Adjustable Fuel Pressure Regulator

bdawg

New Member
Aug 10, 2011
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kirban adj fpr are the best ive found , the bbk didnt work for me, had at least 5 brand new ones that wouldnt even hold pressure for couple seconds ,i even called corp offices in ca
 
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sambodious

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Aug 21, 2019
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Ok.. Say you leave the stock FPR on. it is going to be at 39 -41 psi with the line on and around 34ish with the vacuum line off. So now you replace the stock FPR with an adjustable one. What are you going to set the FP to?

The same 39 psi right with the vacuum line on. There is no reason to change it from stock.

The only difference is the adjustable one has the provision to adjust the FPR up and down to whatever you set it to. NOT so that you SHOULD set it to anything other than factory. Once you install it, it works the same as the factory FPR. People use the adjustable FPR to overcome smaller injectors when they approach full duty cycle by increasing the FP, or if they run larger injectors by decreasing the FP but that is a band aid solution IMO. They do help when you tune a car and when you start adding a power adder.

Now, for full disclosure I have a mild setup,- explorer top end, 70mm TB and MAF, headers, U/D, and I have a Kirban FPR- only because it came free with the other parts I bought.
I realize this thread is old but I've come across it while researching (googling) my current problem. I think this post holds the answer I need but I'm curious if anyone can chime in.

Yesterday morning I got stranded in my 5.0 while taking my kid to baseball camp. I was out cruising 65mph or so on a state highway with the A/C on. Long stretch of road and steady cruise. The car began to exhibit a slight hesitation when I'd apply gentle throttle while keeping my speed steady. Loss of power under throttle application got worse to the point that I was on the side of the road and the motor died. But the car started back up after a few moments! However, I only made it 500 yards or so until I was on the side of the road again, this time for good and finally called for a tow. Now my wallet is a hundred bucks lighter and my pride is hurt lol.

So now I'm thinking about TFI modules and fuel pumps and all kinds of crap but...

After about an hour of the car sitting in my garage I got antsy and went out to look around. I found that the vacuum line to the fuel pressure regulator was *disconnected*. It was my own fault, I had sprung a leak in the coolant line that runs to the EGR spacer (right at the connection to the EGR spacer, so I cut off the leaky section and clamped it back on) just the day before, and later that night I went out to make the repair, I definitely disconnected that vacuum line and obviously never connected it back up.

Well now the car starts right up! I let it idle for 10 straight minutes this morning with no problems whatsoever and it revs perfectly but I'm nervous to take it back out too far from my house i guess i just don't trust it.

See, I'm kinda new to this. But is it not true that vacuum is lowest during constant throttle since the throttle body is open? How does manifold vacuum affect the way the fuel pressure regulator operates? Is it not reasonable to believe that at a gentle part throttle application such as to maintain a 65mph cruising speed in 5th gear... that not having the proper vacuum on the fpr would cause a low fuel pressure situation?

Thanks guys
 

65ShelbyClone

Founding Member
Sep 9, 2000
4,659
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Antelope Valley, SoCal
Firstly, fuel pressure is 39-41psi with the vacuum line off, not with it connected. This is referred to as the base pressure.

Well now the car starts right up! I let it idle for 10 straight minutes this morning with no problems whatsoever and it revs perfectly but I'm nervous to take it back out too far from my house i guess i just don't trust it.

Before gettting excited, know that you can't rule-out heat-related problems like the TFI after everything cooled off like that.

See, I'm kinda new to this. But is it not true that vacuum is lowest during constant throttle since the throttle body is open?

No, vacuum is quite high under light load cruise because the engine speed is elevated and the throttle is open very little.

How does manifold vacuum affect the way the fuel pressure regulator operates?

More vacuum = less fuel pressure.

Is it not reasonable to believe that at a gentle part throttle application such as to maintain a 65mph cruising speed in 5th gear... that not having the proper vacuum on the fpr would cause a low fuel pressure situation?

Thanks guys

No. Think about how much vacuum is on the FPR when the line is disconnected. Atmospheric pressure is zero vacuum, right? What is the lowest vacuum the engine could possibly see with wide open throttle? Zero, right? Therefor, with the FPR line disconnected, the fueling will be closer to correct at high load and richer as vacuum increases at lower load.
 

John Dirks Jr

there is enough sticking out to grab on to
5 Year Member
Jun 28, 2013
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If you drive continually with the vacuum line off the regulator, the condition would be a rich mixture overall. If the O2 sensors are doing their job they would tell the ECU to lower the pulse rate to the injectors to lean it out. During a long cruise this might be managed ok initially but as you change driving conditions and the fuel pressure cant respond accordingly since the vacuum line was not connected. Now you are changing the demands for fuel mixture in the combustion chamber and the ECU cant react quickly enough to continue keeping things in range being able to run properly. If you plug the vacuum line back in, it may correct itself but you will need to drive for a little while so the ECU can re-learn things under the parameters of correct fuel pressure.