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Fuel pump knock?

Discussion in 'The Garage' started by ak bandit, Jun 28, 2006.

  1. ak bandit

    ak bandit 1/2 ton status

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    I bought my engine (396) from a friend who had it professionally rebuilt. It has less than 500 miles on the rebuild. I just got the timing/ignition/mixture tuned in last night. And now it has a slight knock that seems to be coming from the mechanical fuel pump. I'm not positive that its coming from the pump, but it sounds like it.
    When I got the engine from him he had a block off plate as he used an electric fuel pump. I pulled the plate off because I bought a mechanical fuel pump. The fuel pump pushrod was just resting against the block off plate.
    My question is, could this be causing the knock?
     
  2. Roz

    Roz 1/2 ton status

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    check the spacing on it. i belive it should have a space plate on it.(mabe someone can verify) it is possible for it to knock if there is play on the pushrod. usually when u mar the shaft or guid from improper instal or removal. possible the pump is faulty

    the only thing i suggest is pull it and look for abnormal wear if the spacer isnt the issue.
     
  3. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    I'm sure this is a carb'ed engine so you can easily check if it is either the pump or pushrod by pulling the pump and starting it or a minute to see if the knock goes away. Obviously the engine will run long enough since the fuel bowl in the carb still has fuel in it. I would bolt the block off plate back on though just so that you don't get oil leaking out. This will however only rule out pushrod or fuel pump but could be either. When you have the fuel pump off make sure the arm doesn't have any free play in it. Also BBC and SBC use different fuel pumps so i hope you have the correct pump on it.
     
  4. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    BBC doesn't use a plate before the pump like a sbc does so there would be no issues there.
     
  5. ak bandit

    ak bandit 1/2 ton status

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    I'm positive I have the right pump. But do they make different length pushrods? I cant remember if there was any play in the pushrod when I installed the pump. I'm going to do the test that 4x4HIGH suggested tonight.
     
  6. Roz

    Roz 1/2 ton status

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    oh yeah, was thinkin reverse sorry ty


    only one pushrod size i know of. i have gone through 3 pumps in 1 install cause they were defective once
     
  7. diesel4me

    diesel4me 1 ton status Premium Member

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    Noises...

    The pump pushrods are the same on a SBC, and BBC...the pumps themselves LOOK similar,but ARE different where the rocker arm hits the pushrod..using a small block pump will cause a knock at the least--I tried using one I had on a 454,and I made it 1 mile from home before the pump snapped in two at the rocker arm area,and was dangling by the hoses when I looked under the hood to se why it stalled!..:doah:

    The pushrod is driven by a lobe on the camshaft--if its lying down next to the block off plate,its nowhere near touching that lobe,so it shouldn't make any noise..if its bent,it'll make noises with a fuel pump installed..sometimes they get "cupped" like the bottom of a lifter does from riding on the lobe in the same spot all its life--if it gets re-installed with the "concave" part facing the wrong way,it can wipe out the lobe on the cam,or chew the pushrod end up badly..(why did your motor HAVE the OEM pump blocked off??--maybe the lobe WAS worn before you got it???).....

    It could be the timing chain slapping against the timing cover--I've had SBC that wore a hole right thru the cover before!..I've seen broken valve springs make weird knocks,and not affect the running of the engine much,until it reached high RPM's..a "normal" street driven truck might never get "pushed" up to the higher RPM's to make you aware of a weak or broken valve spring..:crazy:
     
  8. ak bandit

    ak bandit 1/2 ton status

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    Well I unbolted the fuel pump, pulled the pushrod out and then bolted the fuel pump back on. I started the engine up and the knock was gone.
    I'm extremely glad that it was just the pump knocking and not an engine knock. The problem is that the pump is a brand new Holley high flow pump. And cost a few bucks. Are these pumps that unreliable?
     
  9. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    How did the push rod look? Any signs of abnormal wear? Is the arm on the pump tight, doesn't flop around?
     
  10. ak bandit

    ak bandit 1/2 ton status

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    The pushrod looked brand new. There were no signs of scratching or wearing. I compared it to the pushrod from my 350 and they are identical. I didnt check to see if the pushrod sat against the arm without any movement. If there is, what could that mean?
     
  11. diesel4me

    diesel4me 1 ton status Premium Member

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    Holley = trouble??

    Maybe its just me,but I don't have much luck with Holley carbs,never used any of their other stuff (unless I had to when fixing someone elses truck that had Holley carbs or other stuff)...

    All the fuel pumps I had that made noises had worn pivot on the rocker arm,and the return or "damper"spring was broken or missing entirely--a new AC-Delco fuel pump is the best one for my money--at least they were,until they started making them in Mexico..not sure how good their quality is now..my 1973 -307, and 1986 -305 motors both still have the ORIGINAL AC fuel pumps on them,and have well over 100,000 miles on them!..:eek1: ..I might start carrying a "spare" with me,I've been wondering how much farther they'll go..:crazy:
     

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