Hi All,
I've got a 1976 Oldsmobile Cutlass Supreme, all original, with 350 and TH350. Runs beautifully, but I noticed when I accelerate, I can hear a strange sound from the car, mostly as the engine is at a high rpm just before the transmission shifts. After shift, it disappears (until the engine rpm goes high again).
The sound is hard to describe, sounds like a hissing sound, possibly a vacuum leak (runs smooth and even at idle, though), but could be a high-rate valve pinging. It does it pretty badly when the engine is cold, then goes away as it warms up. Could be timing (pre-ignition), or even using too low of octane fuel (engine is mostly all original). Engine is mostly original, with possibly 100k or 200k miles on it (not sure, as it's only a 5 digit odometer...), so something could be worn..

Can anyone give me an idea of what to look for to find out what's causing it, based on the symptoms I described?

many thanks, you guys rock, as always!
james

 

Have you done and tran work on it lately, like change the filter? May be the tran pumping air, as not a good seal on filter.

 

Start simple...old hard loose slipping fan belt?

If you suspect ignition (and preignition) do some troubleshooting starting with how old are the wires, cap, rotor and plugs? Don't know? Inspect them. Read the plugs. Are they gapped right? This was the .080" year(s) of plug gap with an extremely lean mechanical curve and carb jetting. Verify the timing. Try some 92 octane and add some octane boost. See if it goes away.
A 76 has a mile of vacuum lines and smog controls(to include a cat converter that can plug up).
Verify you don't have a hardened or broken or missing vac line.
Then work deeper into the problem.

 

Preignition (pinging) usually happens under hard acceleration at lower rpm and doesn't sound like a hiss, more like shaking a can of marbles. I'm sorry I don't know what you mean by valve pinging, never heard that term before. First thing I'd check would be the miles of rubber tubing for a leak.

 

Thank you all for the excellent suggestions! I just changed out the wires (and checked the plugs) this past weekend, all good there! Droldsmorland, you have a very interesting point, I have suspicion that the cat converter is still original on the car... been waiting because I want to switch to dual exhaust, but maybe it'd be better to get that changed asap.

thanks!
james

 

That will cause a hiss but it will also drastically effect performance. The car will be an absolute slug if the cat converter is plugged.A quick n dirty check is to have someone step on the gas while your at the exhaust pipe. You'll hear it and feel the lack of flow.