as well. That regulator was in the intake plenum though(had a torx screw adjustment on it...really kicking myself for having a brain fart on adjusting the LNF fuel pressure). I increased the pressure 6psi on the Blazer and trims were off by 15% so I am surprised they weren't a tad higher on the Solstice.

    http://s1190.photobucket.com/user/hh...egulator%20LNF
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    Great pics!

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    Quote Originally Posted by hhrfreek View Post
    2 photos showing the regulator in the pump(after adjustment). Retaining clip holding it on. Easy to remove with a very small flat blade. Couple pictures showing the regulator before adjustment and 1 after easy to tell. Trims off by ~-3-5% throughout the range(-5ish at idle, -3ish higher loads/wot) so it is easy to see the pressure increase while logging. I think I will bump it up a little more. I put a fuel pump in my 2000 Blazer this winter and adjusted the regulator as well. That regulator was in the intake plenum though(had a torx screw adjustment on it...really kicking myself for having a brain fart on adjusting the LNF fuel pressure). I increased the pressure 6psi on the Blazer and trims were off by 15% so I am surprised they weren't a tad higher on the Solstice.

    http://s1190.photobucket.com/user/hh...egulator%20LNF
    You mentioned something there that some of you might not realize, or it might not be as obvious to some as to others. Bumping the fuel pressure up on the low side pump in a Direct Injected engine will have little effect on the actual fuel trims. It made perfect sense that the trims changed by 15% on your Blazer since it's port injected. That's what increasing fuel pressure in a port injected engine does because it has a direct relationship to the injector flow. In a Direct Injected engine, all bumping the low side fuel pressure is going to do is give a 5th injector more pressure, or help give the entire low and high pressure system more overhead. It will help the high pressure pump put out a little more volume and pressure just because the pressure going into it is higher. That effect will only be shown when you're at the limits of what the HPFP will put out since all other times the pressure is regulated by the ECM. In other words, if you're having issues with high side fuel pressure not holding during WOT pulls, bumping the low side pressure may help. Between having more 5th injector output and higher input to the HPFP, it absolutely made my LNF hold fuel pressure to ~500whp. BTW, I used the SAME LNF low side pump in my LSA up until about 600whp! (I had to finally ditch it when I went full E85/bigger injectors and my power level went to ~700whp) So just in case anyone was wondering, the LNF low side pump with the pressure bumped up to 80psi will support up to 600whp!

    In a constant voltage, pressure regulated fuel system like we're talking about, the pump is essentially pumping a fixed amount of fuel. At 60psi, it's just dumping the extra volume of fuel back through the regulator and into the tank. At 80psi, less of that excess fuel is being dumping and instead is being pushed into the system, raising the pressure. (It's not "free" pressure though, it does make the pump work harder and draw more amperage. I would also recommend putting a remote relay with a large battery feed wire back at the tank, which is what I have on my car.) For the 5th injector setups, it will help offset your injector delta pressure greatly. That's another point that may be obvious to some and not to others... If your low side fuel pressure is only 60psi, and your boost pressure is 30psi, your injector delta pressure is only 30psi. That's not enough to get most port injectors working and spraying properly. Since I was running 30psi boost all the time in my LNF, running at 80psi fuel pressure gave me a 50psi delta pressure over my 5th injector.

    So anyway... you may see a little bit of fuel trim effects from bumping the low side pressure in an LNF, but don't be surprised if you don't see any. If you have a 5th injector, it WILL make the 5th injector flow more and you will have to adjust it's effect on mixtures. Sorry if all that is stupid obvious!

  13. #13
    Thanks for sharing John. This saved me a bunch of money as I was considering a larger fuel lobe cam, but this tip will probably allow me to stick with just a 36lb 5th injector for my additional fueling needs. I am going to do this one more time this weekend to get at least 70psi. Turbo is in the works by fall so I will just get it out of the way now. I will take some better pictures and document this a little better. Should be a breeze since I already did it.
    Solstice GXP
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    Awesome info! I think I had sent GMT a message about this a while back, but it was no big deal since I knew it would be a long time til I got to it. I actually did this on my last motorcycle so I could run E85, and was wondering if it was essentially the same deal. Thanks GMT and all involved, I will be doing this, eventually.

  15. #15
    Ok. As to my other post about the regulator adjustment affect on the tune...It is almost negligible. It had to of been weather causing the trims to be off and it also pointed out an interesting thing, but I will save that for another thread so this doesn't get jumbled up. Posted are 2 idle logs with the tune that I had prior to making any fuel adjustments taken within 1 hour of each other showing the increase from 64psi to 70psi had almost no affect on fuel trims at idle. Notice the log at 64psi idle trim is -.62. At 70psi it is -2.10. Tried to be consistent with both logs...Eng temp very close no fan on no lights on etc. So after all your adjustments they may only be off 1-2% depending on how much pressure you adjust to. Also to keep data simple I will just refer to the pressure you see immediately at key on as you will notice after 2s the pressure ticks down 2-3psi. This only took me 45 minutes the second time so doing this twice is no big deal. Engine was still at 140 degrees when I fired it back up.

    On to the pictures. In the album you will see what the regulator looks like stock at 60psi. The first adjustment the hat is pushed in a little for a pressure of 64psi. The third picture is 70psi. Check pressure at test port before you start so you know what the differential is. This was done on an '07 Solstice GXP. Pop the trunk before you disconnect the battery. Fuel tank is in the trunk. On each side is a 10mm nut that need to be removed. Pull out the drain tubes for the soft top out of each front corner of the trunk by twisting and pulling up. Remove your tire inflator and disconnect the light on the passenger side. It is easiest to pull the carpet up and out starting from the back. After the carpet is out pull the white material back on the drivers side and it will expose the fuel pump access hole. Undo the 7mm bolts holding it on. Slide a flathead under it and work your way around. It has a double sticky tape on it. Do not pry up hard on the cover. It bends very easily. After that is removed you will be looking at the pump. Disconnect the 2 electrical harnesses and fuel lines. Put a rag around the return line and pull it off slowly as it will be under pressure. In one of the pictures I am pointing at the fuel supply line. Do not try to get that off until you have loosened the lock ring and all other hoses are removed. There is not enough room to disconnect it until the pump is free. Once the pump is loose lift and turn it a little and then pull off that line. It will make more sense when you are in there. That may or may not be applicable to a Cobalt. Now pull the pump out slowly and make sure to watch for the gauge float so you don't snag it on the tank. Now that it is out, the regulator is easily identifiable. There is a lock ring holding it on with 3 little tabs. Use a very small flat blade to disconnect these. Now slowly rock/twist the regulator out as it is only being held in place by the o-rings. Pull off the small o-ring and the larger o-ring. Get a razor blade under the green fiber ring and work around it pushing it off(once you have enough space use a flat blade or your fingers). Just be careful with that. Now put the regulator into a 22mm socket. Grab a 12 mm socket and center it on the top of the regulator and put it in a vice. Hold the the regulator and 12mm socket and just make sure they are aligned and pushing in evenly as you crush the top in. I did this slowly, backed it off and looked at it several times during this process to make sure it was nice and even. You get an idea of how far to push the regulator in from my pictures. If John was at close to 80 his was much farther I would guess so there is a lot of room for error here. After that its just reverse order(make sure you connect the fuel supply line before the regulator is in all the way and locked down. Test pressure again. It will take 3 key on cycles to get pressure back and you will see it is much higher.

    http://s1190.photobucket.com/user/hh...%20Walkthrough

    If I made an error or missed something LMK.
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    Last edited by hhrfreek; 04-23-2015 at 06:59 AM.
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    Excellent write-up!

    Note to self - Pop the trunk before disconnecting the battery.

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    This has been a great write up. I would like to keep it going if I may? I would like to know what everyone is using for a fifth injector controller? What are the pros and cons of your controller? Thanks for any info.

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    If a guy wants to run 35lbs of boost, what is his best option. I am looking at the devilsown DVC-100. It controls up to a 100lbs of boost. Any thoughts from the more knowledgeable on this? Thanks