This baffles me to no end

[Pomorza]

Hi all

Three months ago I did an oil change in my Z. I decided to run a lower weight oil (10W40) as, well it was on sale. In the past (I'm sure you guys might remember) I had a rather interesting noise coming from the top end. Well after this oil change I ran a can of seafoam through the intake and the noise all but disappeared.

This evening I did another oil change. I went back to the 20W50 oil I've used in the past as 104 degree summers do nice things engines here. Well the noise is back. Same marble sounds when I release the throttle. Usually appears around 3K or so and lasts until idle. The cam itself is rather loud now.

I don't understand why oil weight has this affect on my car. I ran a tad bit of sea foam through the intake but it didn't solve any of the issues. Does anyone have ANY ideas what this can be? It is driving me nuts when I rev and I hear that BLASTED rattle. It scares me. The chain and tensioner are new, BRAND new. The valves are all adjusted the right way. I've checked and rechecked and rechecked.

Thanks

Jan

[Walter Moore]

How long has it been since the engine was last rebuilt? Is there any risk that you have sludge in your oil passages? Thinner oil will naturally flow better through restricted passages.

[Pomorza]

How long has it been since the engine was last rebuilt? Is there any risk that you have sludge in your oil passages? Thinner oil will naturally flow better through restricted passages.

I don't see sludge being in the oil passages as I've run two different desluging agents in the engine. I've inspected the cam and all the oiling holes look good. When the car is running (with valve cover off) there is equal amounts of oil coming from the cam oiling holes.

Jan

[70 Cam Guy]

My inexperience in Arizona/desert heat is going to be apparent here, but why not go back to the lighter weight oil? Some high quality oil would probably work nicely right?

[70 Cam Guy]

I don't see sludge being in the oil passages as I've run two different desluging agents in the engine. I've inspected the cam and all the oiling holes look good. When the car is running (with valve cover off) there is equal amounts of oil coming from the cam oiling holes.

Jan

Some sludge and carbon buildup in engines can be bad enough that those solvents have no effect on the buildup.

You have obviously inspected the oil flow but perhaps it's restricted enough somewhere that it's affecting oil flow at higher rpm? Pure speculation obviously, just thinking out loud.

[ozconnection]

With all this modern internet technology around............ I know, why don't you post a high quality video/audio of your noise?

Diagnosis can be easier if we can see/hear the 'patient'

Just a thought.

Good luck with it.

[Pomorza]

With all this modern internet technology around............ I know, why don't you post a high quality video/audio of your noise?

Diagnosis can be easier if we can see/hear the 'patient'

Just a thought.

Good luck with it.

I get your point. I'll try to get a video up later this week if I can find the time.

Jan

[Z train]

My inexperience in Arizona/desert heat is going to be apparent here, but why not go back to the lighter weight oil? Some high quality oil would probably work nicely right?

Because the FSM calls for 20/50.

[sblake01]

Because the FSM calls for 20/50.

Look again. I ran 10W-40 in my Z and 810 the entire time I had them. The only wieght the FSM doesn't recommend is 5W-20 for sustained high speed driving.

[Z train]

Sorry,Steve-YOU look again.I am looking at both FSMs(as i type this),one for the 810 and one for the 78 620 and they both call for 20/50 for temperatures encountered here in Tucson.

And as a side note they also call for 140 weight gear lube.

[sblake01]

Okay, I looked again an again I saw the same thing I saw before and every other time I've looked at it. The charts are the same in both FSMs. What am I missing?

Edited September 7, 2010 by sblake01

[doradox]

Think about how the oil pump is driven and that might lead to to a clue as why different weight oil could cause a noise. Thicker oil is harder to pump. Check that the harmonic balancer is tight.

Steve

Edited September 7, 2010 by doradox
Correct spelling