Knocking rods with good oil pressure?

purple81 · 05-10-2006, 04:09:52 PM

I've got a mystery on my hands, I've always thought that if I have good oil pressure, say 40lbs just above idle, that my main and rod bearings must be in good shape. Is this always true?

My daily driver has what sounds like a rod knock when I get on it in gear, but I have good oil pressure. Could it be the rods? Also, my engine is not firing on the #5 cylinder according to the computer, but I haven't been able to figure out why, I'm hoping that that misfire is causing the knock sound, but I have never heard a misfire cause what sounds like a rod knock.

So, what I'm looking for here is "if an engine has good oil pressure, the bearings and clearances MUST be good, correct?"

Thanks

engine · 05-10-2006, 04:32:53 PM

My old work van had a rod knock that lasted about 15k miles before it totally destroyed itself.

When my first 383 totally lost the entire rod bearing ont he #6 (?) rod, I still had 40 psi oil pressure. The entire bearing was in a million pieces in the bottom of the pan.

Twisted_Metal · 05-10-2006, 04:44:33 PM

It depends where your oil pressure gauge is tapped into the engine to tell if you have 40 PSI pressure everywhere.

Piston pins will sometimes knock but they usually have a double-knock sound once they get sloppy. Oil pressure won't be affected.

Are you sure you don't have plug wires 5 & 7 crossed and instead of a rod knock you're hearing a detonation knock from the one cylinder? This could explain both symptoms.

Joekool · 05-10-2006, 04:55:24 PM

A loose wrist pin can cause a rod knock like sound, so can excessive piston slap or a colasped piston skirt. Is the car an automatic? If it is the flex plate could be cracked and that too can sound like a rod knock.

retorq · 05-10-2006, 05:23:50 PM

My buddy's 200ci Ford I6 had a bad rod bearing and the pressure was fine. I was told to check the oil pressure and that I would see it drop when the knock happened, never did though. I was able to pull off the #3 wire and the knock stopped at idle. That was the one with the bad bearing. You can try the wire trick, just be careful?!

Damon · 05-10-2006, 10:19:15 PM

It only takes a few thousands of extra clearance to allow a knock. Not enough many times to significantly reduce oil pressure. If you're running a high volume pump it's even less likely you'll notice a difference. I've run motors around with a lifter hopped up out of its bore and oil POURING out of the BIG oil hole that feed oil to the lifter..... still not had a significant reduction in oil pressure.

Don't jump to conclusions.... a knock can be many things from a shot fuel pump to a loose/cracked flexplate. Don't just assume it's inside the motor.

Cardinal · 05-11-2006, 04:56:32 AM

Is the knock a single or double knock? Single knock is rod bearing, double knock is wrist pin.

Did you say that it didn't do it at all cold?

Before tearing it apart, check the broken flywheel. That give the same symptom that you have.

purple81 · 05-11-2006, 03:56:47 PM

It doesn't knock at idle or in park at all, or at full throttle, only when I accelerate slightly aggressively with about 1/2-3/4 throttle. It sounds weird, it's not like any rod or main knock I've ever heard, that's what throws me, it may not be a rod knock.

The best way I can think of to describe it is that it sounds like a cross between a knock and a rattle.

I'm busting my brain trying to figure this one out.

Joekool · 05-11-2006, 08:33:57 PM

I think you have a case of the detonations, try unplugging your vacuum advance if you have one or taking a few degrees out of the initial timing and see if the noise goes away gets quieter.

For the most part rod knocks and other internal knocks happen when cold or are loudest when cold. Once the oil gets to the bearings or heat expands everything then most knocks tend to get quieter or go away. Audible detonation usually sounds like rocks in a tin can, though if its really bad it can be much louder.