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Erratic fuel pressure

 
Old 06-03-2019, 09:19 AM
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Default Erratic fuel pressure

Took my 70 for a spin and i had to floor it just to keep it running. Got home and noticed my fuel pressure was going from 0-10+ (gauge looked like a dog wagging it tail). I changed the pump hoping that was the issue but i guess not since it's still happening. Since its a mechanical pump there's not a lot going on other than the filter and pump, no regulator, fuses, electric issues etc... I don't have a ton of time to dig into for a couple weeks but i'm wondering what to look for. There was nothing in the filter (i have after market fuel lines to the carb which includes the gauge), I didn't see any kinks in the rubber lines but ill double check that. Any thoughts?

Engine is a 383 w/ a holley 750 and ran fine for an hour ride last week.
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Old 06-03-2019, 09:50 AM
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383's a performance motor ... likely has Non OE cam.

Mismatch between cam material & FP pushrod; subsequent abnormal wear?

Maybe cam's fuel pump eccentric aka lobe worn too small?

FP PR worn too short?
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Old 06-03-2019, 10:02 AM
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I'm of the belief that you should always run a high quality fuel pressure regulator with a built engine. I have a 383 and I run an Aeromotive Fuel Pressure Regulator and an Aeromotive Gauge. The combo has never failed me. Don't scrimp on the FPR. I've personally seen three of the cheap $32 Holley FPR's from Summit fail and leave the vehicle stranded, either from too much pressure or nothing at all. No, the Aeromotive FPR and gauge aren't cheap, but I've had mine for three years with nary a problem. If that doesn't fix it, you know that either the new pump is not working right, or the pushrod running from the engine that drives it is not pushing the arm correctly and the engine is having to pull the fuel in on it's own, or debris in the tank or lines is causing intermittent flow. Unplug the output line and put the end in a container while somebody starts it and see what the flow looks like.


Last edited by F22; 06-03-2019 at 10:03 AM.
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Old 06-03-2019, 10:29 AM
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Originally Posted by jackson View Post
383's a performance motor ... likely has Non OE cam.

Mismatch between cam material & FP pushrod; subsequent abnormal wear?

Maybe cam's fuel pump eccentric aka lobe worn too small?

FP PR worn too short?
Non-OEM cam is correct. It's a crate motor from summit w/ a roller cam alum head and all the goodies installed by the previous owner. The FP pushrod has a bronze end and does not seem to be damaged when i looked it over. i was concerned about the cam being worn but there's only a few thousand miles on it. That doesn't count it out but hopefully its not that. Any easy way to check that?
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Old 06-03-2019, 10:37 AM
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Originally Posted by F22 View Post
I'm of the belief that you should always run a high quality fuel pressure regulator with a built engine. I have a 383 and I run an Aeromotive Fuel Pressure Regulator and an Aeromotive Gauge. The combo has never failed me. Don't scrimp on the FPR. I've personally seen three of the cheap $32 Holley FPR's from Summit fail and leave the vehicle stranded, either from too much pressure or nothing at all. No, the Aeromotive FPR and gauge aren't cheap, but I've had mine for three years with nary a problem. If that doesn't fix it, you know that either the new pump is not working right, or the pushrod running from the engine that drives it is not pushing the arm correctly and the engine is having to pull the fuel in on it's own, or debris in the tank or lines is causing intermittent flow. Unplug the output line and put the end in a container while somebody starts it and see what the flow looks like.
I check the FP push rod and it looks ok, although i wonder if its the correct length. I'd rather not pull it back out to check but that'll be after i check everything else. I think i'll pull the filter and do the bucket trick and see what i find.
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Old 06-03-2019, 01:13 PM
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did you check the sock in the tank?
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Old 06-03-2019, 02:34 PM
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Originally Posted by MelWff View Post
did you check the sock in the tank?
ahhh, the sock. i did not but i'll definitely do that. maybe tonight i fi get a chance. do you know if you can see that from above? i'm pretty sure in 70 it come out the bottom but i could be wrong.
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Old 06-03-2019, 02:54 PM
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Originally Posted by B View Post
ahhh, the sock. i did not but i'll definitely do that. maybe tonight i fi get a chance. do you know if you can see that from above? i'm pretty sure in 70 it come out the bottom but i could be wrong.
You should be able to see it through the filler hole. I have a skinny light I can get through the hole and it shines inside the tank and you can see the end of the tube with maybe the sock is there. I have seen some completely gone also.
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Old 06-03-2019, 03:02 PM
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Originally Posted by Gordonm View Post
You should be able to see it through the filler hole. I have a skinny light I can get through the hole and it shines inside the tank and you can see the end of the tube with maybe the sock is there. I have seen some completely gone also.
I believe this is essentially a pre-filter correct? Meaning i could 'technically' remove it and everything would be ok except maybe the primary fuel filter doing more work.
If i cant quite see it i think the fuel line into a bucket would determine if that's the real issue.
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Old 06-03-2019, 03:30 PM
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Originally Posted by B View Post
I believe this is essentially a pre-filter correct? Meaning i could 'technically' remove it and everything would be ok except maybe the primary fuel filter doing more work.
If i cant quite see it i think the fuel line into a bucket would determine if that's the real issue.
Correct. It is just to not get debris in the lines or the fuel pump. It will run OK without the sock
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Old 06-03-2019, 03:31 PM
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70 has rather large gas fill opening.

Smaller hand-forearm can reach through gas fill to R&R sock (twist-turn & pull-push).
Otherwise, access through bottom.
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Old 06-03-2019, 06:41 PM
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Originally Posted by jackson View Post
70 has rather large gas fill opening.

Smaller hand-forearm can reach through gas fill to R&R sock (twist-turn & pull-push).
Otherwise, access through bottom.
Took a quick look and seems like the sock is in good shape but it's hard to tell just by looking. Its black, not sure if that's stock or not. XL hands so reaching in wont be happening...
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Old 06-04-2019, 10:35 AM
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If you cant find another solution, reexamine the fuel pump. I had a brand new one that came apat internally, I'll look for a picture. I think i may still have the pump to take a pic as well
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Old 06-04-2019, 12:28 PM
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Default Sock

Originally Posted by B View Post
Took a quick look and seems like the sock is in good shape but it's hard to tell just by looking. Its black, not sure if that's stock or not. XL hands so reaching in wont be happening...
Have not seen a sock colored black should be white/cream colored.
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Old 06-04-2019, 07:15 PM
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Originally Posted by MelWff View Post
Have not seen a sock colored black should be white/cream colored.
That's what I was expecting as well...
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Old 06-04-2019, 07:34 PM
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Originally Posted by MelWff View Post
Have not seen a sock colored black should be white/cream colored.
Originally Posted by B View Post



That's what I was expecting as well...

Sure looks like a sock but the few I've hands-on were white/off-white only.
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Old 06-04-2019, 08:37 PM
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Even if the sock is black, I don't see very much dirt there at all. When I got my '74, there was nearly sand dunes down there, surrounding the sock and it still ran! And remember, this car ran fine a week ago.
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Old 06-04-2019, 09:11 PM
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Originally Posted by F22 View Post
Even if the sock is black, I don't see very much dirt there at all. When I got my '74, there was nearly sand dunes down there, surrounding the sock and it still ran! And remember, this car ran fine a week ago.
Agreed, everything looks good in there. I just thought it was odd that it would be black. I'm going to try a few more things and see what I find but it seems like I'm running out of simple things.
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Old 06-04-2019, 09:13 PM
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I have seen plenty of discolored socks. They may have some dirt on them and remember these socks are approaching 40+ years sitting in various kinds of gas. Could change the color. That one looks pretty good.

Last edited by Gordonm; 06-04-2019 at 09:14 PM.
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Old 06-05-2019, 08:44 PM
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I am having the same issue. Looking at adding a filter before the pump and changing the pump. Cant decide if I want an electric or manual. I am running a crate Summit 355 with vortec heads. Not sure about the cam. Any suggestions.
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