l98 knocking help
Thread Starter
Heel & Toe
Joined: Apr 2023
Posts: 18
Likes: 2
l98 knocking help
In december of 2022, knowing nothing about cars, purchased a 1986 corvette only to drive it home and hear it start knocking. Over the winter I dropped the pan, and replaced all rod and main bearings with the exception of the upper half of main bearing #5. I also decided to run 20w-50 oil. I took it for a test drive and the knocking remained. After doing a little more research I decided to replace the harmonic balancer as they have been known to fail. Took it for another test drive and the knocking is still there.
The engine starts quiet and knocking only happens when then oil is up to temp and is only noticeable at idle. Its a quiet knock definitely not a rod knocking but definitely not a tick.
Really need some advice. I'm $800 into parts on a cart that supposedly ran fine and with my limited experience working on cars I cant figure out what the problem is. I did everything I could do without pulling the engine.
Thank you for any feedback or ideas you may have.
The engine starts quiet and knocking only happens when then oil is up to temp and is only noticeable at idle. Its a quiet knock definitely not a rod knocking but definitely not a tick.
Really need some advice. I'm $800 into parts on a cart that supposedly ran fine and with my limited experience working on cars I cant figure out what the problem is. I did everything I could do without pulling the engine.
Thank you for any feedback or ideas you may have.
Advanced
Joined: Apr 2023
Posts: 99
Likes: 14
From: Erie Pa.
I hate to say this. When its cold the tolerances are tighter, as it warms up, heat expands everything, dealt with it many times before. With a knock like that, you cant just go in and replace bearings and think that will fix it, especially only pulling the pan and not measuring all the tolerances. Pull the motor and see what is actually wrong. Only way. And throwing 20/50 just hides the problem until that heats up.
Racer
Joined: Mar 2020
Posts: 387
Likes: 148
From: Langton, Ontario, Canada
2025 C4 of the Year Finalist - Unmodified
Run the engine until it starts knocking and stop it, remove the drive belt first and run the engine again, just for 30 seconds. This will just make sure it's not the water pump or something belt-driven clanking after a warmup.
If that doesn't identify it, then it sounds to me like you may have a lifter and/or camshaft problem. Now that you've opened up the bottom of that engine, it sounds like time to open up the top. Fortunately, you won't have to remove the heads for that. You may be able to point yourself in the right direction by using an infra-red temperature sensor, on the exhaust manifolds, right at the connection to the head. A malfunctioning cylinder will be cooler than a normally operating one. Make sure you check this while the engine is knocking. That 20w50 oil is a bit thick for your L98. I'd stick with 10w30, especially now that you have changed the bearings, so that you get lots of normal oil in there, as opposed to a little thick oil. I wish you would have changed the oil pump while you were in the neighbourhood. Good luck with this!
If that doesn't identify it, then it sounds to me like you may have a lifter and/or camshaft problem. Now that you've opened up the bottom of that engine, it sounds like time to open up the top. Fortunately, you won't have to remove the heads for that. You may be able to point yourself in the right direction by using an infra-red temperature sensor, on the exhaust manifolds, right at the connection to the head. A malfunctioning cylinder will be cooler than a normally operating one. Make sure you check this while the engine is knocking. That 20w50 oil is a bit thick for your L98. I'd stick with 10w30, especially now that you have changed the bearings, so that you get lots of normal oil in there, as opposed to a little thick oil. I wish you would have changed the oil pump while you were in the neighbourhood. Good luck with this!
Thread Starter
Heel & Toe
Joined: Apr 2023
Posts: 18
Likes: 2
Run the engine until it starts knocking and stop it, remove the drive belt first and run the engine again, just for 30 seconds. This will just make sure it's not the water pump or something belt-driven clanking after a warmup.
If that doesn't identify it, then it sounds to me like you may have a lifter and/or camshaft problem. Now that you've opened up the bottom of that engine, it sounds like time to open up the top. Fortunately, you won't have to remove the heads for that. You may be able to point yourself in the right direction by using an infra-red temperature sensor, on the exhaust manifolds, right at the connection to the head. A malfunctioning cylinder will be cooler than a normally operating one. Make sure you check this while the engine is knocking. That 20w50 oil is a bit thick for your L98. I'd stick with 10w30, especially now that you have changed the bearings, so that you get lots of normal oil in there, as opposed to a little thick oil. I wish you would have changed the oil pump while you were in the neighbourhood. Good luck with this!
If that doesn't identify it, then it sounds to me like you may have a lifter and/or camshaft problem. Now that you've opened up the bottom of that engine, it sounds like time to open up the top. Fortunately, you won't have to remove the heads for that. You may be able to point yourself in the right direction by using an infra-red temperature sensor, on the exhaust manifolds, right at the connection to the head. A malfunctioning cylinder will be cooler than a normally operating one. Make sure you check this while the engine is knocking. That 20w50 oil is a bit thick for your L98. I'd stick with 10w30, especially now that you have changed the bearings, so that you get lots of normal oil in there, as opposed to a little thick oil. I wish you would have changed the oil pump while you were in the neighbourhood. Good luck with this!
I did in fact change the oil pump to a high volume oil pump, oil pressure when warm is about 12psi at idle. I checked without the accessory belt and it doesn't make the noise which is why I replaced the harmonic balancer. But I figured once it persisted the belt was causing the crank to wobble possibly.
It would probably be worth mentioning I also have a surging problem while idling (possibly a bad IAC sensor) my idle goes from 400 to 700 rpm's and back again only in park.
thank you for the advice!
