69 engine knock on start up
Thread Starter
Instructor
Joined: Jun 2019
Posts: 126
Likes: 3
From: Hatfield, Pa.
69 engine knock on start up
Hello,
My 69 BB has knocking noise on start up that I hear through the side pipe exhaust but not in the engine compartment. The knocking stops after about 2 minutes and I do not hear it after word, until the next cold engine start up. Any thoughts on the cause of this?
My 69 BB has knocking noise on start up that I hear through the side pipe exhaust but not in the engine compartment. The knocking stops after about 2 minutes and I do not hear it after word, until the next cold engine start up. Any thoughts on the cause of this?
Melting Slicks
Joined: Dec 2014
Posts: 2,786
Likes: 2,328
From: Central UT
Hi Scott, do you have a stethoscope? If you don't, you can try a long screwdriver, a piece of hose, etc. I'd try that and put it up against the valve covers, etc. to see if you can hear it. It could be a number of things. Probably minor since you can't really hear it in the engine compartment, but best to figure it out. BTW, which side do you hear the noise in the exhaust? A heat riser could be causing it if it's out of the right??
It would also help to know which 427 you have - some have more valve train 'noise' than others (like an L71 for example) that turn into music when properly adjusted.
Here's a stethoscope that I have, it works really well and I use it a few times/year to help find stuff. Best, Paul https://smile.amazon.com/Lisle-52750...s%2C178&sr=8-3
It would also help to know which 427 you have - some have more valve train 'noise' than others (like an L71 for example) that turn into music when properly adjusted.
Here's a stethoscope that I have, it works really well and I use it a few times/year to help find stuff. Best, Paul https://smile.amazon.com/Lisle-52750...s%2C178&sr=8-3
Thread Starter
Instructor
Joined: Jun 2019
Posts: 126
Likes: 3
From: Hatfield, Pa.
Thank you for that info! I have the L68 engine and I hear the knock noise out of the drivers side side exhaust port, although when I last checked, i did not even bother checking the passenger side exhaust for knock sound--my mistake!. i will have to pick up that stethoscope you recommend --as I do not currently have one. Thank you again.
Melting Slicks
Joined: Dec 2014
Posts: 2,786
Likes: 2,328
From: Central UT
^^^^ You are most welcome. Since the 'knock' is out of the right side exhaust and it stops after it's warmed up a bit, I'd check the heat riser. The spring on them can wear and cause some 'rattling' until it heats up. It may be something else, but I'd start there and then use your stethoscope.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
a minor rod knock will usually knock 3 to 10 times on startup until oil pressure comes up. a bad rod knock will not stop. this does NOT sound like that. look at the heat riser. see if it flops around. not when hot. wire it open.
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,137
Likes: 4,247
From: US-PNW
Derekderek, could this be piston slap from a worn bore that quiets as thermal expansion removes the excess clearance? I've no direct experience with it myself, so am just throwing it out there.
Scott1959, can you hear it with the hood open and standing next to the engine or just primarily out the exhaust?
Scott1959, can you hear it with the hood open and standing next to the engine or just primarily out the exhaust?
Thread Starter
Instructor
Joined: Jun 2019
Posts: 126
Likes: 3
From: Hatfield, Pa.
Also, forgot to ad that hear it on cold engine start up after car has been undriven for a week. I usually only drive it on weekends. Am i doing engine harm? Just do not have time right now to drive it more often.
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1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,137
Likes: 4,247
From: US-PNW
Since you only hear it out the exhaust, I am leaning away from an internal engine issue. Are all of your pipes securely fastened to their respective places?
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
i had a big block sounded like that at idle. it was the exhaust flappers. mercruiser. thru hull exhaust. but the heat riser is kind of the same thing. i would consider removing it and either cutting out the flapper if you want stock look, or replace it with the eliminator. also possible heat riser is working correctly but passage in intake is coked up and it is a flat out exhaust obstruction and exhaust is popping past it until it opens. unhook vacuum and wire the heat riser open before anything else. https://www.topflightautomotive.com/...hoCfosQAvD_BwE
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,137
Likes: 4,247
From: US-PNW
i had a big block sounded like that at idle. it was the exhaust flappers. mercruiser. thru hull exhaust. but the heat riser is kind of the same thing. i would consider removing it and either cutting out the flapper if you want stock look, or replace it with the eliminator. also possible heat riser is working correctly but passage in intake is coked up and it is a flat out exhaust obstruction and exhaust is popping past it until it opens. unhook vacuum and wire the heat riser open before anything else.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
it could possibly be exhaust popping thru partially blocked intake manifold exhaust passages. wiring it open is a 2 minute job. also, a forged piston engine would be more likely to have cold piston slap. but i don't think it would be heard thru the exhaust. it may be time to pull the intake. stock intake manifold? if so it has that hot slot smile under the carb. tap pipe threads and block it off both sides. needs to go whether it is the issue or not. burns up carb base gaskets and overheats carbs. not everything gm did was a brilliant idea.
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
I would definitely force the crossover valve open and keep it there with some coat hangar wire as a test first. They are problematic at best.
Fouled spark plugs cause a "puff" out the exhaust pipe as they misfire. The plugs might clear off later and run right. Just a thought.
Fouled spark plugs cause a "puff" out the exhaust pipe as they misfire. The plugs might clear off later and run right. Just a thought.
Safety Car
Joined: Sep 2005
Posts: 4,226
Likes: 643
From: Orange County Ca
Change your oil filter and/or oil. Try a different brand. Sometimes a bad or poorly designed anti drain back valve (Fram, etc.) is the problem. Real thick oil in cold weather doesn't help either.
