Torkyturtle

Got a question on my 455. I start the engine and it sounds perfectly sound and after running for 10 minutes or so and it does this gravely rattle sound when you rev up to 1800 - 2k. Its not a constant sound but just as it revs up. It will also do it at light cruise pedal, but only after it has run for 10 or so minutes. I have converted over to a Howell TBI from the Quadrojet. This sound occurred the same way when the Quadro was on it. When its under load this sound does not happen. Anybody have this happen to yours?
Also forgot to mention it has the crossover block off plates installed.

thanks

Vintage Chief

What year and what model vehicle? Is this the stock 455 which came in the vehicle? Heat riser or exhaust manifold or exhaust clamp(s)? Once things heat up they can have a tendency to become noisy. You can use a broom stick (or wooden dowel rod) held up against your ear and probe the engine to see if you can validate where the noise gets louder.

Torkyturtle

Its in a 76 GMC motorhome. It has Thorley headers on it.

Vintage Chief

Those are nice motor homes.

Greg Rogers

I think a 3-4' piece of vacuum hose is even better than a broom stick, etc. Stick on your ear gently and move around running engine. Works really good- a lot better than a stethoscope. (I bought one of those 30 years ago and never use once I started using hose). Good luck, let us know what you find.

sysmg

Gravely rattle sound makes me think of preignition. What fuel are you using? Perhaps a few tanks of premium will tell you if that's the cause. Also what year engine (is it high compression?)?

maxi426

If, by crossover block off plates, you mean the plugs like Edelbrock sells that tap into the heads, could they be loose? Listen there.

CANADIANOLDS

Sounds like a bad wrist pin. That noise happens when a bad wrist pin is worn and not under load.

lite load on the piston allows it to keep going up as the crank changes direction at tdc, you’ll get a lite metallic knock, then at bdc it will do it again

you can make the noise come and go with lite throttling off and on

a bad wrist pin noise doesn’t sound like detonation…they are different

droldsmorland

Hard to diagnose online with minimal data.
Miles on engine?
Metal in oil?
Warm oil pressure?
Could be 100 things. I agree with the block-offs rattling. The better block-offs are a chunk of fitted iron welded into the intake. The cheap stamped steel parts do come loose and rattle.
Could be rod bearings wrist pins skirt slap etc...

gs72

Loose torque convertor bolts?

Torkyturtle

Kinda wondering too on the crossover plates because this seemed to start happening around the time I had those put in. To answer some of the other questions that I should have provided before. I bought the motorhome back in 2001 with both the engine and transmission rebuilt and approx 500 miles on it. I’ve put approx 15k on it since. Oil pressure is 145-150# across the pistons. I’ve had Blackstone Labs look at the oil and no metal out of the ordinary. I do have a question on the wrist pin. If that is the issue would that show scoring on that piston chamber wall? I’ve scoped the piston chambers and saw no signs of wear. The walls looked mirror like as far as I could see.

Thanks,
Tom

Vintage Chief

145-150 psi is your compression/cylinder not your oil pressure

Torkyturtle

Yes correct, sorry, lack of coffee. The oil pressure is running normal. High at startup and it settles on 40lbs when driving and around 7-10 at idle after its warmed up.

Vintage Chief

Compression/cyl & oil pressure are spot-on.

Vintage Chief

I’d probe w/ a dowel rod, broom stick or vacuum hose all over the engine.

Oldsguy

If it is related to the engine heating up frist maybe air in the coolant? When the t-stat opens then that air starts running throught the radiator and engine and may be making the noise. Just another stab at it, could be wrong.

Vintage Chief

^x2^ Possibly. Ever hear the noise a hot water radiator(s) makes in a home as it evacuates air in the system and the water heats up? Kinda/Sorta could be?

BangScreech4-4-2

Rod bearings, all internet diagnosis caveats notwithstanding.

Have somebody repeatedly rev the engine from idle to 2500-3000 RPM (in neutral/park with brake set, of course) while you listen through the aforementioned vacuum hose/stethoscope. If you get a grinding or scraping sound, there's your culprit.

CANADIANOLDS

that low of oil pressure at idle isn’t good. Bearings may be gone out of it

take your oil filter off and cut it open…look for glitter

CANADIANOLDS

No you won’t see scoring if a pin is bad

droldsmorland

7-10 oil PSI at warn idle is very low.
What weight of oil are you using?

sysmg

Oldsmobile specs are 10psi per 1000 RPM. So 7 at 750RPM meets oldsmobile's specifications. Low, but not problematic as long as it goes up at higher RPM.

BangScreech4-4-2

Bare minimum, though. I wouldn't be happy about it.

Torkyturtle

The oil pressure does come up as it revs. I’ll kick on my EBL display and get the recorded oil pressure to make sure. Was giving that pressure from the OEM oil pressure gauge. First thing I’ll do is take belts off and run it briefly to make sure its not power steering or water pump or alternator. Appreciate all the ideas guys!

Tom

Torkyturtle

sorry, missed this question. The engine they built for these was done at a lower compression ratio to allow for lower grades of gas. But its worth a try. I believe I’ve tried this and had no change in condition.

TG

Donaldbabineau

Rattling gravelly sound is exactly what my engine sounded like under light throttle before I put a restrictor on the vacuum advance limiting it to 10* of advance.

Torkyturtle

I’ll look into it but its been converted over to TBI from the Quad carb so not sure this would apply but I could check.

Thanks!
Tom