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Hello,

My 74 450 started running very weak after leaving the car parked for 6 weeks.

I went through the usual; vacuum, ignition, etc., fuel flow and it seems I have low fuel pressure (also tested the fuel flow and got exactly the amount as per the manual - 1 liter in 30 sec.).

The pressure at the cold start valve was tested, and yes, low pressure, under 1 BAR.

Perhaps fuel pressure of the fuel pump be tested as well (at the fire wall) to determine if the issue is the pump or the FPR.

I will attempt to adjust the fuel pressure regulator, has anyone done this before and could share some advice?



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In this video, I show the k-jet pump rig that has a back-to-tank-bleeder-valve that I can use to reduce the pressure. Totally closing the valve allows the d-jet fuel pressure regulator to do its job and set the pressure no higher than ~30 psi. This rig allowed me to get confidence that this d-jet fuel pressure regulator is doing its job.


Just one concept/method of testing (which only required I take the supply and return lines and hook them to my rig).


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Hello,

My 74 450 started running very weak after leaving the car parked for 6 weeks.

I went through the usual; vacuum, ignition, etc., fuel flow and it seems I have low fuel pressure (also tested the fuel flow and got exactly the amount as per the manual - 1 liter in 30 sec.).

The pressure at the cold start valve was tested, and yes, low pressure, under 1 BAR.

Perhaps fuel pressure of the fuel pump be tested as well (at the fire wall) to determine if the issue is the pump or the FPR.

I will attempt to adjust the fuel pressure regulator, has anyone done this before and could share some advice?



Sent from my MAR-LX1M using Tapatalk
Yes, I have adjusted the FPR - just rotate the adjustment screw. But I would question why it would have gone out of adjustment? Maybe the internal diaphragm developed a leak.

Maybe think about why you only got 1bar at cold start valve. Could be due to leaks through injectors, leak back through supply line and pump check valve, leak through FPR. You could clamp off some hoses to find out. There is a section in the shop manual that covers how to test for fuel pressure. http://www.oncebitter.com/r107/r107CD1/Program/Engine/107/M117_45/074-120.pdf
 

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Test your system before you mess with the FPR.
Get a hose shutoff clamp, and put it in place on the fuel return line after the FPR (I mean Dampener) and before the hard line but do not clamp it yet.
Remove hose from Cold Start Valve, and put your pressure dial in there. I use a long hose so I can lay the dial up against the windshield.
Bump the key 3 or 4 times but do not go all the way over to start/crank position and see the high/low as the pump works.
Now clamp down the return line and try it again 3 or 4 times, it should hit and hold about 60psi (bars are for burgers and brews) it should hold 60psi for about 5min's.
if it;s not check for leaks from front 2 rear. Check Every Clamp turn twist and curve for leaks. If you are satisfied you have no leaks remove the line that feeds the fuel rail and place your tester there, (this will save you from pulling the injectors for now) do the bump test again.
The car runs on around 30psi but the pump will put out 60+
If you have and hold 60 problem is up front, if not it could be as easy as the check valve on the pump it's self.
This is based on your statement that the volume test is dead on. I have a check valve you are welcome to have good luck on getting a new one, but you know once you will replace it the 40+ year old pump will need to be replaced it's just Murphy's Law
 

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Test your system before you mess with the FPR. (I mean Dampener)
Trog - There is a FPR and a Dampener. Only FPR can be adjusted.
What you described is an extension of what is in manual link I posted earlier. He should just do the tests! Not surprising he isn't getting enough fuel if pressure is less than 1bar!
 

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Trog - There is a FPR and a Dampener. Only FPR can be adjusted.
What you described is an extension of what is in manual link I posted earlier. He should just do the tests! Not surprising he isn't getting enough fuel if pressure is less than 1bar!
Agreed! I missed the fact you posted a link, the( dampener) was a correction I put in the wrong spot on my way out this am, it's fixed now.
If the FPR has failed adjustment of it is pointless, the FPR is not going to go 2/3 out of wack just sitting there, but a chunk of rubber hose will give you a hard time
 
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