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Afpr changing pressure?

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lil corb

15+ Year Contributor
758
7
Sep 7, 2005
Woodbridge, Virginia
I initially set my fuel pressure on my fuel lab regulator to 38 psi many months ago. Well yesterday I was trying to do some open loop tuning and glanced at my liquid filled gauge and realized my pressure was low ( 30psi with vacuum line capped) at idle. So I adjusted it and re tuned (lowered Maf sliders) and today I start the car up and see my pressure has increased at operating temp to 50psi with line capped at idle. So I re adjusted the line to 38psi capped and going to check it another day to see where it's at.

Question is I have never noticed it drop before like that after doing pulls but is it possible that the afpr can drop pressure and regain itself and I happen to catch it while it was at low pressure?

Re wired 190lph, fuel lab afpr, and 1150c fic. I want to get a better understanding of what's going on so I can resume my tuning and not worry about my fuel pressure changing .
 
I am doing tuning here and there, in between "life". And i have not seen one change at all in my fuel lap AFPR. And i check often. I know it's not the exact answer you are looking for, but i figured i would share, for comparisons sake.

I did notice in between starting up car and cooling down my mtxl wide band read e9 (low voltage) could have maybe dropped voltage causing low fuel pressure?
 
Could be a faulty gauge. Do you have a secondary gauge or anything to confirm? I usually run an in cabin electric fuel pressure gauge to confirm. Could also be the first signs of a dying fuel pump. Check your fuel filter as well.
 
Fuel filter is new but I know the pump has been on since at least 2005 that I know
Of. I have a fp 255 sitting around, maybe it's time to update
 
An update, checked fuel pump via link. Drove car doing some closed loop tuning then rechecked fuel pressure, down a couple of psi. Adjusted it again dtove around again checked fuel pressure, again down a couple PSI. It seems feel pressure goes down as pump gets hotter . Confirm signs of dying fuel pump?
 
An update, checked fuel pump via link. Drove car doing some closed loop tuning then rechecked fuel pressure, down a couple of psi. Adjusted it again dtove around again checked fuel pressure, again down a couple PSI. Confirm signs of dying fuel pump?

Either the pump or the oring that seals the pump to the sending unit is deteriorating.
 
Before you go spending more time and money keep in mind the liquid filled gauges that are under the hood are HORRIBLE at reading pressure accurately after a few minutes. These are ONLY good for the first couple of minutes, as the temps rise under the hood the liquid thins out and the needle will drop. Hence why you noticed it was really high right after start up and yet was really low after you drove around for a while.

Set it on initial start up and only read it again after a long cooling period or the next day at start up. This will save you $ and time, been here and got burned too. :thumb:
 
Before you go spending more time and money keep in mind the liquid filled gauges that are under the hood are HORRIBLE at reading pressure accurately after a few minutes. These are ONLY good for the first couple of minutes, as the temps rise under the hood the liquid thins out and the needle will drop. Hence why you noticed it was really high right after start up and yet was really low after you drove around for a while.

Set it on initial start up and only read it again after a long cooling period or the next day at start up. This will save you $ and time, been here and got burned too. :thumb:

The liquid doesn't affect what the needle says. The liquid is only there to dampen vibration on the needle.
 
Update* Just changed the pump (apparently I had a GSS143 (255hp) for another 255hp while only managing to break one stud on the sending unit :thumb:

So far the pump is a lot quieter (which leads me to believe the old one was dying) and I have started and let the car idle multiple times and then checked fuel pressure. It's holding steady at 37-38psi :thumb:

Going to do some more WOT tuning and re check the Pressure but everything is looking good so far.

Will update after Open loop tuning.

Thanks
 
Before you go spending more time and money keep in mind the liquid filled gauges that are under the hood are HORRIBLE at reading pressure accurately after a few minutes. These are ONLY good for the first couple of minutes, as the temps rise under the hood the liquid thins out and the needle will drop. Hence why you noticed it was really high right after start up and yet was really low after you drove around for a while.

Set it on initial start up and only read it again after a long cooling period or the next day at start up. This will save you $ and time, been here and got burned too. :thumb:

Well, you might have something. Went for a drive again after new pump install and after almost an hour or driving noticed the pressure dropping again. Now, I was due for a new pump since old one was at least 10 yrs old.

Now what the guage is reading (if it is because of heat in engine bay) cause any tuning issues? I'm noticing my ltftlo is adding fuel (positive %) coinciding with the drop of psi I'm seeing. Or is there something else going on with my fuel set up? Afpr holds pressure properly after car is off and only has about 5k miles on it so I'm sure the afpr is fine. Also o ring was properly install and lubed.

Any ideas?
 
Well, you might have something. Went for a drive again after new pump install and after almost an hour or driving noticed the pressure dropping again. Now, I was due for a new pump since old one was at least 10 yrs old.

Now what the guage is reading (if it is because of heat in engine bay) cause any tuning issues? I'm noticing my ltftlo is adding fuel (positive %) coinciding with the drop of psi I'm seeing. Or is there something else going on with my fuel set up? Afpr holds pressure properly after car is off and only has about 5k miles on it so I'm sure the afpr is fine. Also o ring was properly install and lubed.

Any ideas?

If it werejust the gauge, the tune wouldn't change at all. Is the exhaust run close to the fuel line under the car?
 
If it werejust the gauge, the tune wouldn't change at all. Is the exhaust run close to the fuel line under the car?

It's a rre dp, tp and n1 catback so it runs at stock location. Never noticed any issues prior to this and want to run car at track next Sunday.
 
It's a rre dp, tp and n1 catback so it runs at stock location. Never noticed any issues prior to this and want to run car at track next Sunday.

What do you have for fuel pump wiring? Good ground? Rewired?

Also, is the fuel tank vapor line still functional or do you have it blocked off?
 
Guys liquid filled gauges have a temperature/pressure indicated variance. When the engine bay is hot it will read different then cold. As long as you set it cold and it stays in the same place when cold there is nothing to worry about. I ran into this years ago and after some research found it to be normal.
 
Guys liquid filled gauges have a temperature/pressure indicated variance. When the engine bay is hot it will read different then cold. As long as you set it cold and it stays in the same place when cold there is nothing to worry about. I ran into this years ago and after some research found it to be normal.

The autometer liquid filled gauge I used had a vented case and I never had any variations from hot or cold. Regardless though, he said his fuel trims started increasing which prompted him to look at the gauge. That to me would say it isn't a gauge issue.
 
What do you have for fuel pump wiring? Good ground? Rewired?

Also, is the fuel tank vapor line still functional or do you have it blocked off?

I have a STM rewire kit. The ground I have for it is bolted between the gold lower trunk holder and it's black bracket.
Now they both are metal where it's bolted but I'm questioning now if that grounding spot is good or not.
 
I have a STM rewire kit. The ground I have for it is bolted between the gold lower trunk holder and it's black bracket.
Now they both are metal where it's bolted but I'm questioning now if that grounding spot is good or not.

Next time you see the pressure dropping, go back there and see it the wires are hot. If so, try changing the ground location.
 
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