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Strange Engine Knock

Discussion in 'Chevy Trucks' started by bowtie726, Dec 30, 2005.

  1. bowtie726

    bowtie726 Junior Member
    Messages: 15

    When I bought my 74 K10 it had a sweet running hopped up 350 in it.... I've since blown that motor and had to stick a 305 in it just so the truck would still be usable. I only paid $175 for it so I didn't expect anything great, but I've got this strange knock and was wondering where I should start looking for damage. It only knocks when the engine is under load. Cruising at between 25-40 is fine, but when under acceleration, or at speeds over 45, or when towing, going up a hill....you get the picture.... it knocks. I've never had any experience in diagnosing knocks, so i really can't narrow down whether it's the top end or bottom end. Oh, and when the truck's in neutal/park you can rev the engine up to near red line and won't hear the knock at all, which is really what throws me off. Anyone know where I should start? Pull the oil pan and check out the main bearings, or the intake manifold to check out the rods/lifters, or what?? ANY IDEAS??:confused:
  2. schnitz

    schnitz Member
    Messages: 98

    Long shot, but try looking at the torque converter bolts. Seems to me it should knock in the motor at all times if it's an internal engine problem.

    In a while, Chet.
  3. jrm123180

    jrm123180 Senior Member
    Messages: 156

    Try a higher octane fuel...recently had this problem and it turned out to be that the fuel octane rating wasn't enough for the timing I was running at
  4. Detroitdan

    Detroitdan PlowSite.com Addict
    Messages: 1,937

    Top end knock is twice engine speed, bottom end knock is engine speed. A bad rod or main bearing will knock at first only under load. Don't worry, it will get worse. I think it is more likely a rod if it's only under load, the rods are more likely to reflect knock under varying engine loads. A main bearing is less likely to be affected by load, they will usually knock all the time. Which is not to say that it couldn't be a cracked torque converter or flexplate, or low octane as mentioned above. Even bad timing can make the top end run far enough out that it will knock, because the fuel is ignited at the wrong time. I hate to tell you to spend money tuning it up when it could be terminally ill, but that's really the only choice. Or run it til it quits. At least you aren't out a ton of money on the motor. I personally have had great luck with 305s, people badmouth them but I think they are a great motor. Best of luck with it.
  5. bowtie726

    bowtie726 Junior Member
    Messages: 15

    Tomarrow I'll take a look at the torque converter bolts. And I don't think it needs a tune up because when i stuck the motor in I put new sparkplugs, cap, rotor, and wires in with it... The other day while i was monkeyin' around underneath the truck I thought I saw one of the transmission to block bolts loose...haven't had time to check it out though, could this cause a knock? So you think I should check for a bent rod first then? Oh, one thing I didn't mention was that when I swapped the 350 for the 305 I used the aftermarket raised intake manifold off the 350. I had looked over the aftermarket and stock manifolds and they seemed to be the same thing except for the raised carb. mounting. And then I also put the holly street avenger dual bowled 4 barrel carb on. Could that have anything to do with it??
    Last edited: Dec 30, 2005
  6. lorentzlawnsnow

    lorentzlawnsnow Senior Member
    Messages: 143

    start simple.. check the timing.
  7. Midwest BuildIt Inc

    Midwest BuildIt Inc PlowSite.com Addict
    Messages: 1,280

    i would say that it is a rod, i had a ford that did the same thing before it came apart. the rod only nocked under load, and slowly got worse.
  8. Jpocket

    Jpocket Senior Member
    Messages: 302

    I'd say run it till it blows, have another motor siting on stand by. a 305/350 is gonna knock a long time before it blows.
  9. jrm123180

    jrm123180 Senior Member
    Messages: 156

    Just start with the simple things first....suck as timing and/or using a higher octane fuel, loose or converter bolts, or even a cracked flex plate.

    I would then suggest getting a stethoscope from your local auto store and listen to the motor so that you would at least have an idea as to where the noise is coming from.

    Good Luck!!!
  10. R Thomas

    R Thomas Junior Member
    Messages: 6

    On the older Chevy motors the spring in the fuel pump would go bad, sometimes it would sound just like a rod knock because it would let the fuel pump rod beat against the cam. Probably not it but it's a thought.
  11. derekbroerse

    derekbroerse 2000 Club Member
    Messages: 2,377

    a) Are you sure the distributor got set in the right place and didn't move?

    b) If tranny-to-block bolts are loose, could the torque convertor bolts be too? Under load, the engine rocks a bit, if its loose, maybe the alignment of it all could be out of whack and causing something. Inspect the flexplate itself well, they crack fairly regularly... usually around the crank flange somewhere. They tend to sound like a mild rod knock.

    Manifolds fit either engine no problem. Exhaust manifolds are where sometimes the fit problems come from. But neither makes knocks... sometimes exhaust leaks, but no knocks.
  12. bowtie726

    bowtie726 Junior Member
    Messages: 15

    Yeah, I also stuck the headers off the 350 onto the 305, They leaked on the 350, but got them to seal on the 305....
    I crawled under there today and checked out those transmission bolts, only found one loose, so I tightened it up and drove it, and at first it didn't seem to be knocking so bad, hardly at all.... but later on in the trip it started doing it again, but not quite as loud as it was before
  13. yzf1000_rider

    yzf1000_rider Senior Member
    Messages: 147

    just a guess but how about egr valve sticking?
  14. bowtie726

    bowtie726 Junior Member
    Messages: 15

    Could be I suppose, I haven't ever replaced them, but how's that make a knock? It may explain why I can't get it to seal up and stop leaking oil, but how's it create a knock?
  15. ljrce

    ljrce Senior Member
    Messages: 181

    what abot the knock sensor i dotn kwno if they had them back in those years or not
  16. bowtie726

    bowtie726 Junior Member
    Messages: 15

    well the 305 has the knock sensor with the connetion sticking out, but have nothing to plug in to it...
  17. derekbroerse

    derekbroerse 2000 Club Member
    Messages: 2,377

    The 305 is obviously a lot newer than your truck. They started using them back in the early/mid 80's. In your case, it will be nothing but a fancy pipe plug :D

    Have you confirmed the engine timing yet?
  18. derekbroerse

    derekbroerse 2000 Club Member
    Messages: 2,377

    Duplicate post.
    Last edited: Jan 5, 2006
  19. BigBurban

    BigBurban Member
    Messages: 31

    Check your timing as mentioned several times....

    unhook your vaccume advance and plug it and take it for a ride and see if it goes away. The advance may be stuck or too steep of a curve.