In the last few months my 240 has been loosing power and also getting a rattle/vibration from somewhere under the car. It otherwise runs fine. It seem this happened after I replaced the header pipe with one from a junker. What are the chances an old deposit from the pipe disloged and partially plugged the cat? It is the original cat on a car with 295k on it. How about any good ways of testing it? Appriciate any information. Brad

There should be a test port on the downpipe. You can remove the plug and test the backpressure, but you would need that equipment, and I don't know what the numbers should be. You can also see if your car will make a little more power with the plug removed, but I don't know how much difference that will make, aside from being loud.

Greg

The rattle test is the easiest to perform, but if it doesnt give you definite answer you can test the pressure with a fuel pressure/vacuum gauge. Figure out a way to attach it to the test port and look for about 2 lbs to 4 lbs pressure. This is acceptable and in a non turbo car actually helps the car run better at low rpm than if you had 0 backpressure.

Odds are the ceramic insert in the cat is loose/broken and is restricting exaust flow. Crawl under and hit the cat with your hand. (cold) If you get a rattling/jingling noise, it's time for a new cat.