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What does "detonation" sound like? Engine "knock"?

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by Big6ft6, Jul 19, 2009.

  1. Big6ft6

    Big6ft6 New Member

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    So I'm afraid I either have some detonation or "knock" in my new engine in my 89 2500 burb 350 TBI.

    Sometimes...usually shortly after I start the truck when I'm accelerating up an incline (where more torque is on the engine) I get a sound that sounds like my radiator fan is whacking into my fan shroud. I thought the extra torque was "tilting" the engine and the fan was running into something.

    But when I had my finace sit in the truck with and hit the gas and the brakes at the same to time to make the noise...the noise happened, but the fan wasn't hitting anything! It sounded more like it was coming from the engine. Only happens under load, starting from a stop un a hill etc.

    The other thing I noticed was it seems to go away once the truck warmed up.:shocked:

    This makes me nervous. I'm not sure why my engine would be detonating unless the distributor is shot or was slipping??? But what is "engine" or "rod" knock....I've heard it used all the time but I guess I've never stopped to know for sure what it is?

    Am I experiencing detonation? Or rod knock? What should I do about it?
    #1
  2. 2COR517

    2COR517 New Member

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    Can you elaborate on the "new engine" I'm not sure I can describe pinging.
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  3. tlperry68

    tlperry68 New Member

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    The only detontions I've done sounded like KAAABOOOOOOOOOOM. They weren't on motors.

    Rod knock is virtually alway pesent. If you only hear it s the RPM increases you might want to check the motor mounts. Sounds like something is moving and causing an impact into another moving part.

    How new is the motor?
    #3
  4. bill190

    bill190 New Member

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    If you have heard a Mercedes diesel engine at a stop light or a big truck diesel engine, that is the sound of knocking. Or a United Parcel Service Truck. I know when they are here from the sound of the truck!

    This can be caused by the timing being too advanced. The timing is all done by the engine computer. In general, sensors which input information to the computer which can change the timing are for the following...

    Crankshaft position
    Engine speed (RPM)
    Engine load (manifold pressure or vacuum)
    Atmospheric (barometric) pressure
    Engine coolant temperature
    Intake air temperature (retards timing when hot)

    (Your specific vehicle may not have sensors for all the above)

    Then also there should be a knock sensor. When the engine computer detects knocking via this sensor, it should retard the timing, then slowly advance it to the best setting without knocking. So possible problem with knock sensor not working too.

    Then there can be other problems which cause knocking. Older vehicles have this problem...
    http://www.lemonlaw.com/gm-engine-problem.html

    A GM Factory Service Manual Set would give instructions for checking each of the above sensors. (Order from helminc.com or a GM dealer.)

    Or take it to a GM dealer and they could put it on their fancy electronic testing gizmo's and tell what is going on.
    Last edited: Jul 20, 2009
    #4
  5. 2COR517

    2COR517 New Member

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    You have to set the timing on the 89 TBI. I would start there first. Get a Chiltons or Haynes to get the procedure. He has a knock sensor, but if the distributor is way off, it may not be able to correct.
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  6. Big6ft6

    Big6ft6 New Member

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    Thanks guys,

    I did set the timing the proper way (disconnect the timing computer wire etc) when I installed the engine (just a rebuilt stock TBI 350 motor) so it was at least o.k. to begin with. So if this is pinging from too advanced ignition timing then either the distributor was not tightened down enough and has moved, or something is malfuncitoning.

    Also, if mytiming was too advanced wouldn't this happend all the time? Not just when the engine was cold?

    Thanks for the description of engine knock...like a diesel...definitly not that sound, this sounded excactly like the very tip of my metal cooling fan was just barely tapping against something, about that speed and exactly that sound.

    Any other ideas?
    #6
  7. Jimmiee

    Jimmiee New Member

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    You could remove the fan belt. Start it up cold and drive it around the block. That would prove it one way or the other. Usually an engine will ping more when hot rather than cold. Another quick check is to loosen up the distributor and back it off about 1". That will retard the timing and it should remove the ping if in fact it is pinging.
    #7
  8. Big6ft6

    Big6ft6 New Member

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    Thanks Jimmie! Godd idea...of course since it is a serpentine belt removing it would mean no water pump or power steering? Is that a good idea?

    But that would for sure remove the fan from the suspect list.
    #8
  9. Jimmiee

    Jimmiee New Member

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    It wont hurt for a short trip.1 or 2 minutes with a cold engine. Now if you had a Ford it would spit out the PS fluid when turning but with a GM you should be OK. Just don't let it get hot!
    #9
  10. rdnkdrummer

    rdnkdrummer New Member

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    Another way to check, without all the part removal, is to put super unleaded gas in your tank, or a bottle of octane booster, if you're usually running regular. If the pinging stops under the same circumstances, it could be a fuel delievery problem. or, it could just be bad gas in your area.

    i have an 85 carbureted, and an 89 TBI, and both HATE 87 gas from valero or conoco and some other cheapy stations. They will "ping" like crazy and i have to add a bottle of octane boost to make it go away.
    #10
  11. unplugged

    unplugged New Member

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    I'd bet my money on piston slap. The clue pointing me in that direction is that it seems to go away when the engine warmed up. Piston slap is nothing to worry about. Let the engine warm up and then drive it if it bothers you that much. Search these forums for more info on piston slap.

    Piston slap sounds something like this:

    [ame="http://www.youtube.com/watch?v=RuiJffXOJTY&feature=related"]YouTube - CA18det piston slap?????[/ame]
    #11
  12. 2COR517

    2COR517 New Member

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    Yeah, that make sense.
    #12
  13. phoebeisis

    phoebeisis New Member

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    To me detonation sounds like a few rocks/pebbles being shaken in a metal can- or maybe more like the can being rolled with rocks pebbles in it.
    Luck,
    Charlie
    #13
  14. Bikeman

    Bikeman New Member

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    Could be an exhust leak at the manifold, they sometimes go away when the metals heat up and expand a little.
    #14
  15. theW

    theW New Member

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    Any update on this? I think that I have the same thing on my 2002 Tahoe. I noticed it when towing my camper this past week.
    #15
  16. Big6ft6

    Big6ft6 New Member

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    *******UPDATE*******

    Still unresolved. I did remove the fan (actually unbolted it from the pump) and drove around, no effect noise still happens.

    I have driven the truck a bit more to study the noise.

    The noise ONLY happens when I'm pulling away from a COMPLETE stop. It happens from about 900-1500 rpms and it happens everytime. Sounds like someine is taking a wrench and whacking against my bellhousing, there is a metalic "ting" to it...maybe more like a wrench against my Y-pipe of the exhaust system.

    The key symptom is that it is related to starting from a COMPLETE stop. Just a small amount of speed is enough to keep the noise from happening at all. For example I can be going 3mph..then floor it...no noise. I have to come to a complete stop. I can floor the gas pedal while driving going up a hill...tranny will downshift and everything...no noise.

    It doesn't happen in reverse only in drive.

    It is definitly related to overcoming inertia (an object at rest wants to stay at rest blah blah blah). Since this would be the point of the greatest torque on the engine mounts etc...I'm leaning towards me failing to torque a bolt down somewhere in the engine/tranny connection when I installed the engine, and now it is coming loose and under extreme torque pulling away from a dead stop something is moving/flexing enough to cause a bolt on something rotating member to interfere with something. (like a bolt on the flexplate or toque converter or on the torque converter cover).

    Again, the noise sounds like a wrench wacking against a heavy metalic object...it is pretty fast ...ting..ting...ting..ting..ting..I think it speeds up with RPM...then it slows down a little and stops after 1500rpms..then I never hear again until the next complete stop. It is worse if I'm staring on an uphill...quieter on level ground and almost non-existant starting on a downlope. The harder I start..the louder the noise...so I'm trying to start everytime by "feathering" the gas pedal as slow as possible...but noise happens anyway, just not as loud.

    The only other thing I can think of is a faulty torque converter itself, but this was relaced new when I installed the engine and only has ~1000 miles on it.

    Also what if the flexing was causing the shift linkage to push against the "edge" of the D position..

    This obviously is very disheartening.
    Last edited: Jul 27, 2009
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  17. 2COR517

    2COR517 New Member

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    I would check your motor/transmission mounts, especially where you have a shifter alignment issue. Could also be a flywheel and/or torque converter. A company I worked for had a 84ish 1 ton that chewed up flywheels. I never drove it when it was broken, but they said it was "ticking".

    And, from a dead stop is when TC torque multiplication is at it's highest.
    #17
  18. Big6ft6

    Big6ft6 New Member

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    something just occurred to me.

    Why wouldn't this happen in reverse? The flywheel and torque converter are still rotating in the same direction when in reverse.

    This must then be related to actual torque form the driveshaft torquing the transmission casing somehow...becuase only output shaft is changing direction between drive and reverse.

    Hmmmm???
    #18
  19. Crawdaddy

    Crawdaddy Moderator

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    U-joints maybe? I just diagnosed my cousin's 2000 Silverado and turns out the front u-joint was GONE. He would have a weird ticking starting off the line and when he'd slow down to a stop. Judging from the fact that you have an 89, I'd bet it's not a u-joint for the front axle, since it was a real transfer case back then, but it could be the front u-joint on the rear driveshaft.
    #19
  20. Big6ft6

    Big6ft6 New Member

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    Thanks for the idea Craw...but I can get it to happen when the car isn't moving by stepping on the brakes and given her some gas. So the "ticking" happens even when the drive shaft isn't moving.

    It really sounds like something rotating that is contacting something else. so what is all rotating when the truck is in gear, but not moving? flywheel, torque converter, tranny pump? Anything else?

    It seems to be coming from the tranny area, but that is the best I can tell.

    What happens if the shift linkage connection is getting torqued around...would that cause clicking in the tranny?
    #20

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