Hi all, this is my first post!

My question is regarding why my beloved Monte has lost acceleration power. It seems to take longer to get up to speed than it used to, and sometimes I get the RPMS up to 3500-4000 just to try and get up to 55-60 mph in about 10 seconds.

Took it into the dealership, they told me it was a clogged cat. Well, not wanting to pay 450 bucks for them to replace it, I went out and bought a universal fit Magnaflow for 100 bucks, and had a friend put it on. After he removed the old cat, we inspected it only to find it really wasn't clogged, (dealership said it was VERY clogged, to the point where if I kept driving it I wouldn't be able to drive faster than 20mph....)

Anyway, put a new cat on. Then I became desperate. Used a bottle of Lucas Complete Fuel System Cleaner when I filled the gas tank, and the next day I even replaced my air filter. Well, the car only barely seems to be performing better now.

I have no problems driving once I'm up to speed, it just seems like it takes more work and longer for it to get up to speed, unless I really give it gas. But I don't like driving the RPMS clear up to 4000.

No check engine light is on. Car has 142k miles on it. Pulled a couple spark plugs, they looked clean.

Also, one more thing. The engine smells pretty gassy, like it's burning rich when I start it and accelerate.

Anyone have any ideas what might be the cause of these problems? Thanks!

 

I'd check to make sure the fuel pump is working properly. You could have a stuck or bad injector. Regardless, I'd be willing to bet it's something fuel related, seeing as the cat has been ruled out.

 

I would change the fuel filter and think about the o2 sensor

 

THose symptoms some times line up with a cat, but that's no longer an issue since you replaced it.
I would consider some basic maintenance at this point (unless this stuff has been done).
- Change plugs and wires (the factory ones are only rated for about 100,000 maybe 150,000 tops). DO NOT USE BOSCH PLUGS (they are garbage on GM V6's and have a history is issues). I've used AC Delco and NGK plugs with great success.
- Change the air and fuel filters (a clogged air filter can cause poor performance and a clogged fuel filter will stress the fuel pump).
- Replace the upstream O2 sensor (one on the rear exhaust manifold, stick with an AC Delco if you can).

Those are some basic items and hopefully easy DIY things to try.

 

I also would have suggested the cat but you have already done that. Another possibility could be that since you put an aftermarket cat on, the O2 sensors are now screwing fuels trims up so bad trying to get emissions where they need to be, that it's now not running right. Try unplugging the o2 sensors and see if it runs any better.


I may be completely wrong, just my .02

 

You don't need to unplug an o2 sensor, just drive the car while it's cold because until it heats up the o2 sensors are in Open-Loop and don't send signals.

That is a good point though, disconnect the battery for an hour to reset your fuel trims and let the car learn new ones.