Fuel Pressure Regulator test - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 36 (permalink) Old 06-10-2019, 08:49 PM Thread Starter
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Fuel Pressure Regulator test

I am trying to find the problem with my car running rough and no power under load. It's a 75 Z and has run wonderfully for the 20 years I have owned it. I started with checking fuel pressure after the fuel filter. I have a solid 36 psi not running. Running it drops to 29 psi and steady there. It should be 32 I think. Is that much difference a problem? In checking the fuel pressure from the regulator I am getting 36 psi and again a solid 29 psi running. After I shut it down, I am getting very little bleed off of pressure, maybe a pound in an hour.
I guess for now, I am curious if the 29 psi fuel pressure while running is possibly a problem.

Additionally, here is what I have done to the car over the last few months:

1. Pulled injectors and cleaned them - all are functioning basically the same
2. Replaced injector connectors with new ones
3. Checked each connector for electric pulse
4. New fuel rubber hoses for the fuel rail and system under hood
5. New fuel filter
6. New water temp sensor
7. New Distributor Cap and rotor
8. New NGK spark plugs
9. New gas in the tank
10. I have cleaned all electrical connections

So now I am checking fuel pump and fuel pressure regulator and that is where I am at.
I'd appreciate any thoughts.

Thanks,
GT75Z
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post #2 of 36 (permalink) Old 06-11-2019, 10:25 PM
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Originally Posted by GT75Z View Post
I am trying to find the problem with my car running rough and no power under load. It's a 75 Z and has run wonderfully for the 20 years I have owned it. I started with checking fuel pressure after the fuel filter. I have a solid 36 psi not running. Running it drops to 29 psi and steady there. It should be 32 I think. Is that much difference a problem? In checking the fuel pressure from the regulator I am getting 36 psi and again a solid 29 psi running. After I shut it down, I am getting very little bleed off of pressure, maybe a pound in an hour.
I guess for now, I am curious if the 29 psi fuel pressure while running is possibly a problem.

Additionally, here is what I have done to the car over the last few months:

1. Pulled injectors and cleaned them - all are functioning basically the same
2. Replaced injector connectors with new ones
3. Checked each connector for electric pulse
4. New fuel rubber hoses for the fuel rail and system under hood
5. New fuel filter
6. New water temp sensor
7. New Distributor Cap and rotor
8. New NGK spark plugs
9. New gas in the tank
10. I have cleaned all electrical connections

So now I am checking fuel pump and fuel pressure regulator and that is where I am at.
I'd appreciate any thoughts.

Thanks,
GT75Z
Running fuel pressure should be between 36 and 38 psi. Pull the vacuum line to your regulator after the car warms up and see if any fuel drips out. This would indicate your fuel pressure regulator is toast.

1978 280z - stock L28 engine - Dallas, TX
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post #3 of 36 (permalink) Old 06-11-2019, 10:57 PM Thread Starter
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Running fuel pressure should be between 36 and 38 psi. Pull the vacuum line to your regulator after the car warms up and see if any fuel drips out. This would indicate your fuel pressure regulator is toast.
Fuel pressure not running should be 36 psi. From what I have seen, fuel pressure while running is less than that, being 32 psi. I have pulled the vacuum line from the fpr after warm up and it is dry. No fuel present. I can't find any problems with the fpr. I don't really want to throw $125 at a new one if that is not the problem. My next move is to do the tests out of the FSM. I don't understand a lot of those procedures but that is my only place to go now.
Thanks, GT75Z
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post #4 of 36 (permalink) Old 06-12-2019, 01:05 PM
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your FP is correct both running and not.
check the TVS see the FSM for the procedure.


I presume you checked the air filter?
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post #5 of 36 (permalink) Old 06-12-2019, 02:37 PM Thread Starter
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your FP is correct both running and not.
check the TVS see the FSM for the procedure.


I presume you checked the air filter?
So 29 psi is OK for fp while running, correct?
I will look for the TVS procedure in the FSM, thanks.
By Air Filter are you referring to the AFM? I have not run tests on that yet.

Thanks again,
GT75Z
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post #6 of 36 (permalink) Old 06-12-2019, 03:50 PM Thread Starter
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Originally Posted by GT75Z View Post
So 29 psi is OK for fp while running, correct?
I will look for the TVS procedure in the FSM, thanks.
By Air Filter are you referring to the AFM? I have not run tests on that yet.

Thanks again,
GT75Z
OK, I have not done this before and have a question on how to read the check procedures in the FSM. On the left side of the chart we have "Condition" on the right side we have "Check Item Number" and then we just see two or three lines starting with 1, 2 or 3 - followed by a list of numbers in parenthesis (see attached photo from the FSM). I understand that these are probably identifying ports on the 35 pin connector. But I am not sure what to do with them. Do I check each port for connectivity to ground? And if continuity does not exist, do I determine what element that number is relative too.
Also, when you pull the 35 pin connector, I can't seem to find anything that tells me which pin is #1 and which would be #35 .

Thank you,
GT75Z
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File Type: jpg FSM01.jpg (34.9 KB, 5 views)
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post #7 of 36 (permalink) Old 06-12-2019, 04:12 PM Thread Starter
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Originally Posted by GT75Z View Post
OK, I have not done this before and have a question on how to read the check procedures in the FSM. On the left side of the chart we have "Condition" on the right side we have "Check Item Number" and then we just see two or three lines starting with 1, 2 or 3 - followed by a list of numbers in parenthesis (see attached photo from the FSM). I understand that these are probably identifying ports on the 35 pin connector. But I am not sure what to do with them. Do I check each port for connectivity to ground? And if continuity does not exist, do I determine what element that number is relative too.
Also, when you pull the 35 pin connector, I can't seem to find anything that tells me which pin is #1 and which would be #35 .

Thank you,
GT75Z
I think I have the numbers figured out. Run the corresponding test if that number is shown for a condition. I got confused and didn't see that right off. Still curious about how the counting goes on the 35 pin connector.
Thanks,
GT75Z
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post #8 of 36 (permalink) Old 06-12-2019, 04:59 PM
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So 29 psi is OK for fp while running, correct?
I will look for the TVS procedure in the FSM, thanks.
By Air Filter are you referring to the AFM? I have not run tests on that yet.

Thanks again,
GT75Z

I am talking about the air filter. I assume its new and not clogged. always check the easy stuff 1st , its it getting air (air filter) is it getting gas (gas in tank) etc...
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post #9 of 36 (permalink) Old 06-12-2019, 05:01 PM
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the FSM has two kinds of test, one with a volt meter one with a continuity test. Some checks use both.
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post #10 of 36 (permalink) Old 06-12-2019, 08:25 PM
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+2 on the air filter and checking cheap, free easy stuff first. I would also pay close attention to the vacuum system. A leak in that system can call all kinds of havoc. What's your vacuum at idle and is the needle steady? 'No power under load' also suggests a malfunctioning distributor vacuum plate. Its common for these to get sticky and lose the little ball bearings they ride on after this many years. Traced a similar prob to a '76 I used to have and after fixing the difference in power was dramatic.
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