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Discussion Starter #1
I have a 1996 GMC Suburban with a 5.7l. When started, cold or hot, the engine has a knocking sound coming from the front of the engine, around the water pump area.
Symptoms include:
Knocking sound
Belt visibly moving, and tensioner pulley jumping with the noise.

This noise is repeatable any time the belt is on the vehicle. However, if I remove the belt, all knocking sound goes away. This leads me to believe the engine itself is fine, as it runs perfect without the belt, and even WITH the belt sans knocking. If I add tension to the belt with a wrench, the knocking goes away as well.


I have replaced the AC Compressor as it needed to be done anyway, same with the water pump. The tensioner pulley and idler pulley and belt have also been replaced as they need it as well.
I am at a complete loss for what this is.

Video for sound of problem
 

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If your 1996 suburban has a harmonic balancer, that is my first suspect. It's most probably the rubber has separated.
 

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Discussion Starter #3
Would there be any indication from just inspecting it? Any vibration or wobbling?
 

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If the rubber has separated, you will see the pulley wobble while engine running.
I wouldn't run it. Take off the belt and try to move the pulley outer sheave sideways. It shd not move.
 

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Discussion Starter #5
Removed the belt, the pulley is solid, and the harmonic balancer looks intact as well. Removed the pulley, the rubber doesn't appear to be cracked, or failing in any other way. Could it be the key/keyway has cracked or failed?
 

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That's good.
Could be something else in the front belt driven.
What's the condition of the key or keyway?
 

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Discussion Starter #7
Without pulling the balancer off (my puller has disappeared, probably let someone borrow it) it's hard to tell. However just by feel it doesn't seem like it's spun or moved and opened the keyway bigger.
 

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I watched the video closely and it looked like the bottom crank pulley is jumping up and down with the knocking sound.
It's definitely down there in the crank.
 

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Discussion Starter #9
Thank you! Getting a puller in the morning to better inspect the harmonic balancer. Also will be checking the pulley itself for cracks.
 

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Pick up one of those stethoscopes to trace down the noise source. Have you ruled out the power steering pully and pump? The video is awfully short. If it was longer and moving to either side it might help.
 

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Discussion Starter #11
It turns out to be the PS Pump itself. The shaft was moving just enough up and down to cause the noise. There was no indication on the pulley itself. So I put a new pump in and the sound has gone away.

Now if I could fix the AC. It leaks down and doesn't hold a charge. I've checked with dye in the system and can't find a leak, so that leads me to believe that it's in the back half. There are lines that run down the frame to the back heater core/ac thingy. I will probably buy an AC Rear Delete kit that caps off the hoses..
 

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2000 Silverado Z71 4x4 5.3L 460k+ miles w/ GM rebuilt motor and trans
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I will probably buy an AC Rear Delete kit that caps off the hoses..
I would fix it instead of cap it. The rear air helps a lot with the overall climate of the vehicle without freezing out the front row. We were able to get by with broken front AC for a couple months in FL using just the rear air.
 

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It turns out to be the PS Pump itself. The shaft was moving just enough up and down to cause the noise. There was no indication on the pulley itself. So I put a new pump in and the sound has gone away.

Now if I could fix the AC. It leaks down and doesn't hold a charge. I've checked with dye in the system and can't find a leak, so that leads me to believe that it's in the back half. There are lines that run down the frame to the back heater core/ac thingy. I will probably buy an AC Rear Delete kit that caps off the hoses..
A sniffer is needed for most AC systems in cars. Slow leaks means it's on the low pressure side and most likely it's an evaporator core. You'd be surprise how hot it can get i the back without one. I has a 2002 Astro van and the rear evaporator blew out but the front still worked. On the high pressure side the lines do split. If I recall there is a pressure switch just before or after the core. I'm amazed how the system works as two different units with one compressor. If you open the panel to get access to it you may see the core sitting in condensation water that furthers its deterioration.
 
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