95 GTP Catalytic Converter Question

Hi everybody,

I have a 95 Grand Prix GTP, DOHC 3.4L, dual exhaust, and I've been
having the classic symptoms of a bad catalytic converter. I took it to
Midas and they said that if it is OBD I, the catalytic converter can be
removed and replaced by a straightpipe (which is actually legal here in
Saskatchewan) since there would be no oxygen sensor found after the
converter. My question is, would this have any adverse effects other
than increased emissions? I have read numerous posts of people saying
that removing the catalytic converter can sometimes increase power
because of reduced backpressure, but some say it actually reduces the
power since the backpressure is required for low-end torque. On my car
in particular, which would likely be true? Any comments would be great.

407

Aaron407 wrote:
> Hi everybody,
>
> I have a 95 Grand Prix GTP, DOHC 3.4L, dual exhaust, and I've been
> having the classic symptoms of a bad catalytic converter. I took it to
> Midas and they said that if it is OBD I, the catalytic converter can be
> removed and replaced by a straightpipe (which is actually legal here in
> Saskatchewan) since there would be no oxygen sensor found after the
> converter. My question is, would this have any adverse effects other
> than increased emissions? I have read numerous posts of people saying
> that removing the catalytic converter can sometimes increase power
> because of reduced backpressure, but some say it actually reduces the
> power since the backpressure is required for low-end torque. On my car
> in particular, which would likely be true? Any comments would be great.
>
> 407

I would be surprised if it were true that there is no legal
ramification to removing your cat, even in the case of an OBDI vehicle.
Perhaps the truth is that, in your province, a safety/emission
inspection isn't required. That doesn't necessarily make it legal to
modify your emissions system.

If the cat is actually plugged (what are the 'classic symptoms of a bad
cat', anyway?) it most likely got plugged due to a meltdown from poor
fuel management conditions. This root cause will need to be addressed
to achieve a proper running car regardless of the converter situation.

Finally, please explain how low backpressure (the kind realized from
removing a converter) negatively affects low end torque? I know, I
should have asked 'some', since 'some say that'...

Toyota MDT in MO

I've looked into emissions regulations for Saskatchewan, and it seems
that you cannot remove a working catalytic converter nor replace any
catalytic converter with a used one. However, once one is worn out, it
can be removed and does not need to be replaced. I'm not sure why the
law is like this, but it is.

As for the symptoms, I have significant lost power, rattling coming
from the cat, poor idling, an 'air sucking' sound coming at WOT, and
higher than normal average engine temperature. From what I've read it
seems that the catalytic converter is worn out.

and about low backpressure causing loss of low end torque - I'd like to
hear a good explanation too, I don't know where the numerous people got
this information from.

Can anybody else throw in their opinions?

407

Aaron407 wrote:
> I've looked into emissions regulations for Saskatchewan, and it seems
> that you cannot remove a working catalytic converter nor replace any
> catalytic converter with a used one. However, once one is worn out, it
> can be removed and does not need to be replaced. I'm not sure why the
> law is like this, but it is.
>
> As for the symptoms, I have significant lost power, rattling coming
> from the cat, poor idling, an 'air sucking' sound coming at WOT, and
> higher than normal average engine temperature. From what I've read it
> seems that the catalytic converter is worn out.
>
> and about low backpressure causing loss of low end torque - I'd like to
> hear a good explanation too, I don't know where the numerous people got
> this information from.
>
> Can anybody else throw in their opinions?
>
> 407

There's a possibility of multiple problems as I stated earlier, but one
of them could be that your cam gears are out of time. These engines
are hard to set up the gears on during a timing belt/ front cover
service. There is no keyway for the cam gears to line up exactly on
their respective cams, so they can be put on any old way. A few
"teeth" off and you suffer bizarre drivability problems, the most
likely being a loss of power. Also, if a cam gear *was* to slip while
driving, it would slip in the retarded direction - really causing a
loss of power.

Ofcourse you could remove the headpipe at the most convenient pre cat
location and drive the car for an A/B comparison before condemning the
cat.

Toyota MDT in MO