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sgellis
Samba Member
Joined: July 06, 2016
Posts: 2196
Location: SW Nova Scotia, Canada
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 Posted: Sat Aug 07, 2021 7:11 pm Post subject: Sudden loud knock on 4k engine |
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1600 sp rebuilt 4k miles ago. New piston/cylinders, heads, connecting rods, bearings, push rods, filter pump and block redone.
Has run perfectly from rebuild in 64 camper.
Two days ago we stoped and engine cooled. Started and moved twice. All perfectly normal. Today started it with no throttle and instantly very loud knock. Shut off immediately. Turned over by hand easily. Removed coil wire and cranked normally with no knock and even compression. Put coil wire back and started with loud knock. Shut off immediately. Sounded exactly like a very bad rod knock. Didn't run again as it sounded like something was about to disassemble in a dramatic fashion.
It has never knocked before.
Drained oil. Looked about the correct amount and not to bad looking.Pulled motor and disassembled expecting to see a broken rod bolt or some other major failure. Found nothing. All case bolts torqued. All rods torqued correctly. Spun nice and felt good on crank. All bearings look like they are not even worn in yet. Pushrods look good. Cam looked good. Gland nut was tight. Pistons look good. Wrist pins look good. Cut open the filter and very clean. Nothing in base. Alternator spins free with fan in shroud.
Engine did have a fair amount of carbon in it for 4k. Oil in number one I think but nothing in exhaust so maybe from moving engine around.
No idea what to do next except put it back together and run it until we find the problem. Maybe it was just a freak thing but it certainly didn't sound like it
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cbeck
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Joined: January 14, 2014
Posts: 2570
Location: high ridge, mo
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sgellis
Samba Member
Joined: July 06, 2016
Posts: 2196
Location: SW Nova Scotia, Canada
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danielzink
Samba Member
Joined: August 03, 2004
Posts: 289
Location: Lansing Michigan
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| Posted: Sun Aug 08, 2021 6:22 am Post subject: Re: Sudden loud knock on 4k engine |
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With it running did anyone bother to grab the fan shroud and rock fore and aft to see if it was possibly the fan scraping ?
<don't laugh...stranger thing have happened>
Dan
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sgellis
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Joined: July 06, 2016
Posts: 2196
Location: SW Nova Scotia, Canada
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raygreenwood
Samba Member
Joined: November 24, 2008
Posts: 23205
Location: Oklahoma City
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| Posted: Sun Aug 08, 2021 8:31 am Post subject: Re: Sudden loud knock on 4k engine |
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Possibilities in my mind.....if you have found nothing out of sorts inside.....sometimes exhaust issues can sound like knocks.
Loose exhaust stud between head and beat exchanger....exhaust leak
Crack in muffler.
Head to cylinder leak. Did you run a compression test before tearing down?
Ray |
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Frank Bassman
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Joined: July 01, 2012
Posts: 903
Location: Miami
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| Posted: Sun Aug 08, 2021 9:11 am Post subject: Re: Sudden loud knock on 4k engine |
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Cam thrust bearing to cam play? Sometimes they make a racket...
-Frank |
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sgellis
Samba Member
Joined: July 06, 2016
Posts: 2196
Location: SW Nova Scotia, Canada
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sgellis
Samba Member
Joined: July 06, 2016
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Location: SW Nova Scotia, Canada
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Brian_e 
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Joined: July 28, 2009
Posts: 4053
Location: Rapid City, South Dakota
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| Posted: Sun Aug 08, 2021 10:56 am Post subject: Re: Sudden loud knock on 4k engine |
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Check the cam gear backlash. I had two aftermarket aluminum gears go to shit on me. They sound exactly like a rod knock at idle.
A proper fitting cam gear will have no backlash, but also no tight spots, and it won’t lift out of the case half when the crank is turned backwards. The factory had 15 different cam gear sizes to make the backlash perfect on their new cases. Aftermarket only has one size.
Brian
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evanfrucht
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Joined: July 24, 2016
Posts: 2186
Location: Laurel Canyon, CA
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| Posted: Sun Aug 08, 2021 10:58 am Post subject: Re: Sudden loud knock on 4k engine |
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Perhaps something wrong with starter or timing way off by accident, something wrong with ignition. Violent backfire of some sort as it's trying to start?
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Dan the workingstiff
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Joined: July 03, 2006
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Location: Downriver, MI
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| Posted: Sun Aug 08, 2021 11:02 am Post subject: Re: Sudden loud knock on 4k engine |
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| Brian_e wrote: |
The factory had 15 different cam gear sizes to make the backlash perfect on their new cases. Aftermarket only has one size.
Brian |
Interesting!
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sgellis
Samba Member
Joined: July 06, 2016
Posts: 2196
Location: SW Nova Scotia, Canada
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| Posted: Sun Aug 08, 2021 11:06 am Post subject: Re: Sudden loud knock on 4k engine |
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| Brian_e wrote: |
Check the cam gear backlash. I had two aftermarket aluminum gears go to shit on me. They sound exactly like a rod knock at idle.
A proper fitting cam gear will have no backlash, but also no tight spots, and it won’t lift out of the case half when the crank is turned backwards. The factory had 15 different cam gear sizes to make the backlash perfect on their new cases. Aftermarket only has one size.
Brian |
Should be good. Original VW cam in great shape and was working. We turned the crank backwards and stayed put. No sign of looseness or movement on cam gear. Cam bearings looked new.
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sgellis
Samba Member
Joined: July 06, 2016
Posts: 2196
Location: SW Nova Scotia, Canada
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| Posted: Sun Aug 08, 2021 11:25 am Post subject: Re: Sudden loud knock on 4k engine |
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| evanfrucht wrote: |
| Perhaps something wrong with starter or timing way off by accident, something wrong with ignition. Violent backfire of some sort as it's trying to start? |
It started immediately and didn't notice it running ruff but the noise was a bit distracting so maybe it was. I suppose it is theatrically possible for the timing to get so far out to knock that bad and still run but doubt it would start so well.
I was considering what could go wrong that would fire a cylinder at the wrong spot. That could make a hell of a knock. Distributor cap and button looked good. I didn't inspect wires but they would need to have quite bad damage to cross fire I would think. We should have noticed that when removing them. All wires were plugged in good and cap was secure.
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My camper interior build
My 54 standard restoration
The Nova Scotian Barndoor (YouTube)
The Nova Scotian Barndoor (CAD drawings) |
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Miyagi
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Joined: September 08, 2019
Posts: 7
Location: CHILE
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| Posted: Sun Aug 08, 2021 11:54 am Post subject: Re: Sudden loud knock on 4k engine |
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hello, I'm going to tell you a story.
Years ago at a VW club meeting, a T2 arrived with a problem similar to yours. The engine was reconditioned and it had like 5 or 6 thousand km. Sometimes when starting the engine loud knocks were heard. The driver cut the ignition and started once or twice and the noise disappeared and the vehicle ran normal.
After many opinions to confirm ours conclusions, we called our friend and best master mechanic in Air-cooled, and he came to listen to T2 until the fault occurred (loud noise behind the engine).
He said, excessive play of the crankshaft and back support of the main bearing in the block (case) is o are damaged. OMG, T2 go right now on a crane to workshop and disassembly the engine.
The facts was, the dowel pin that holds the crankshaft main bearing was soft, shorter, and broke the housing hole in the main bearing. The soft metal would sometimes turn or rotate under the crankshaft rotation and return to its position and made the noise, and some case damage too... Bad selection of the pin, its size and fit.
No damage in crankshaft, piston, heads, push rod, oil pump.
Have a nice day |
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sgellis
Samba Member
Joined: July 06, 2016
Posts: 2196
Location: SW Nova Scotia, Canada
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| Posted: Sun Aug 08, 2021 1:45 pm Post subject: Re: Sudden loud knock on 4k engine |
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| Miyagi wrote: |
The facts was, the dowel pin that holds the crankshaft main bearing was soft, shorter, and broke the housing hole in the main bearing. The soft metal would sometimes turn or rotate under the crankshaft rotation and return to its position and made the noise, and some case damage too... Bad selection of the pin, its size and fit.
No damage in crankshaft, piston, heads, push rod, oil pump.
Have a nice day |
Thanks! We will take a very good look at the dowels and bearing fit in case. they were stuck a bit in case so definitely not too far from correct fit. All looked good but didn't really look close at the dowels and their fit.
_________________
My 64 Kombi restoration
My camper interior build
My 54 standard restoration
The Nova Scotian Barndoor (YouTube)
The Nova Scotian Barndoor (CAD drawings) |
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Dougy Dee
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Joined: August 21, 2004
Posts: 1763
Location: Niagara Region, CANADA
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| Posted: Sun Aug 08, 2021 5:09 pm Post subject: Re: Sudden loud knock on 4k engine |
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| A plug blown out will sound like that too... |
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evanfrucht
Samba Member
Joined: July 24, 2016
Posts: 2186
Location: Laurel Canyon, CA
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| Posted: Sun Aug 08, 2021 5:21 pm Post subject: Re: Sudden loud knock on 4k engine |
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| sgellis wrote: |
| evanfrucht wrote: |
| Perhaps something wrong with starter or timing way off by accident, something wrong with ignition. Violent backfire of some sort as it's trying to start? |
It started immediately and didn't notice it running ruff but the noise was a bit distracting so maybe it was. I suppose it is theatrically possible for the timing to get so far out to knock that bad and still run but doubt it would start so well.
I was considering what could go wrong that would fire a cylinder at the wrong spot. That could make a hell of a knock. Distributor cap and button looked good. I didn't inspect wires but they would need to have quite bad damage to cross fire I would think. We should have noticed that when removing them. All wires were plugged in good and cap was secure. |
Could plug wires have gotten switched around either at the cap or the plugs on either side?
I've heard a bad starter hit a flywheel but it makes a very loud bell like sound, not really a knock. I don't think that's what's going on but just throwing it out there.
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1964 Fury Wagon ( the pavement shredder ) |
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sgellis
Samba Member
Joined: July 06, 2016
Posts: 2196
Location: SW Nova Scotia, Canada
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| Posted: Sun Aug 08, 2021 5:38 pm Post subject: Re: Sudden loud knock on 4k engine |
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| evanfrucht wrote: |
| sgellis wrote: |
| evanfrucht wrote: |
| Perhaps something wrong with starter or timing way off by accident, something wrong with ignition. Violent backfire of some sort as it's trying to start? |
It started immediately and didn't notice it running ruff but the noise was a bit distracting so maybe it was. I suppose it is theatrically possible for the timing to get so far out to knock that bad and still run but doubt it would start so well.
I was considering what could go wrong that would fire a cylinder at the wrong spot. That could make a hell of a knock. Distributor cap and button looked good. I didn't inspect wires but they would need to have quite bad damage to cross fire I would think. We should have noticed that when removing them. All wires were plugged in good and cap was secure. |
Could plug wires have gotten switched around either at the cap or the plugs on either side?
I've heard a bad starter hit a flywheel but it makes a very loud bell like sound, not really a knock. I don't think that's what's going on but just throwing it out there. |
I suppose it is possible someone switched the wires on me but it wasn't that kind of crowd and an isolated spot.
_________________
My 64 Kombi restoration
My camper interior build
My 54 standard restoration
The Nova Scotian Barndoor (YouTube)
The Nova Scotian Barndoor (CAD drawings) |
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sgellis
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Joined: July 06, 2016
Posts: 2196
Location: SW Nova Scotia, Canada
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