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GT40 Tech - Engines/Induction/Exhaust Motors and engine related - right here!

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Old 17th April 2017, 03:19 PM   #1
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Knock Knock

Unfortunately, this is not the start of a joke thread! While driving around the other day, I developed a pretty nasty mechanical knocking that started low and progressed much louder over just a couple of miles. Engine sputters and lurches, but I have not lost fluids or noticeably blown anything apart. I was able to limp the car back to the house, albeit not looking or sounding very cool.

Before I start tearing this thing apart, I am looking for any and all educated guesses on what something could be. The thought being that if I get too many that are really expensive to fix, I may just commit to rebuilding the other 302 I have and swap it out. If things come in fairly benign, maybe I try a few and see where it takes me.

My Father has told me to check the harmonic balance, so that is one. I am sure you fine group of Gentlemen have questions, so fire away.

Thanks in advance!
Robert
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Old 17th April 2017, 03:28 PM   #2
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Re: Knock Knock

Does the knock speed up when engine is reved ? if so sounds like big ends ...
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Old 17th April 2017, 03:55 PM   #3
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Re: Knock Knock

Robert, when I see a post like yours I usually check recent posts.
You have had a slow WP leak, replaced timing cover, waterpump and mentioned your engine placement is 'nose down'. I have to ask, have you fitted coolant/air bleed lines to the rear of your intake manifold. You could have a ign timing problem with the lurching etc or a piston partially seizing due to a coolant air lock or the Balancer could be coming loose. Check to see if timing has moved, do a bolt check, did you get a tank full of low octane by mistake.... I could go on!
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Old 17th April 2017, 04:51 PM   #4
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Re: Knock Knock

Check your oil first
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Old 19th April 2017, 08:46 AM   #5
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Re: Knock Knock

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Originally Posted by johnny cobra View Post
Does the knock speed up when engine is reved ? if so sounds like big ends ...
Yes, the knock does speed up with increased RPMs.
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Old 19th April 2017, 09:04 AM   #6
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Re: Knock Knock

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Originally Posted by jac mac View Post
Robert, when I see a post like yours I usually check recent posts.
You have had a slow WP leak, replaced timing cover, waterpump and mentioned your engine placement is 'nose down'. I have to ask, have you fitted coolant/air bleed lines to the rear of your intake manifold. You could have a ign timing problem with the lurching etc or a piston partially seizing due to a coolant air lock or the Balancer could be coming loose. Check to see if timing has moved, do a bolt check, did you get a tank full of low octane by mistake.... I could go on!
The balancer was my Father's first guess since he helped with whole front end overhaul. We did not put in any additional bleed offs anywhere and I don't believe the fuel is an issue. The car has been driven since then, and no timing issues have come up.
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Old 19th April 2017, 09:52 AM   #7
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Re: Knock Knock

I'd pull the valve covers and look to see if the push-rods are OK. I once drove a GTO with much the same symptoms you mentioned and thought it would need to have the bottom end rebuilt; imagine my pleasant surprise when the Pontiac dealership called in 3 hours to say it was running again...a bent pushrod. It might be worth the time to check.

Cheers!

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Old 19th April 2017, 10:47 AM   #8
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Re: Knock Knock

Robert, From what you have described, loud knocking noise, it makes me believe its something related with the rotating assembly, piston, rod or main bearing. If you are convinced it's internal engine noise and not balancer or flywheel/clutch noise I would start by pulling the spark plugs just to verify something did not get on top of a piston. Look for damage to the end of the spark plug a small screw or something. You said the noise started low and got louder so I would not think you will find anything but it's easy to check before looking deeper. Next I would pull the oil pan. Look for metal in the pan and a discolored rod bearing cap. If I see any metal in the bottom of the pan I would commit to pulling the engine and a rebuild if possible.
That's just my thought process, I'm not there and hearing the knock so you are the one committing to all the work and the direction you take.
Good Luck,
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Old 19th April 2017, 06:32 PM   #9
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Re: Knock Knock

Not hard to diagnose if I was there but drop the oil and check for metal and remove oil filter, cut around the top with a hack saw, check media for pos. metal.
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Old 19th April 2017, 07:49 PM   #10
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Re: Knock Knock

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Originally Posted by bill kearley View Post
Not hard to diagnose if I was there but drop the oil and check for metal and remove oil filter, cut around the top with a hack saw, check media for pos. metal.
Better to punch a hole in the can with a chisel, then use a tin-snips.
Cutting the can with the saw puts metal in the pleats of the filter.
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Old 20th April 2017, 10:53 AM   #11
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Re: Knock Knock

Sounds like a spun rod bearing... Try disconnecting one spark plug wire at a time to help isolate which cylinder is suspect.
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Old 20th April 2017, 04:40 PM   #12
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Re: Knock Knock

If you keep running it to try some of the suggestions above its quite likely the problem will get bigger than when first heard, even the 'must' drive home probably will have seen to that.
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Old 20th April 2017, 04:44 PM   #13
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Re: Knock Knock

I agree that it sounds like a spun bearing however it could be anything and more likely something done recently like you say HB. Good/bad news is that bearing should show in oil so I agree, that is step one. Also if/when pulling plugs for inspection do a leak down while you're at it, could uncover something. Removing valve covers and turning engine over by hand to try and find source is another option before pulling stuff off that requires more work - be sure to check trunions for damage. Do you have solid or hydrl lifter? could be collapsed/damaged lifter. Another good guess with the timing but you say checks out. Good Luck.

Last edited by LBCportagee; 20th April 2017 at 05:32 PM.
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Old 20th April 2017, 06:13 PM   #14
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Re: Knock Knock

Double check your pulleys bolted to the harmonic balancer.
I bought a truck once with "a rod knock" and it turned out that the bolts holding the pulleys to the HB had backed off..
same holds true for flywheel bolts - but they're obviously more difficult to check.
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Old 21st April 2017, 09:10 AM   #15
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Re: Knock Knock

Thanks guys for the suggestions.

Robert
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Old 24th April 2017, 10:09 PM   #16
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Re: Knock Knock

Stop running engine. At this point you might still have salvageable parts like pistons, crank, cam, and rods. Pumping bearing material all around the system isn't going to make anything better. It is soft compared to the steel stuff like the cam and crank and these can be used again if the damage isn't too bad. Over size bearings, crank grind, etc. can fix it as good as new.

Take off filter, pore out a couple of ounces through a white paper towel. Look for brass/silver/metal material. If you have any then drop the oil pan next and check again. Spun bearings will deposit bearing material in the oil pan. If you have it there then the whole motor is in need of a rebuild.

Sorry no way around that. The good news is that a rod bolt didn't let go, you have something left to rebuild and bearings and gaskets are cheap.

Last edited by Howard Jones; 24th April 2017 at 10:16 PM.
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Old 8th May 2017, 01:25 PM   #17
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Re: Knock Knock

Gents,

Good news and a bullet dodged!!!

I discovered that one of the roller rocker nuts had snapped the head off. The rocker backed it's way off far enough to turn 20 degrees and come completely off the valve stem and push rod. Unfortunately it bent the rod, but other than that, I am back in business. This explains why it went from bad to worse as I nursed it home. It took a while for that exhaust valve to completely stop working.

Thanks again for all the help!

Robert
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Old 8th May 2017, 01:39 PM   #18
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Re: Knock Knock

Quote:
Originally Posted by RChianese View Post
Gents,

Good news and a bullet dodged!!!

I discovered that one of the roller rocker nuts had snapped the head off. The rocker backed it's way off far enough to turn 20 degrees and come completely off the valve stem and push rod. Unfortunately it bent the rod, but other than that, I am back in business. This explains why it went from bad to worse as I nursed it home. It took a while for that exhaust valve to completely stop working.

Thanks again for all the help!

Robert
When you say "...bent the rod", you mean the pushrod, not a con-rod, right?

Sounds like you had the same problem I had with my GTO...and I agree, I was SO relieved when it didn't require tearing the engine down to figure out what was wrong internally! I'm glad it worked out as well for you

...I hate to ask, though...did you find the part that snapped off the roller rocker nut? If not...well, you will want to!

Cheers!

Doug
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Old 8th May 2017, 02:31 PM   #19
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Re: Knock Knock

That could be partially good news. I had a feeling that since you drove it home and it didn't completely grenade on you that it could be valvetrain related. I think you got lucky there anyway. So is your expectation that you had a faulty part or did you diagnose a cause? Improper lash setup? Have you had engine work lately new cam? could piston have hit valve as this could snap the nut off and bend the push rod. Also this could have been your knock that once it actually broke progressed as you said. Could also be an oiling issue.

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Old 8th May 2017, 02:51 PM   #20
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Re: Knock Knock

This was very similar to what happened to me a few years back. Stopped at an AutoZone, removed the valve covers and immediately saw the problem (except no broken studs), removed the rocker and bent pushrod, and off to the house I drove. All went back together with a new pushrod and guide plate without nary a problem since. Never understood or could determine the cause of it in the first place other than a weak pushrod.
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