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#1 |
Registered User
Join Date: May 2012
Location: Littlerock,Ca
Posts: 13
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Sounds like rod knocking
I have a 71 gmc with an early 80s 350 and a 3 speed with granny gear. I recently started to get a knocking sound from my truck but only in 2nd and 3rd only when gassing if I let off it stops and if I'm parked in neutral I can free rev with no noise. Any advise on where I can start looking.
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#2 |
Registered User
Join Date: Feb 2000
Location: Center City, MN, USA
Posts: 3,253
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Re: Sounds like rod knocking
Spark knock?
Maybe try higher octane or pour octane booster in what you already have.
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'70 cab, '71 chassis, 383, TH350, NP205. '71 Malibu convertible '72 Malibu hard top Center City, MN Last edited by mrein3; 02-21-2013 at 03:19 PM. Reason: spelling |
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#3 |
Registered User
Join Date: May 2012
Location: Littlerock,Ca
Posts: 13
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Re: Sounds like rod knocking
I run 91 just cause I feel it runs better
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#4 |
Registered User
![]() Join Date: Sep 2012
Location: Factory
Posts: 681
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Re: Sounds like rod knocking
timing too far advanced would be my guess. does it sound sort of like valves rattling when you are up to speed (like 50 mph) and trying to cruise along? no knocking/rattling when you are accelerating under load like when you are starting off from a light???
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#5 |
Registered User
![]() Join Date: Sep 2012
Location: Factory
Posts: 681
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Re: Sounds like rod knocking
did you just buy the truck or do any work on the motor recently?
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#6 |
Cluster King
![]() Join Date: Nov 2012
Location: Junction City, OR
Posts: 5,263
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Re: Sounds like rod knocking
Usually a rod will make some noise on a free rev. So it only knocks under a load? It it a deep thump or a tinny tic noise, like a lifter? Possible main bearing knock. They are a little hard to diagnose. It is a deep thump like noise and nothing like a valve train noise.
Have you checked the end play on the crank. If you can get both hands on the balancer and push/pull on it and hear any knock then it has to much end play. If you can't move it back and forth, then it is most likey OK. Also, you may have broke a gear tooth in the trans which will also cause a deep thump type noise under load. |
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#7 |
Registered User
Join Date: May 2012
Location: Littlerock,Ca
Posts: 13
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Re: Sounds like rod knocking
No when I take off in second (first) I don't get any noise. As soon as I shift to second(third) it sounds almost like a knock I have not done any engine work. It idles fine
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#8 |
Registered User
Join Date: May 2012
Location: Littlerock,Ca
Posts: 13
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Re: Sounds like rod knocking
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#9 | |
Registered User
Join Date: May 2012
Location: Littlerock,Ca
Posts: 13
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Re: Sounds like rod knocking
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#10 |
Registered User
Join Date: May 2012
Location: Littlerock,Ca
Posts: 13
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Re: Sounds like rod knocking
If its a bearing wouldn't it be thru he whole rpm range ?
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#11 |
Registered User
![]() Join Date: Sep 2012
Location: Factory
Posts: 681
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Re: Sounds like rod knocking
Could be any number of things. (cracked flywheel, drive shaft, anything). But if it's something that just started happening. Meaning you have had the truck and it didn't do it in the past and it's not a supplemental effect from some engine work you've done makes me think that something broke.
I had a buddy the other day that had a timing issue where when he was up to speed it sounded like every valve in his motor was rattling. Turned out it was too much advance on his distributor and when he got to cruise speed and the vacuum advance kicked back in it was causing predetonation. Try unhooking your vacuum advance on your distributor and plug the line coming out of the carb/intake. Then take it for a drive. You will get a little rough idle and a decreased throttle response. But when you get it up to speed top gear for your granny trans....see if you still hear it. If you do, plug vacuum advance can back up and rule that out. Super easy check just to see if it is timing curve. If no knocking....then you know where to go look. |
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#12 |
Cluster King
![]() Join Date: Nov 2012
Location: Junction City, OR
Posts: 5,263
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Re: Sounds like rod knocking
Not always. Main knocks are similar to rod knocks and they have a sweet spot where they make more noise.
It is one of those things that are hard to diagnose unless I can actually hear it. Check all of the obvious things like the flywheel like you mentioned. There are many things that it could be and hopefully we can help you narrow it down. |
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#13 | |
Registered User
Join Date: May 2012
Location: Littlerock,Ca
Posts: 13
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Re: Sounds like rod knocking
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#14 |
Registered User
Join Date: Nov 2010
Location: Las Vegas, Nevada
Posts: 160
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Re: Sounds like rod knocking
It's a manual trans with gears in it. Nobody else mentioned that maybe there may be some chipped teeth on 2nd and 3rd gears. If first is quiet, there may be no engine trouble. You can chip a gear tooth and every time it goes around, it will knock. Sometimes you can feel it in the shifter, sometimes you can't. And it can knock on accelleration, and not on coast, or vice-versa.
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T.Y.N Custom Works..... Because "Stock" Sucks! 1971 K 10 GMC Long Bed 1977 Formula T/A Frame off Resto-Mod ![]() 1995 FXDL Harley Low Rider 1985 C10 ShortWide 91 Burb front. |
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#15 |
Special Order
![]() ![]() Join Date: Nov 2004
Location: Mt Airy, MD
Posts: 85,859
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Re: Sounds like rod knocking
Does it knock in neutral? Sounds like your trans.
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"BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~ |
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