Bizarre and growing sequence of symptoms-- bad cat? Worse?

'99, 175k miles, bone stock
I have a strange and growing sequence of problems. I *think* I know what is going on, but I wanted to run it by the experts first. Sorry this is so long.

First, I noticed I was losing coolant (yes, I've already ordered LIM gasket, more on that.) Then, oil sludge under rad cap but no water in the crankcase. Then my EGR valve failed.. took me a couple weeks to get to it, and the car started getting a little sluggish with a growing resonant "groan" that tracks with engine speed. I assumed this noise was the EGR getting stuck wide open, so I replaced that and the EGR code never came back after reset. The noise only got a little better, though, and the car didn't run any better. Then, the resonating "groan" got louder, almost raspy. I cannot really tell where it is coming from, but I have been operating under the working theory that the catalytic converter element is breaking down--- and if the cat is clogged, that MIGHT explain the failed EGR valve.

AND.... now the car won't rev over (exactly) 3800 RPM. It is acting exactly like a rev limiter is kicking in, like the injectors are cutting out rhythmically. I thought-- a bad cat *might* explain this too, but how would it be limited to 3800 rpm with such laser-like precision?

So right now, I'm assuming that 1) I need to replace my lower intake manifold gasket, stat and 2) possible bad catalytic converter. The CEL is NOT on, but someone has my code reader till tomorrow so I haven't checked for soft codes.

Am I on the right track? Or is it possibly something far worse? Why am I rev limited to 3800rpm?

Last question-- I was looking under the car at the cat. We have no inspections at all here in SC, so I'd like to just rod out the cat (or have it piped through) and put an extender on the downstream O2. I saw the mounting flange on the front end of the cat, but the back end seems to be crimped into the muffler inlet. Will this rear junction come apart, or would it have to be cut? Everything looks factory original, but I've only had the car for 40k.

Thank you everyone!
Dave

Cat cant control anything like fuel cutoff.
Has the harmonic balancer even been replaced on the car? Some strange noises can be heard when this is failing and moving around.
MY cat was replaced with a 3" straight pipe when i had a down pipe installed. Would be a good time to do this small mod as well. Wouldnt hurt anything.

Edit: wonder if it could be transmission related as well. Does the noise stop if you drop the car into neutral while moving?

I had also considered the harmonic balancer. Have never has one go bad, so I don't really know the symptoms. Noise is worse while driving, especially under acceleration, but does not go away entirely when revving in neutral. I have not tried popping into neutral while moving, will give that a try. What is the benefit of a downpipe replacement? Do you think I'll have any CEL issues with cat removal if I don't reprogram?

14years wrote:

I had also considered the harmonic balancer. Have never has one go bad, so I don't really know the symptoms. Noise is worse while driving, especially under acceleration, but does not go away entirely when revving in neutral. I have not tried popping into neutral while moving, will give that a try.  What is the benefit of a downpipe replacement? Do you think I'll have any CEL issues with cat removal if I don't reprogram?

Downpipe helps relieve some exhaust restriction making the engine run easier. Will gain some performance, more with other things like an intake.
You will get a CEL with the Cat removed unless you can turn the code off with a tuner or get a device that tricks the sensor into thinking the cat is working.