Knock Knock!!!

[SmokeyVette]

I tell ya, it never fails, it NEVER freakin' fails, just when you get one persistent problem lined out another one rears it's ugly head.

I've got a knock, using the screwdriver-to-the-ear method, we've determined that it's in cylinder 1 or 3. It's a double-knock, not real loud yet but constant and speeds up with the engine.

For the past two years I've had low oil pressure at idle, I know, I know, but the engine still ran like a scalded dog and I still had at least 40 lbs at speed, so I figured it was just an old gauge. Anyway, the other day, while making my 30 mile commute I noticed a knock so I parked her. We pulled the valve covers off and fired her up to check things out on Saturday morning. The knock was gone but it took about 3 minutes for the oil to start trickling out of the holes in the roller rockers. Once the engine was good and warm the knock appeared. I've already cleared the funds with the wifey and we're getting a short-block assembly from Scoggin-Dickey (we live in Lubbock literally five miles from them:bounce but I'm wanting to know what is wrong with the old one before I tear into it.

Now I'm thinking that the knock is rod related, and I'm thinking that the low oil pressure and just barely trickling oil from the rocker arms is worn cam bearings.

What say ye?

 

[Twin_Turbo]

pull the plugwire from cyl 1 or 3 and see if it goes away, without a power stroke the knock should not be audible if it's rod bearings.

 

[SmokeyVette]

Good call, I'll try that, thanks.

What do you think about the low oil pressure problem?

 

[Twin_Turbo]

worn or a lot of clearance on rod bearings will result in low OP

 

[TimAT]

Cam bearings too. Too much clearance there, and the rest of the engine sees low pressure too.

 

[Twin_Turbo]

Yes, cam bearings too but they usually are not worn as they have relative small loads on them in street applications but do check

 

[SmokeyVette]

So what would cause the cam bearings to wear prematurely?

I believe it to be cam bearings because about a year ago I had a problem with foreign matter introducing itself into number 8 cylinder and this required a replacement of the number 8 piston, at that time I also replaced all rod bearing which actually looked pretty good...

I began seeing problems with fluctuating oil pressure about two years ago, while at highway speeds, about four months later I changed cams on a quest for more power.

Doing some math, we approximate that the engine has gotten about 20k miles a year on it, and this leads us to think that in the past five years the engine has bout 100k on it. I drive it often (daily for most of it's life) and I drive it hard, but what would cause bad cam bearings at about 60k miles?

 

[SmokeyVette]

Well, I didn't have a chance to run it again before we pulled the engine, but I DID find something out.

We got it out and on the engine stand, with no spark plugs in it I was rolling it by hand, when I went to roll it the other way I found "something" when I turn the harmonic balancer back-and-forth two or three degrees you can feel and most definitely hear a solid "thunk" at the end of each turn. Without pulling the oil pan I'm more than sure that the problem is serious and it's in the bottom-end.

This means that the crank is more-than-likely screwed right?

I guess I'll have the machine shop look at the block, if all is well then we'll probably turn her out to 40, and since I'll need a new crank anyway, I might as well stroke it for the next project...