Fuel pressure drop - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 18 (permalink) Old 09-06-2019, 11:44 AM Thread Starter
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Fuel pressure drop

Any obvious theories on my my fuel pressure drops when I give throttle? I replaced my fuel pump with aftermarket. Rest of my EFI is stock by the way. 1975 S30.
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post #2 of 18 (permalink) Old 09-06-2019, 02:07 PM
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Any obvious theories on my my fuel pressure drops when I give throttle? I replaced my fuel pump with aftermarket. Rest of my EFI is stock by the way. 1975 S30.

FPR not working right.


what is the pressure with the pump on, engine off? should be about 36 psi.


Hookup a vacuum pump to the hose that goes to the FPR, pull a vacuum see if the pressure drops as the vacuum deepens. A small hand held mini vac the kind used for bleeding brakes is prefect.
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post #3 of 18 (permalink) Old 09-07-2019, 01:36 AM
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I believe this has been discussed before in these parts.

The crux has been that 36lb. is the expected static pressure, but that operation pressures are lower. These vintage Bosch L-Jetronic systems function at pressures that while considered high (compared to the 3, 5, or 10lb. of street carburetors) then, are insignificant in today's high pressure environment.

The more cogent question would be; are you experiencing symptoms of fuel starvation, leading you to ask your question, or just curious?



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post #4 of 18 (permalink) Old 09-07-2019, 01:35 PM Thread Starter
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I believe this has been discussed before in these parts.

The crux has been that 36lb. is the expected static pressure, but that operation pressures are lower. These vintage Bosch L-Jetronic systems function at pressures that while considered high (compared to the 3, 5, or 10lb. of street carburetors) then, are insignificant in today's high pressure environment.j

The more cogent question would be; are you experiencing symptoms of fuel starvation, leading you to ask your question, or just curious?
Yes I have a lot of issues going on..... very rough idle, exhaust smells, etc. I just pulled plugs and they have caked oil on all of them. Also I just did compression test and all cylinders are at 148-158. Several right at 150. I had head rebuilt and we did lash etc. Did new timing chain perhaps that has loosened already.
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post #5 of 18 (permalink) Old 09-08-2019, 01:48 AM
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So, the answer is "no, I have no symptoms of fuel starvation".

Probably would have been more fruitful to have lead off with your last post.

Based on it, you seem to be having a bad time with a recent rebuild. On the scant info, one might conclude that your rings are not seating, but that's shooting from the hip.

Maybe you should tell us more about it, so the readers can better understand your real problems.



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post #6 of 18 (permalink) Old 09-08-2019, 04:00 PM
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Yes I have a lot of issues going on..... very rough idle, exhaust smells, etc. I just pulled plugs and they have caked oil on all of them. Also I just did compression test and all cylinders are at 148-158. Several right at 150. I had head rebuilt and we did lash etc. Did new timing chain perhaps that has loosened already.
Oil on the plugs isn't good. I agree with the previous post that symptoms point away from fuel pressure to mechanical issues. The compression is good enough that rings are unlikely, but that poses the question of where the excess oil is coming from.

Are the plugs the right heat range? It would be good news if a change of plugs to a hotter heat range could clean up the oil.

If the timing chain was installed and timed correctly (good thing to check) I doubt it has stretched or otherwise changed.

Last edited by pilgrim; 09-08-2019 at 04:05 PM.
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post #7 of 18 (permalink) Old 09-09-2019, 01:29 AM
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If it were me, I'd be concentrating on resolving the source of the oil in the combustion chambers, instead of finding a way to cover it up.

Good compression and good oil sealing are not the same thing. And there aren't enough oil passages in the head to wet all the cylinders.

Certainly, oil fouling can produce a rough idle.

How many miles on the re-build? How deep did you go?

Inquiring minds want to know...



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post #8 of 18 (permalink) Old 09-09-2019, 01:05 PM
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If it were me, I'd be concentrating on resolving the source of the oil in the combustion chambers, instead of finding a way to cover it up.

Good compression and good oil sealing are not the same thing. And there aren't enough oil passages in the head to wet all the cylinders.

Certainly, oil fouling can produce a rough idle.

How many miles on the re-build? How deep did you go?

Inquiring minds want to know...
All good points. Compression can be good but oil rings can be trashed or installed incorrectly (upside down, for instance.) The result is high oil usage. But I think he'll need to drive it a bit for that to become visible as oil consumption. I really hope that's NOT the problem.

Another candidate for excessive oil use would probably be valve stem seals. It's just possible that those were missed during a rebuild.
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post #9 of 18 (permalink) Old 09-09-2019, 09:07 PM Thread Starter
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All good points. Compression can be good but oil rings can be trashed or installed incorrectly (upside down, for instance.) The result is high oil usage. But I think he'll need to drive it a bit for that to become visible as oil consumption. I really hope that's NOT the problem.

Another candidate for excessive oil use would probably be valve stem seals. It's just possible that those were missed during a rebuild.
Hi thank you for the input! On the rebuild, just the head, not the block. The block is still stock. Only thing I did there was clean all of the carbon off the top of the pistons (there was a lot!). The reason I did the head, the CAM was damaged so I decided to get an upgraded CAM from REbello and spring kit. The rockers had to be redone as well as 2 were shot. We replaced the oil seals on head too with kit from Zcardepot. I just realized today that when I did compression test, I did not use throttle, so my readings likely are lower than they should be I plan to redo this weekend. I messed that up! Yes, my main symptoms are running really roughly at idle especially when cold it almost doesn't run, and of course the fouling plugs. Car seems to have decent power and will actually burn out. When I started this post, was trying to figure out why the fuel pressure was dropping with throttle... that's still happening. I think it could be the fuel filter perhaps ? I have already replaced it twice in a span of a couple years. There's maybe 500 miles on the rebuild. But again, main issue is the very rough idle and plugs, and my old 4 speed but that's another issue...
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post #10 of 18 (permalink) Old 09-09-2019, 10:10 PM
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The more I think about it the more I think you might ignore the oil for the moment, IF it's not outright fouling plugs.


But...you say the plugs have caked oil on them, and that sounds like fouling. How many miles on the plugs since installation? if only a few hundred, then that's a significant problem.


I once had a 1966 GTO I bought used that had been run hard - so hard the oil rings had lost their temper. It burned a quart of oil every 50 miles but never fouled a plug. that tells me you can burn a lot of oil without making the spark plugs malfunction.

Last edited by pilgrim; 09-10-2019 at 01:35 PM.
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