I have a couple symptoms on my 76 Valiant that I think might suggest a plugged up Cat.

- First, on cold mornings, it doesn't idle smoothly during the warm-up period when the choke is doing its thing.

- Second, I'm having a heck of a time getting my exhaust collector to seal up properly. Both of the surfaces seem straight and smooth and I've tried various remedies. (Haven't tried the RemFlex gasket yet though). I'm thinking this might be some backpressure from a plugged up Cat.

I know that both of these things could be caused by other issues, but I think I'm starting to build a case against the Cat. It's the original equipment that came on the car. And, considering the condition of the original engine when I pulled it out, I can only speculate about the nastiness that was pumped into the exhaust system during that period.

Any opinions on my theory? The Cat will be coming out sooner or later regardless. Since money is tight right now, I'm just trying to gauge which it should be (sooner....or later).

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1954 Ford, Tudor Mainline
1976 Plymouth Valiant, 4 door


www.automotive-ninja.com

Okay, first things first.

Disclaimer: The following information is for temporary diagnostic procedures only.

I would remove the cat and install a piece of exhaust pipe in it's place and see if that fixes your condition.

I agree with your reasoning on the way your slant is behaving. I'm not sure that the cat is the culprit, but I would try this first.

_________________
"Ja, Ich fahre ein altes auto."
'78 Volare 225
'67 Charger 318

I would bet on a plugged cat. Hold your hand over the tail pipe, Good Flow?
Frank

_________________
Scrapple: Because a mind is a terrible thing to waste.
73 Duster - Race Car
66 Dart Wagon - DD
178" FED
82 D150
All Slant powered

Don't know about where you live, but the muffler shop around the corner can check backpressure for a couple of bucks. They drill a hole in your exhaust pipe before and after the cat, crank it up, and measure pressure differential. I had it done and it was indeed clogged up. But since I am emissions exempt anyhow, I just busted out the sawzall...

_________________
If it was easy, everybody would be doing it.

Sounds like I'll be cutting it out with a sawzall.

I need to find a length of pipe with the flares on the ends....and the appropriate clamps. (don't have a welder at present)

I think I saw that sort of material the last time I was in OReilly....or maybe NAPA.

_________________
1954 Ford, Tudor Mainline
1976 Plymouth Valiant, 4 door


www.automotive-ninja.com

O'Reilly's and Auto Zone should have what you need, maybe NAPA too.

_________________
"Ja, Ich fahre ein altes auto."
'78 Volare 225
'67 Charger 318

Partially plugged cat or choked exhaust can limit the engine RPM regardless of gears? I'm wondering too because max rpm I can get is 3,500 under load.

Cheers, Wizard

Think of it this way: go jogging with an apple in your mouth. If you walked or just stood there you could breath fine, but you wouldn't be able to run very fast or far just breathing through your nose!

A clogged converter is like a spud stuck in the tailpipe; the engine has to work hard just to push out the exhaust. If you rev the motor, that creates more exhaust. Eventually the intake charge is so diluted it won't fire.

_________________
If it was easy, everybody would be doing it.