Olds 455 engine noise help!
 

I have a GMC motorhome that has a Olds 455 in it. When I bought it 20 years ago it had been rebuilt and just had 500 miles on it. Since then I’ve put maybe 15k on it and it runs pretty good for the exception of a gravely sounds that occurs briefly at shift points and light gas pedal use cruising on the highway. Oil pressure is normal and temps are normal. It starts happening only after it gets driven for 10 min or more. Using the stethoscope it sounds loudest around the harmonic balancer shaft. Any ideas of what this may be?

thanks,
tom

Your motor is pinging at those times due to a lean fuel mixture condition.

that taking place in between gears will do no harm, but under light throttle load conditions it eventually will.

Hi Tom,
I too own a 1975 GMC Motorhome and am having the same issues with valve train noise. I'm wondering if you ever got to the bottom of your issue?
Thanks

You issue is that all the cylinders are pinging at light part throttle.

this is most likely caused by the vacuum advance still being engaged and making for too much total timing .

disconnect your vacuum advance and plug it off and take a test drive and I bet that issue stops.

if it does then you need to install a aftermarket adjustable vacuum advance can .

i would say to then start off with it being set to add about 15 degrees of vacuum advance and see what that does for you.

you will need to mark your balancer for added timing marks to check this out, or you could get a timing tape to stick on it.

if no tape is available here’s how to mark the balancer.

lets say it diameter is 8 inches.
8 time 3.14 gives a perimeter of 25.120”.

25.120” divided by 360 is .069”

So each added degree above TDC is about 1/16”.

so I would make marks at 2.34” ( for 34 degrees)
also at 2,900” ( for 42 degrees) and then up at 3.45” for a total of 50 degrees.

yes the original vacuum advance could give you a total timing of over 40 degrees.

note that part throttle light load ping is not normally distructive unless the motor is in that condition for minutes on end, and that backing off on the added timing that vacuum advance brings in will effect your part throttle cruse gas mileage to some degree, so this is a balancing act to get things dead nuts right.

Thanks for that quick response!

Yes, pinging on light load part throttle, no noise on acceleration or slowing down, just at cruise. Multiple valve lifter adjustments have changed nothing.

I've left out quite a bit of background info. First, I've owned this Motorhome for 30 years and after 200k miles have just rebuilt the engine, everything up to new specs. I did get convinced into a "RV" camshaft, the Melling MTO-1. It's a mild, low end torque cam.

With the original OEM engine I was very used to hunting down the timing as you suggested, compromising the power curve for cruise vs pickup etc., vac can connected/disconnected etc. However, I never encountered much valve clatter at cruise like I am now. Yes, some in a hill climb etc, but not at cruise. So, I'm attributing this finicky new characteristic to the cam. At the same time, I'm hunting down any other possibilities that others might have experienced.

As you said, living with a little is ok, I certainly don't want to compromise my new engine with a lot.