Questions and Answers

Cadillac Sedan DeVille Exhaust Noise Q. Hello, I've taken my 1976 Cadillac Sedan DeVille 500 cid engine, owned since new, to exhaust shops, general repair places, machine shops and posted on several boards but no one has been able to come up with an answer to this. While idling in neutral, especially raising the RPM's to around 800, there is a fairly loud deep exhaust roar.

There's also a sputtering on coasting. Listening with a stethoscope the noise is only in the muffler. The exhaust system from the converter back was new (Walker) a couple of years ago, nevertheless we tried three mufflers and even replaced the converter with no progress.

The car has new plugs, cap, rotor, vacuum advance, plug wires, air cleaner filter, evap cannister filter, temperature sensor in air cleaner; EGR cleaned and tested for open passages, EFE (heat riser) seems okay, cleaned breather, timing okay, 6° BTDC at 600 RPM or less. Car performs extremely well (except stumbles when cold and idle is rough sometimes and smooth other times).

One muffler shop said there may be a leak in either the exhaust manifold or crossover. I do notice a soft spitting sound on accelerating when cold. I use Shell 93 octane, but recently mixed it with half 89 octane to see what happened, but no change.

Interestingly enough no knock either, engine designed for 91, which would be what, I assume, I would get with half 93 and half 89. Any idea what might be going on here? We're really stumped!

Thanks...

A. I think in this case you are trying to cure the symptom and not the disease.

This engine has an Air Injection Reaction (AIR) system and I think you'll find there is a problem with it. The air injection system supplies the oxygen needed to ignite and burn any unused fuel in the exhaust system by injecting fresh air into the exhaust system, near the exhaust valves, via the air manifolds, hoses and injection tubes.

There is a diverter, or air by-pass valve, that prevents backfiring, when triggered by a sharp increase in manifold vacuum, by shutting off the injected air to the exhaust port areas during this richer period.

There are also two check valves that are inline between the AIR manifold and the exhaust ports. The check valve portion of the check and delay valve (if equipped) provides a rapid balancing of chamber pressure when there is a sudden decrease in vacuum during acceleration. It was fairly common for either the diverter valve or check valves(s) to go bad and cause the problem you describe.

You can find the procedure for checking the AIR system in the Motors Emissions Service Manual, which you can find at the public library.