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Erratic fuel pressure

 
Old 06-07-2019, 12:38 PM
  #21  
cardo0
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Originally Posted by B View Post
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.

Those little peanut fuel pressure gauges all junk! Just do a forum search to read about them. Get a real fuel pressure gauge mounted away from heat (tape it to the windshield temporarily) to check the fuel pressure. You may have been chasing a bad reading pressure gauge - not bad pressure.
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Old 06-07-2019, 08:18 PM
  #22  
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Originally Posted by cardo0 View Post
Those little peanut fuel pressure gauges all junk! Just do a forum search to read about them. Get a real fuel pressure gauge mounted away from heat (tape it to the windshield temporarily) to check the fuel pressure. You may have been chasing a bad reading pressure gauge - not bad pressure.
This weekend I'll be putting a hose in a bucket to eliminate the sock being an issue. After that I'm starting to think you're right. Other than the cam lobe for the fuel pump im almost out of things to check. I'll try another gauge if its not the sock and see.
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Old 06-07-2019, 09:50 PM
  #23  
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If its a mechanical analog gauge, get a liquid filled one, it eliminates bouncing due to engine and pump vibrations. If you go electrical, check the reviews, dont get a cheap one. Summit, aem, and another one got bad reviews on the summit website. Appears only autometer got good reviews but they start above $200. Ive got a summit gauge i may be returning, too bad cause it looks great
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Old 06-07-2019, 10:35 PM
  #24  
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mechanical pressure gauges:

brand ASHCROFT is among the best and priced accordingly.
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Old 06-09-2019, 04:48 PM
  #25  
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Going to try running the car off a bucket. Any place that is better to run the line from? I was thinking to disconnect the rubber line from the tank to the steel line then run a line from the steel line to a gas can. Thoughts?
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Old 06-10-2019, 06:33 AM
  #26  
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away from heat and flame, should be fine. Do You have a helper with a fire extinguisher or are you going to keep one by your side, just in case? One of those," IF you have it, you wont need it" situations. As for flow the tank is uphill of the pump and provides a downhill flow to the pump, at least on my 68 it is so try and keep the bucket level with the frame or slightly higher so the siphoning effect is easier to maintain. When you disconnect the hose, dont let too much gas pour of the line going to the pump as you attach the temporary fuel line to it. And and your going to have to crimp the rubber line thats still attached to the tank so that doesnt keep flowing out.
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Old 06-10-2019, 04:17 PM
  #27  
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Here's what ive tried:

Check filter on the carb
Replaced fuel pump
Bypass gas tank
Bypass fuel line (connect directly to fuel pump)

I put a real fuel gauge on it with the same results, bouncy fuel pressure. Other than the cam lobe I dont know what else to check. Is there any easy way to check that w/o removing the cam?
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Old 06-10-2019, 06:05 PM
  #28  
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Originally Posted by B View Post
Here's what ive tried:

Check filter on the carb
Replaced fuel pump
Bypass gas tank
Bypass fuel line (connect directly to fuel pump)

I put a real fuel gauge on it with the same results, bouncy fuel pressure. Other than the cam lobe I dont know what else to check. Is there any easy way to check that w/o removing the cam?
Using the bucket, did it come out sporadically? Did the pressure increase and weaken as you watched it fill the bucket? Or is that what the fuel gauge indicated?
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Old 06-10-2019, 06:11 PM
  #29  
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Originally Posted by F22 View Post
Using the bucket, did it come out sporadically? Did the pressure increase and weaken as you watched it fill the bucket? Or is that what the fuel gauge indicated?
I attached the gauge between the fuel pump and carb. It starts but runs really rough and the gauge goes from 0-12-0 crazy fast. Like a dog wagging its tail just like before.
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Old 06-10-2019, 06:30 PM
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Originally Posted by B View Post
I attached the gauge between the fuel pump and carb. It starts but runs really rough and the gauge goes from 0-12-0 crazy fast. Like a dog wagging its tail just like before.
But what does it do, when you've got it going to the bucket?
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Old 06-10-2019, 07:00 PM
  #31  
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Originally Posted by F22 View Post
But what does it do, when you've got it going to the bucket?
Not sure what you mean. It wont really idle on it's own so I cant look in the container while its running. It behaves exactly as if it's running off the tank.

What should I be looking for when it's running off the container?
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Old 06-11-2019, 09:32 AM
  #32  
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You'd do the bucket test while it's cranking, not while it's running. Hook the three or four foot rubber line to what comes off the fuel pump to the carb and put the other end in the bucket. You're going to look for erratic pressure coming out of the hose into the bucket. I still would recommend, what I initially recommended, a good fuel pressure regulator and a gauge. Pro race cars use Aeromotive FPR's and gauges. Mine has been trouble free for years and years.
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Old 06-11-2019, 09:46 AM
  #33  
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Originally Posted by F22 View Post
You'd do the bucket test while it's cranking, not while it's running. Hook the three or four foot rubber line to what comes off the fuel pump to the carb and put the other end in the bucket. You're going to look for erratic pressure coming out of the hose into the bucket. I still would recommend, what I initially recommended, a good fuel pressure regulator and a gauge. Pro race cars use Aeromotive FPR's and gauges. Mine has been trouble free for years and years.
ok, into a bucket while its cranking, i misunderstood. i can try that today.
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Old 06-11-2019, 12:01 PM
  #34  
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Make sure you're not around any open flames, like the water heater or anybody smoking. Have a fire extinguisher standing by and only do this for ten to twenty seconds. Have a clean bucket and be ready with a funnel to put it back in the tank when done. The idea is, to watch how the fuel stream behaves. If it's a strong and steady stream, you're good to go. If it's intermittent, like hardly pumping anything for a half second then sudden hard pressure the next and back again, then it's saying that you're fuel pressure gauge is telling you the truth. But somehow, I doubt that. If you get good results, then it's time to get a good FPR and gauge. The Aeromotive setup was $150, but the gauge is liquid filled and highly accurate. That goes for the Fuel Pressure Regulator too.
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Old 06-11-2019, 01:11 PM
  #35  
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Originally Posted by B View Post
ok, into a bucket while its cranking, i misunderstood. i can try that today.
I had pretty much the same problem on a -68 Camaro some years ago. It took me quite some time to figure it out.
It turned out to be a hole (corroded) in the fuel line just over the rear axle. So it was sucking air and it became progressively worse. Fuel pressure fluctuating 5-15 psi.
But if you had a short suction line in a bucket direct to the pump you can eliminate that, just didnít understand what you did with the bucket.
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Old 06-11-2019, 05:40 PM
  #36  
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I put a hose in a bucket and it spurts gas. Some, none, some none.... now what?

How far should the cam lobe push the fuel pump rod?
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Old 06-11-2019, 07:11 PM
  #37  
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Originally Posted by B View Post
I put a hose in a bucket and it spurts gas. Some, none, some none.... now what?

How far should the cam lobe push the fuel pump rod?
OK, so now we know that the fuel gauge is telling the truth. My guess from looking at the internet is that a Chevy SBC cranks at least 80 rpm. So that pump has to be being pushed in and out at less than a second per revolution. If it was running right, there's little possibility of seeing the erratic fuel flow you're seeing.

According to this thread on Camaro's . net. The travel was 0.395"
https://www.camaros.net/forums/18-en...lift-spec.html

"measured installed with pump rod the travel is 0.395". That is on a brand new Crane Cam with zero wear. you will not get the correct measurement by measuring the cam lobe. It is round (approximately 1.500" measured on a new Straub cam) and offset ground like a rod journal. I happen to be putting two engines together right now and am at the perfect stage on both to measure and inspect parts."
Joe


Let's switch gears here and talk about the installation. When putting in the fuel pump into the block, you've got to make sure the push rod is going to come back down right on top of the fuel arm. Because the push rod can lay down at an angle in that area (if some of you out there think I'm wrong on this, please chime in!). I've used heavy grease to hold it in it's highest position and I've also done a quick 'hold the rod end with your finger, shove it up and stab the pump in' method, hoping that I did it right (I did and it worked, but there were doubts... A good suggestion from someone here was to use the dipstick to hold it in the correct, all the way up position, because it's so thin. Then letting the rod come down on the arm. It could be that the pump you have is defective.

Here's a pretty good description that I wasn't aware of, on installing a fuel pump and keeping the rod in the upper position
REMOVAL & INSTALLATION

  • Disconnect the fuel intake and outlet lines at the pump and plug the pump intake line.
  • On small block V8 engines, remove the upper bolt from the right front mounting boss. Insert a long bolt (3/8 in.-16 x 2 in.) in this hole to hold the fuel pump pushrod.
  • Remove the two pump mounting bolts and lockwashers; remove the pump and its gasket.
  • If the fuel pump pushrod is to be removed from V8s, remove the two adapter bolts and lockwashers and remove the adapter and its gasket from small blocks and remove the pipe plug and pushrod from 454 cu. in. engines.
  • Install the fuel pump with a new gasket reversing the removal procedure. Coat the mating surfaces with sealer.
  • Connect the fuel lines and check for leaks.
I just went out and took a picture of the block that I pulled that ran the old mechanical fuel pump. The long bolt method bolded above seems to do the trick. I used a drill bit to push against the fuel pump pushrod and indeed, it did hold it in it's upward position. Gently tightening that bolt against it, as described above would do the same thing.

Forgive the blurry pic, but the natural tendency of this pushrod is to drop down to it's lowest position and most of us, don't want that to happen when putting in the fuel pump (not sure if you had considered that or not, but let's cover this anyway, because the bolt method is pretty neat, versus the dipstick or finger-stab method.

Fuel Pump Pushrod dropping by itself.





Fuel Pump Pushrod, held in upper position with pressure through the front boss hole on engine block.








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Old 06-12-2019, 01:00 AM
  #38  
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Originally Posted by B View Post
I put a hose in a bucket and it spurts gas. Some, none, some none.... now what?

How far should the cam lobe push the fuel pump rod?
What I ment was to have the suction line in a bucket/canister of gasoline, that way you eliminate any problem up to the pump. If you have a leaking hose or line anywhere between the tank and pump you will have this exact problem with fluctuating pressure. It doesnít necessarily leak out/dripping on the floor but it may very well draw air to the pump.
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Old 06-12-2019, 08:28 AM
  #39  
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When in installed the new pump I put a screw into that hole in the block to hold the pushrod up, same concept as the drill bit i guess. This was a problem with the old pump so i wouldn't expect it to be an install issue since it happened before and after i put the new pump in. I ran the car drawing fuel from a bucket at the rear of the metal fuel line where it normally attaches to the tank. I was trying to eliminate the sock in the tank as an issue, no change. I then ran the car drawing fuel from a bucket into the fuel pump directly hoping to eliminate a leak in the fuel line, no change. I could run the car drawing from a bucket directly from the pump to the carb but that has a flared metal fitting that goes up to a rubber line from the metal line to the carb fuel lines.

At this point i'm leaning toward the cam lobe. I'm out of town until sunday then i'll change the oil and see if there's metal in there. I feel like i can but a dial indicator on the pump rod and see how far it moves. There's not a lot of room to operate there so this could be fun. Anyone know of another way to check the cam lobe or any other suggestions?
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Old 06-12-2019, 03:04 PM
  #40  
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Originally Posted by B View Post
When in installed the new pump I put a screw into that hole in the block to hold the pushrod up
...
And you removed the screw and replaced it with the plug, making sure the plug can't reach the pushrod?

Last edited by Danish Shark; 06-12-2019 at 03:12 PM.
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