Weird Sound - Rocks Tumbling - Is that Catalytic Converter?

[Stoeger]

On startup, at ide, sounds like rocks are tumbling in the exhaust.

I thought I read somewhere that's what a cat sounds like when it's breaking apart... is that true?


Sound goes away when I rev up over idle. Less noticable after engine is warm.

* 97 TJ. 4 cyl. 85K.

[cruizer]

Yep, the honeycomb in the cat is broken up inside. Quick way to check, hit it with your hand(exhaust cold) and if it rattles, you need a cat.

[Stoeger]

I took it in, had the cat replaced with a test pipe, and a new o2 sensor down there.

but now... sometimes it hesitates when accelerating from a stop. (bogs down, but doesn't stall) it's fine once going.

not throwing any codes yet, only happened a couple times, but it was scary (i was in traffic) could the lack of cat be causing this bogging?

[fredjacksonsan]

When you take out the cat and replace with a test pipe, I believe that you are reducing the backpressure. When reducing backpressure, you will gain HP on the high end, but lose torque on the low end. So the bog you are getting is a direct result of losing the torque.

I know, cats suck...but if you install a new one you will probably return the low end torque, which for the 2.5 will be a good thing as it didn't have much to start with.

[Stoeger]

I think you're right about the backpressure thing... and I think I've narrowed down my symptoms.... (and now I don't think my problem has to do with my cat)

I only bog when taking off right after startup. Even if it's only been sitting maybe a half-hour, sometimes it will bog. Not all the time, but only some of the time. And once it's warmed up, it's completely fine, even in bumper to bumper traffic.

Does that help diagonse? I'm going to price out a new set of wires & cap & rotor...

[Stoeger]

I guess 'bog' isn't best term, not so much just less power... but it's more of a 'stumble'... it almost stalls (which is very scary in traffic)! I press the gas pedal and I go like 10 feet, and then the engine doesn't rev up anymore, rpm's go very low, it spits and rumbles and all of a sudden it will take off... And then, I better keep on the gas or it will do it again. (Until it warms up, then it's fine.)

[fredjacksonsan]

Tune up couldn't hurt.