My 1997 Catera, 175,000 has been running great. On a 250 mile trip last weekend, it started getting rough and noisy. So noisy I thought there must be a hole in the muffler. I took it to my local guy, not a Caddy dealer, he found nothing physical, and checked the computer.

So I started driving today. When letting off the gas, the engine became noisy, and it felt like a backpressure roar. At very low speeds it actually backfired and stopped the car.

Any ideas on what causes this?

Do you have the timing belt history on this vehicle? I own a 97 catera and that's the ONLY thing i don't have history on. The engine was rebuilt around 51,000 miles and recently my oil cooler was shot. When they did the rebuild, they used the same oil cooler! I'm hoping they didn't use the same timing belt!

That's the only thing that could come to mind, unless it's a head gasket problem. I would also like to ask if you have thought about using a fuel injector cleaner/octane booster. If possible please post a video of engine at idle and using the throttle to revv the engine. Lots of stuff i know but sounds like the problem could be found/fixed or something else may be going on.

Hope that helps!!