Knocking on a cold crank

[stingray72]

My truck knocks pretty hard in the morning when I crank my truck. It's only on a cold start and only when the truck is idleing. When I give it gas the knocking stops and after a few seconds of warm up it doesn't knock while it's idleing anymore. This is a pretty loud knock and you can feel it shake the truck

Anyone have any ideas as to what this could be so I can start figuring out how to fix this problem.

Thanks

[Classic Heartbeat]

Kinda sounds like a main bearing to me. When the truck warms up, the oil becomes more fluid, and when you rev the engine you bring the oil presser up, quieting the knock. What kind of oil pressure are you maintaining at an idle once your truck is warm? WES www.ClassicHeartbeat.com

[stingray72]

I'll have to check and see what the oil pressure is.

pardon my ignorance but what does the main bearing do and why would it going bad cause this problem? Also.. how hard is it to replace?

Sounds like there not any oil in the pistons on a cold start and it knocks until some is pumped in there. That's just a guess though

[lukecp]

Sure it isn't an exhaust leak? Sometimes the exhaust gasket will expand once it gets warm....would be worth looking into.

[stingray72]

I have an exhaust leak.. I know that. I'm going to try and fix it this weekend I hope. Would the knocking get any less as the engine warms up though?

[Classic Heartbeat]

The main bearings hold your crank in place. They have verry close tollerances and is in part what will determine your oil pressure. When they are worn out they knock and eventually will seize or spin, usually distroying the crank in the process. WES www.ClassicHeartbeat.com

[cableguy0]

the reason they go bad is they just wear out over time and miles its not that your motor is junk or anything it happens to most engines just means its time for a rebuild and a bunch of good excuses to get it to make more power

[rage'nrat638]

make sure your not using a fram filter or some other junky filter

[stingray72]

Ok.. I think I've figured out the problem. The manifold is broken where it connects to the tail pipes. One of the bolts is also broken which is causing the rattling (let's hope). I'm going to get headers and put them on and we'll see if that solves the problem

[old Rusty C10]

a bearing will not go away like that i was thinking that maybe the oil pump was not sucking so quick do to dirt in the system or the oil filter was bad fram does suck ya know, or some people have run thick oil in winter and ya dont need to do that in real cold weather thinner is a little better for real cold areas check the owners manual

[stingray72]

Sorry to bring this back from the dead but I finally got the new manifold on and fixed the exhaust leak. Well, there's still the knocking. Now it's at low rpms. When I crank the truck and put it in reverse or drive with just the brake applied the knocking is very pronounced. It sounds like its coming from under the cab (not the engine). Might be the other side of the exhuast or maybe a motor mount or something. I dunno. I'm going to have to find this... it's driving me crazy!

[tom hand]

Check your torqe converter bolts where it bolts to the flywheel....I have seen people start to pull a motor they thought was bad, only to stop when they get under the truck to loosen the flywheel bolts

[Huck]

Old mechanic trick---get a stick about 2 feet long and place it squarely on the block with the engine running. Put your good ear right up against the stick and listen. If you hear a strong THUD every time the knock occurs, it is NOT exhaust, loose bolts etc. It is a very worn bearing or rod. That being the case, you need to do something VERY SOON as the engine is telling you it will self destruct. Some may say you can drive it "mine knocked for years" but---it will die when you least expect it to---rules of "MURPHY'S LAW"!
You may be able to save the block by getting it to a good repair shop where bearings could be replaced but the best cure is a rebuild or engine replacement. Kind of like a pay me now or pay me later. Once an engine starts to knock due to wear, there is no cure short of replacing the worn parts---period! Sorry

[PR454SS]

One other thing to check is the crank thrust bearing, I know from recent experience that it is very hard to pinpoint this problem.
to check it start your motor and wait for the knock to start, cut the motor off and push/pull on the crank pulley, you should have very little movement.

[cableguy0]

huck is exactly right the problem your describing sounds like its rebuild time. usually if convertor or flywheel bolts are loose they will knock all the time not just when its cold. and what you are describing is a rod or main bearing on its way out. but as huck suck use a long stick or screwdriver and put it up to each cylinder and take a listen you will hear it and it will be amplified through the stick

[stingray72]

The engine is only 2 years old.. I don't see how this could be happening. I'll perform all these checks to be sure but this doesn't seem right that it's happening this early in a new engine.

Also.. once it starts to knock.. wouldn't it do it all the time?

[PR454SS]

That's the thing that was hard to tell about the thrust bearing as the crank would move forward and backward the knock would come and go. drove me nuts till I found it.

[biglou55]

Start your truck up let it idle put your arm underneath the truck and grab the exhaust pipe (with gloves on) and move it around i bet your knock will go away. at idle your engine is setting up enough resonate vibration to allow the exhaust to chatter and hit something. probably the bottom of the cab. just my 0.2 cents..

[BLACK AND BLUE 67-72]

Check your torque convertor bolts!!! If you put it into gear and hold the brake it gets louder and sounds like it's coming from under the cab, then this is a definite possibility.

[old Rusty C10]

make sure youre not using too thick an oil too if your ein a cold climate sometimes too thick of oil will do that

[stingray72]

welp.. I checked all the stuff mentioned and couldn't find the problem so I broke down and took it to my mechanic. Flywheel is the problem he says. I'll ask what exactly it was tomorrow when I get the truck back. Thanks for all the help!

[Cat Fuzz]

I've had that happen where I went to pull a motor and found egged out flexplate bolt holes. That makes more of a high pitched rattling sound that goes away when you rev the motor.

[Crank]

Okay, so I just bought a 76 Chevy 3/4 2wd for a hundred and fifty bucks. It's got teh big ol' Corp. 14 bolt under it and junk. But when ya start it up, it knocks, until the oil gets warmed up i reckon, and thins out, my bro told me this was a main bearing goin out right? Ya reckon that's what it is? And do you think they put 4 bolt main blocks in these trucks factory? If so, i'll build it to a 383 stroker. hehe.
Brandon

[ZOOMY]

A cracked or badly worn piston will knock when it's cold, but quiet down when it warms up and expands.

[jmanz69]

Make it a 383 anyway. You don't need 4 bolt mains for a standard or even mild 383. You will want them if you have a ton of HP and torque. Very rarely do 2 bolt main blocks give out like everyone thinks they will. 4 bolt mains are better, but 2 bolt mains are fine too. Not like you're building a 10,000 RPM drag motor or anything.