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Thread: Mystery Fuel Leak and Knocking noise

  1. #1
    Join Date
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    Independence, KY, USA
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    Default Mystery Fuel Leak and Knocking noise

    I had an o-ring fail on the side of the injection pump on my 94 6.5 K2500 Suburban. I found pulling the intake manifold was the easiest way to gain access to the passenger side of the pump to remove the cover and change the o-ring. Got everything back together and started it up. Since that point, I have 2 new issues. I still have a fuel leak, but can't figure out where it is coming from. It drips off the rear of the engine. The valley looks oily, but not particularly fresh - it just turned 200K. Any common points of leakage that aren't so easy to find?

    Also, it started knocking. It sounds worst at idle. Oil pressure is just as it was and the noise seems to subside while rolling down the interstate. I swear at times - its gone at idle, but does return. Runs fine and just as smooth as it did before repairing the first leak. Did I get air trapped in the fuel system somewhere to cause the noise?

    Any advice would be appreciated. I love this truck and drive it daily. For a 94, its in great shape. I just don't have a ton of time right now to spend tracing the leak and trying various solutions without a general direction.

    I'd be tempted to take it to a shop to get it resolved, but my faith in shops is low and where would I find a competent and trustworthy mechanic that knows the 'ol 6.5?

    Thanks.
    2001 Silverado 2500HD CC Dmax 6spd<br />TST PowerMax<br />Forum #1321 Sold

    1994 GMC Suburban K2500 6.5

    2014 GMC Sierra 2500HD 6.6

  2. #2
    Join Date
    Dec 2003
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    Default

    The valley drains out the back.
    Check your fuel filter housing they have been known to rot out on the bottom.
    Any air that you introduced would be gone already,but it may be pulling in air through the leak point.
    90 Chev 3500 c/c 4x4,6.2na,400 auto,4:10 gears.DSG Timing gears,main girdle, isspro tach, pyro,boost,oil and trany temp.Dual Tstats, High volume peninsular pump,on shelf, Custom turbo and intercooler 85%complete. Change of plans for the dually, it's going to get a Cummins. Both trucks are Blue 90 4x4 crews

  3. #3
    Join Date
    Jul 2020
    Location
    Montana
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    Default

    Common fuel leaks in the valley are thus:

    1. Injection pump throttle linkage O-Rings

    2. Injection pump fuel inlet hose.

    3. Injection pump inlet nipple.

    4. Injection pump lines in the valley.

  4. #4
    Join Date
    Feb 2000
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    Montana
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    Default

    Quote Originally Posted by DmaxCC6spd View Post
    I had an o-ring fail on the side of the injection pump on my 94 6.5 K2500 Suburban. I found pulling the intake manifold was the easiest way to gain access to the passenger side of the pump to remove the cover and change the o-ring. Got everything back together and started it up. Since that point, I have 2 new issues. I still have a fuel leak, but can't figure out where it is coming from. It drips off the rear of the engine. The valley looks oily, but not particularly fresh - it just turned 200K. Any common points of leakage that aren't so easy to find?

    Also, it started knocking. It sounds worst at idle. Oil pressure is just as it was and the noise seems to subside while rolling down the interstate. I swear at times - its gone at idle, but does return. Runs fine and just as smooth as it did before repairing the first leak. Did I get air trapped in the fuel system somewhere to cause the noise?

    Any advice would be appreciated. I love this truck and drive it daily. For a 94, its in great shape. I just don't have a ton of time right now to spend tracing the leak and trying various solutions without a general direction.

    I'd be tempted to take it to a shop to get it resolved, but my faith in shops is low and where would I find a competent and trustworthy mechanic that knows the 'ol 6.5?

    Thanks.
    You can use baby powder to sprinkle in the area where you think the leak originates, and the powder can help to pinpoint the location.

    Sounds like you may have an injector fuel knock. This is an injector that leaks fuel between injection cycles. These knocks can sound exactly like a bad rod bearing. If the knock goes away as you rev the engine, only to return once it idles for a short time, it's a fuel knock. You can also loosen one fuel injector line fitting (one at a time) to find the culprit. That's sorta like pulling spark plug wires to find the dead cylinder on a gas engine. If that's it, you'll need to replace that injector... soon...

    Jim

  5. #5
    Join Date
    Aug 2000
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    Independence, KY, USA
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    Update - It only seems to leak after being shut off. I had it idling in the driveway for about 5 minutes and there wasn't a drop!

    What is "injector fuel knock". This only started after replacing the o-ring on the pump. Could I have air trapped in the injector? Sometimes the noise goes away and I don't hear it interstate cruising. Also, I don't notice it from outside the vehicle. It is most prominent inside at idle.
    2001 Silverado 2500HD CC Dmax 6spd<br />TST PowerMax<br />Forum #1321 Sold

    1994 GMC Suburban K2500 6.5

    2014 GMC Sierra 2500HD 6.6

  6. #6
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    A fuel leak at the rear of the valley, accompanied with a miss, is often caused by a loose injector line nut at the rear of the injection pump. If so, that line, and any below it, should be wet with fuel. A miss is not the same as a fuel knock, but can be mistaken by those who are unfamiliar. An engine with a dead-cylinder knock will rock the engine in-time, and can cause "knocks" against worn out engine mounts, exhaust components, etc.

    (if not leaking fuel at the injection pump lines)
    Can you hear it with the hood up? If so, you can do as previously suggested. At idle, while you can hear the knock, loosen each injector fuel line nut a little (about 1/4 turn) until fuel weeps a little, one at a time (and tighten after each). Loosening the fuel line nut takes away the fuel for that cylinder (like pulling spark plug boots). When you do this on the suspect cylinder, the knock should stop. On non-suspect cylinders, it will only miss (because you "killed" that cylinder). Most often, if you are able to identify a unique cylinder, the problem is that injector. You can confirm if it is that injector, or something wrong with that cylinder, by swapping that injector with one on the opposite bank, then retest those cylinders. If the knock follows the injector, then the injector is bad. If it stays with the cylinder, then the problem is with that cylinder. There can be other results, but stick with that for the moment and let us know what you find.

    Another possible cause of a "knock", is the accessory drive pulley. Very common, especially as these trucks age. If the rubber isolator (similar to the damper) is gone, it will clang or clunk, metal to metal, and can also sound like a knock. Check the pulley for hard, cracked, or missing chunks of the rubber. If so, it needs to be replaced. Easy job, just remove the belt, unbolt and replace the pulley (bolts to the damper), then reinstall the belt.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  7. #7
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    Independence, KY, USA
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    Unfortunately I haven't had a chance to look for the leak in detail yet. It's just so odd the as loud as the knock is, I really only notice it inside the cab. Converter bolt suddenly loose? That's the point of the engine closest to the interior. The weather is getting warmer so i should be able to look more closely while it is running and listen for the knock.
    2001 Silverado 2500HD CC Dmax 6spd<br />TST PowerMax<br />Forum #1321 Sold

    1994 GMC Suburban K2500 6.5

    2014 GMC Sierra 2500HD 6.6

  8. #8
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    Yukon Canada
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    Try adding quart of Marvel Mystery Oil in the engine oil
    My signature truck developed a knock years ago on a trip,i thought there was something going to come out of the engine.
    Had a mechanic buddy with me and he thought it was toast as well.We did some investigation on the side of the road and ruled out the ussall suspects.Has some MMO with me so i dded a quart or so and continued on our trip.After about 30 miles the knock all but dissapeared.I drove the truck for a few more years and the knock never did come back.
    A shot in the dark but hey maybe you will have the same result.
    90 Chev 3500 c/c 4x4,6.2na,400 auto,4:10 gears.DSG Timing gears,main girdle, isspro tach, pyro,boost,oil and trany temp.Dual Tstats, High volume peninsular pump,on shelf, Custom turbo and intercooler 85%complete. Change of plans for the dually, it's going to get a Cummins. Both trucks are Blue 90 4x4 crews

  9. #9
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    Knoxville,Tennessee
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    Default

    Have you checked the harmonic balancer ?
    "The Constitution is not an instrument for the government to restrain the people, it is an instrument for the people to restrain the government."
    -Patrick Henry


    A5150nut
    2006 K3500 D/A
    94 6.5 4x4 5spd Sold

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