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Epic loss of power going uphill after catalytic converter replaced

Got a check engine code for catalytic converter. Had it replaced tonight. On the way up a very large hill, got momentum at the bottom, about 45mph, half way up the hill RPM’s remained the same, speed slowly kept decreasing.almost down to 35 mph at the top. Engine idle is fine. when accelerating now, has low, flat, loud engine noise. almost no power, especially up hill.

More than likely there’s something wrong with the way engine is operating.

Catalytic converters don’t die.

They’re killed by the engine.


I wouldn’t doubt it if the engine had killed the cat, the engine burns oil heavily (Like I have to add 1.5 quarts a week). That’s why when the codes told me cat, I figured it wouldn’t hurt to replace it. But other than that, I have no clues at to why the engine is in a bad state since the only CEL I got was for the cat.

If it accelerated fine before the new cat, sounds like a bad new cat (or its intallation). The loud engine noise could be an exhaust leak.

One idea, temporarily remove the cat and see if the acceleration returns to normal. If so, at least you’ve narrowed it down to the cat replacement. Other common things that cause performance loss at higher engine loads are the fuel pump, the fuel filter, and the engine air filter. MAF/MAP problems can cause it too. But those usually toss a code. I expect you already know this, but if you are actually burning 1.5 quarts a week through the engine, the new cat isn’t going to last very long. Seems a waste of a new cat. That amount of oil burning usually indicates an engine rebuild is in order. You might could get away with replacing the rings if you feel lucky. But that’s still a pretty big and expensive job and might not work.

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If the transmission didn’t downshift, then that means a clutch is slipping. If everything is working correctly, the engine RPM is directly proportional to the car’s speed, for any gear.

edit: a CVT is the exception, it would behave as you describe.

Does this car happen to have a CVT transmission? The RPM situation might be normal in this case. I agree that the oil burning is going to be a problem for you. This isn’t like you are adding a quart every 3000 miles. This thing is drinking oil!

I would also check the oxygen sensors. These will also be fouled by all that oil burning and can result in poor performance as you describe. I have also never seen a bad sensor throw a code so there must be a HUGE range these things can be within before triggering a check engine light. The suddenness of these symptoms make me think it may be the new cat.

I guess I was super by AutoZone. I had the code checked they and they said it was the cat. They weren’t entirely wrong. The problem is, they said it was the secondary cat, which I replaced. It’s the first car/exhaust manifold assembly

Thanks guys! This community is awesome!