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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.
 

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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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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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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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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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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.
 

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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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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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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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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.
 

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Mr.Farmerz:

Note: what follows is simply one bloke's view...


Now, that's the way to start a conversation! All that's needed for a Big Picture view is a few more facts...

- What is the current odometer reading?

- A little history of the engine would be helpful.

- What was the condition of the exhaust manifold's end bolts. Did you install a new dual manifold gasket when you put the head back on?

- Did you pay attention to the clearances of all the valves in their guides? Explain.

- What do the cyl. bores look like?


Forensically speaking, the highest likelyhood is that the cam (and lifters) did not "fail", per se; the oil spray bar has probably been cloggy for quite some time. Hope you cleaned it well.

Frankly, the black crust on the piston crowns should have been a big red flag, with the plug conditions as a chorus of affirmation.

However, let us not jump the gun; fill out the picture, so we can make some cogent observations.

PS: I'd advise you to forget the "fuel pressure" thingy (it drops in operation, period); it's distracting you from facing some real issues.
 

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When you flush the toilet, the pressure in the shower drops. Same holds for the fuel pressure when you open the throttle--the pressure above the injectors falls when you reduce the flow resistance downstream to permit more flow.
 

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Mr.Farmerz:

Note: what follows is simply one bloke's view...


Now, that's the way to start a conversation! All that's needed for a Big Picture view is a few more facts...

- What is the current odometer reading?
I don’t know the car sat for about 10 years prior to me getting it. It’s either 105k or 205k.
- A little history of the engine would be helpful.

- What was the condition of the exhaust manifold's end bolts. Did you install a new dual manifold gasket when you put the head back on?
I replaced the stock exhaust which had a ton of leaks with 6-2-1 headers and 2.5” piping. All the bolts were replaced too. None of the originals broke off I was able to get them all off. They did have a little rust on the ends.
- Did you pay attention to the clearances of all the valves in their guides? Explain.
I had a head shop install the cam and the springs and check the valve lash. I then had a local experienced mechanic tune it. Still I guess something could still be wrong..
- What do the cyl. bores look like?
They all looked good

Forensically speaking, the highest likelyhood is that the cam (and lifters) did not "fail", per se; the oil spray bar has probably been cloggy for quite some time. Hope you cleaned it well.
** I did NOT think to clean the oil spray bar and will do that!
Frankly, the black crust on the piston crowns should have been a big red flag, with the plug conditions as a chorus of affirmation.
*with the compression numbers all being close to the same there should not be an issue with rings right ?
However, let us not jump the gun; fill out the picture, so we can make some cogent observations.
Thank you
PS: I'd advise you to forget the "fuel pressure" thingy (it drops in operation, period); it's distracting you from facing some real issues.
Great will do !
 

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Mr.Farmerz

OK, let's continue to exploit new info.

Age: So far, only you are in a position, with intimate first hand experience, to make that guess.
However, your evaluation should include knowledge of it's formative environment. Thirty years in Cal. implies one thing, in Indiana, another.

I don't want to add pressure to your deliberations, but keep in mind that that 100K mi., one way or another, would have serious implications for the condition of say, the bearings and rings.

Speaking of which: do you know the age of the bearings and rings? Have you dropped the oil pan to at least check whether the oil pickup screen is clean (this can be a very real problem with cars that have sat (and start/idle-to-warm doesn't count) for extended periods, more so it the oil is not fresh and clean) If so, did you take the opportunity to reach up and rattle the rods and play with the crank?

Can you say specifically; a) how long have you owned the car? b) "500mi. since rebuild", but how many miles, in toto, have you put on it? c) ever notice smoke/wet tailpipe? d) when did the idling problem become annoying? e) when did you first notice the plug oiling? f) is the idling problem better, same, or worse since the head work?

In the search for the sources of a rough idle beyond the obvious excessive oiling, one must consider changes; in this case, the exhaust manifold. The obvious question here is, did you (or anyone) check the mating face for "flat"? And whose mating gasket did you use?

Head work: It would seem the answer to my question is "no, I don't know if anyone checked valve guide clearances". By the same token, I would ask a) in what kind of shape were the existing intake valve stem seals, and b) did the guys that worked the head replace them with new valve stem oil seals. And if your contractor promised a "ready to install" head, one might think they should have thoroughly cleaned and tested the oil spray bar, but without specific acknowledgement.....

Also, did "tuning" include working the running valve clearances (a necessary exercise that would also verify spray bar operation)?

More Also: have all the injectors been serviced?

With no offense intended, would you know if the cyl. bores did not look "good"?

More: Does the engine smoke under deceleration? What does the tailpipe look/smell like?

I'm sticking with "compression and oil sealing are not the same thing".


Finally, I will risk the boredom of moralizing. There is a school of thought that says when one adopts a forty-year old engine of a very uncertain history/mileage, prudence demands a comprehensive knowledge of the existing conditions, right down to the bearings and rod bolts. Only in this way can one understand how to achieve a sustainable harmony in the engine and avoid uncertainty in use. Of course, this principle applies to all of the car's systems, but it is critical in the case of the source of locomotion.

The Moral: It is not unreasonable to believe that you quit investing in engine refurbishment way too soon.

Of course, one could adhere to the "it's cheaper to wait 'til something breaks to fix it", but this will almost inevitably fetch you up against another truth of life with cars: "pay me now, or pay me more later (with interest)".

$.02, no, $.04 from an oil-stained peanut gallery.
 

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Mr.Farmerz

OK, let's continue to exploit new info.

Age: So far, only you are in a position, with intimate first hand experience, to make that guess.
However, your evaluation should include knowledge of it's formative environment. Thirty years in Cal. implies one thing, in Indiana, another.

I don't want to add pressure to your deliberations, but keep in mind that that 100K mi., one way or another, would have serious implications for the condition of say, the bearings and rings.

Speaking of which: do you know the age of the bearings and rings? Have you dropped the oil pan to at least check whether the oil pickup screen is clean (this can be a very real problem with cars that have sat (and start/idle-to-warm doesn't count) for extended periods, more so it the oil is not fresh and clean) If so, did you take the opportunity to reach up and rattle the rods and play with the crank?

Can you say specifically; a) how long have you owned the car? b) "500mi. since rebuild", but how many miles, in toto, have you put on it? c) ever notice smoke/wet tailpipe? d) when did the idling problem become annoying? e) when did you first notice the plug oiling? f) is the idling problem better, same, or worse since the head work?

In the search for the sources of a rough idle beyond the obvious excessive oiling, one must consider changes; in this case, the exhaust manifold. The obvious question here is, did you (or anyone) check the mating face for "flat"? And whose mating gasket did you use?

Head work: It would seem the answer to my question is "no, I don't know if anyone checked valve guide clearances". By the same token, I would ask a) in what kind of shape were the existing intake valve stem seals, and b) did the guys that worked the head replace them with new valve stem oil seals. And if your contractor promised a "ready to install" head, one might think they should have thoroughly cleaned and tested the oil spray bar, but without specific acknowledgement.....

Also, did "tuning" include working the running valve clearances (a necessary exercise that would also verify spray bar operation)?

More Also: have all the injectors been serviced?

With no offense intended, would you know if the cyl. bores did not look "good"?

More: Does the engine smoke under deceleration? What does the tailpipe look/smell like?

I'm sticking with "compression and oil sealing are not the same thing".


Finally, I will risk the boredom of moralizing. There is a school of thought that says when one adopts a forty-year old engine of a very uncertain history/mileage, prudence demands a comprehensive knowledge of the existing conditions, right down to the bearings and rod bolts. Only in this way can one understand how to achieve a sustainable harmony in the engine and avoid uncertainty in use. Of course, this principle applies to all of the car's systems, but it is critical in the case of the source of locomotion.

The Moral: It is not unreasonable to believe that you quit investing in engine refurbishment way too soon.

Of course, one could adhere to the "it's cheaper to wait 'til something breaks to fix it", but this will almost inevitably fetch you up against another truth of life with cars: "pay me now, or pay me more later (with interest)".

$.02, no, $.04 from an oil-stained peanut gallery.
New information - I removed valve cover to clean the oil spray bar and noted a couple big items - 1. oil smells like gas which tells me the car is running rich... 2. the valves apparently were not adjusted properly.
I have adjusted the valve lash and now thinking this all maybe my cold start valve stuck open? Or temp sensor not working properly...
 

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Mr.Farmerz:

Clearly, my contributions are not needed here.

I do apologize for wasting your time.
 

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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...

If your fuel pressure drops instead of rising when you hit the throttle then that is a problem. It's going to be either a defective pressure regulator, OR you have a restriction in the fuel delivery to the FPR. It could be a restriction in the feed line or hose, a partially plugged fuel filter, or even a plugged pickup screen inside the tank. At this point it's a secondary problem because you first need to find out why your plugs are so dirty and you smell bad exhaust etc. That sounds like a possible injector problem. If your injectors are squirting liquid or too much fuel then they're washing out the cylinders which is going to lower compression a little, and cause the plugs to cake up. This scenario could be from dirty injectors, cheap injectors like Sorenson's, even a defective ECU or AFM. Now there could be a connection between the two problems. Partially plugged pickup in the tank, and possibly putting rust into the system, if that rust is getting to the injectors, or if gasahole (deliberate mispelling) has been sitting in the car for any length of time that can also cause rust in the injection system including the injectors. By the way, actual fuel pressure on these cars was set at 28-30 psi at idle and 34-36 on accel. It can be raised to 34 at idle but you shouldn't go any higher than that. For those of you that let any car or internal combustion engine sit for more than a month you should be using a circulated mix of Seafoam gas treatment in you fuel tank. That will the fuel stabilized up to 2 years even if it's gasahole. That will prevent a multitude of bad scenarios.
Z man of Washington
360-668-2979
www.datsunstore.com
 
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