351 v6 noisy?

[Troyport]

Hello all, I have a 351 V6 in my 68 GMC. Runs great, but rattles a little at hot idle. Just seems noisy, but can't put a finger on it. The other day while standing about 15 feet behind it, could hear a tinny rapping sound, like when you click your tongue. For fun I put in a bottle of STP oil treatment and it seems quieter. Oil pressure at 700rpm idle before STP was 30. Now about 35. Previous owner used 15w40. Oil was fresh so I haven't changed it yet. Thoughts?

[Funky61]

Hello Troyport;

Perhaps an exhaust leak/manifold or valves need adjusting?

[Troyport]

Maybe... well, definitely have a "ticker" on the driver's side, but that isn't the same sound I hear. And correction, I have 50psi at hot 700rpm idle, and if I drop to 500 I have about 40psi. Vacuum gauge flutters at idle, so probably should adjust the valves before I jump to conclusions.

[AZKen]

Originally Posted by Funky61

Hello Troyport;

Perhaps an exhaust leak/manifold or valves need adjusting?

I second that emotion.

[George Bongert]

Originally Posted by Troyport

Maybe... well, definitely have a "ticker" on the driver's side, but that isn't the same sound I hear. And correction, I have 50psi at hot 700rpm idle, and if I drop to 500 I have about 40psi. Vacuum gauge flutters at idle, so probably should adjust the valves before I jump to conclusions.

Greetings, Troyport!

Given that you have 50 psi oil pressure @ 700 RPM hot, and 40 psi @ 500 RPM hot is well within normal range, and is indicative that you do not have any loose (badly worn) crankshaft or camshaft bearings. I (along with others) suggest checking valve clearances.

[Troyport]

Thanks for the reassuring replies. I will check valve clearances... What viscosity oil would you suggest?

[AZKen]

Everybody has their own idea of what oil brand and viscosity. Read up on it and decide for yourself. You have to get educated by study, not by opinion on certain things. Oil is oil, synthentic is synthetic. Viscosity is about climate and region. I.E. 10W40, W= Winter.

[George Bongert]

Originally Posted by Troyport

Thanks for the reassuring replies. I will check valve clearances... What viscosity oil would you suggest?

Personally, I would recommend a 10 W 30 oil with ZDDP (Zinc) to protect the flat tappet cam that these engines are equipped with. You can use either a good name brand engine oil, or the more expensive synthetic oil in your engine. Personally, I prefer to use the more expensive synthetic oil. It's a matter of personal choice. Synthetic oil lasts longer, and provides for easier and better cold weather starting, since it does not thicken as much as standard crude base oils do in cold weather.

[Prowbar]

One might find this chart handy to select a suitable oil viscosity.

[Troyport]

Adjusted valves, all were fine except exhaust on cyl 1 was very loose. Probably put 3/4 turn on it. I will assume PO adjusted incorrectly and I don't have a cam lobe burning up. I suppose time will tell.