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Knock, Knock, Knock. WHY????

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Nate87k5, Jun 9, 2000.

  1. Nate87k5

    Nate87k5 Registered Member

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    Since the temp. out here and Phoenix has risen to the 108-112 mark I have been getting alot of spark knock. Seems like it only happens when the engine is running hot(205-210)and under a load. Never had a problem with this during the winter and never have any problem with it until the outside temp gets 100 or over which causes the engine to run hotter. Now I have checked and changed several things to no luck: Checked timing, spark plugs(looked good with good gap)cap and rotor(both looked good), wires, and I run 87 octane fuel. Replaced with in 8 months: MAP sensor,EGR valve, Knock sensor, O2 sensor, ECM, most vacuum lines, one injector, and coolant sensor, trans, heads ect. Now I run AC Delco plugs(nothing specail) and I'm not sure what heat range they are. I have been told that if the plugs get to hot the can cause preignition which will cause knock. I thought that the Knock sensor is suppose to prevent Knock. I am wondering if that is true or not and if I should change my brand of plug and heat range or get another Knock sensor. Maybe you guys can give me something else to check or replace. Although I would like to fix this problem because my wallet is getting alittle thin from paying for all the other stuff. I am not getting the "Service Engine" light warning. I'm stumped. Please help. 87 k5 TBI 700r4
     
  2. HarryH3

    HarryH3 1 ton status Author

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    Did you disconnect the timing jumper before checking the timing? If not, then it'll be way off. The knock sensor will only pull a few degrees out of the timing when it senses knock. If the timing is already too far advanced, it can't always make enough difference to stop the knock.

    Higher air temps and lower humidity both increase the tendency to spark knock. You've got plenty of both in Phoenix. [​IMG]

    <font color=black>HarryH3 - '75 K5</font color=black>
    http://ThunderTruck.ColoradoK5.com
     
  3. Nate87k5

    Nate87k5 Registered Member

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    HarryH, I did disconnect the timing jumper before checking the timing. Read 0 degrees just like it should according to the sticker on the front.
    You are correct on the High temp and low humidity here in Phoenix. In what way would that conribute to spark knock. Am I going in the right direction with changing to a cooler plug??? Thanks
     
  4. HarryH3

    HarryH3 1 ton status Author

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    Nate, spark knock is caused when the fuel ignites before the spark plug fires, while the piston is still rising in the bore.

    Hotter fuel ignites easier so as the temp rises, knock increases. Humidity in the air cools the air as it evaporates (which it does quickly at engine temps!). With less humidity there's less cooling, so again, hotter air/fuel mixture.

    The stock plug should be fine. I had an '87 K5 when I lived in Houston. Not as hot and dry as Phoenix, but still brutal summers. Spark knock wasn't a problem with that truck in that weather.

    Something is not working right on your engine. Running too lean can cause spark knock. A clogged injector, weak fuel pump or dirty fuel filter can cause the engine to run lean. If it runs too lean for too long, the ECM will set the check engine light.

    Has the knock sensor been removed for any reason? I read in a car mag recently that they have to be installed at the proper torque setting or they don't function quite right.

    You could also have carbon in the cylinders that's getting hot and igniting the fuel, causing the knock. GM dealers sell a top-end cleaner chemical that's supposed to do a good job of cleaning out any carbon, just be sure to follow the instructions carefully.

    <font color=black>HarryH3 - '75 K5</font color=black>
    http://ThunderTruck.ColoradoK5.com
     
  5. Nate87k5

    Nate87k5 Registered Member

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    Thanks for the help HarryH. I just wish I had a clear cut answer for what to fix or replace. Maybe I'll start with the easiest and cheapest and work my way up. I just hate to do that because it can be such a pain and so hard on the wallet. Again thanks.
     
  6. dumbfounded

    dumbfounded 1/2 ton status

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    When you replaced your knock sensor did you torque it properly, if you didn't this could be an easy fix. Those knock sensors can be a little touchy....

    Wasn't me.............
     
  7. brucenjudy

    brucenjudy Newbie

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    Nate, I also live in Phoenix and have been pulling my hair out trying to figure out what that dam knocking is. This is the first summer driving my 90 K5 and this knocking has been getting worse with the heat. Please post your results. I havn't done anything yet to fix this - hopefully it will be cheap.
     
  8. Mike's85K5

    Mike's85K5 Registered Member

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    Just one more simple thing, Nate. Since camshafts aren't degreed at the factory and timing chains do stretch try setting the timing a couple of degrees on either side of 0. Sometimes just this little bit of movement can eliminate the spark knock because the cam aligns itself to true 0 degrees.

    Mike
     
  9. Nate87k5

    Nate87k5 Registered Member

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    Just to update everyone on my Knock problem....I changed fuel grades and went up to 92 octane. This has help and the knock is hardly noticable. I'm on the second tank and will go back to 87 octane when I am done to see if the Knock comes back(cant keep buying 92 octane on my budget). When I installed the Knock Sensor I did not torque it down to specs so I will do that this weekend. Mike85 I think you maybe correct on the timing chain. I think mine is ready to be replaced. Engine has 145,000 on it and the Front main seal is leaking and I was thinking about putting on a new chain the same time I change the seal. Thanks for all the input on this matter. You guys/gals are great.

    PS. Can anyone suggest a brand of Timing Chain or Front Main Seal?
     

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