Clogged Catalytic Converter
 

if the nuts were old and rusty on the O2 sensor studs just ahead of the cat, would a plugged/clogged cat "blow them off?"

Happened just after I got my 94 4Runner up and running. Put new nuts on, it's fine. Getting horrible gas mileage on the highway, 10MPG, also can smell something like propane turning off the truck and getting out.

Are these symptoms the cat needs replacement?

When the cat plugged up on my El Camino it just felt like I was pulling a big trailer.

For what it's worth......my 03 Tacoma did the same thing.
I could hear an exhaust leak and after some investigation discovered my rear O2 sensor just
laying on the studs....no nuts. How they came off i have no idea. I rethreaded the studs (ASE)
and put new nuts on it. My cat. is fine by the way.

P.S. my 95 4 runner exhaust always smelled like rotten eggs.

You can quasi-check the cat efficiency by checking with an infrared thermometer before and after the cat. After warmup, it should be 50-100deg hotter after the cat which show the molecules are getting burned up inside.

Try taking out the O2 sensor and see if it drives better.
The O2 is only used after the engine is warm and in closed-loop.

I once started getting terrrible performance from a daily driver until the rusted 2 y.o. Midas muffler fell off. Ran good after that.

My 928 ran rich for years and DESTROYED one side of the 928S4 cat that I installed on my car....at a cost of $500! It is plugged, the ceramic matrix melted and caused a blockage.

I want to put a pair of Random-tech cats on my car, since they cause almost no back pressure...and my rich-running condition is finally solved, but they are $500 each...

N!

all good to know, thanks. A friend also said the cat glows hot when plugged up.

So I don't think I have a bad cat. THis engine is the worst Toyota ever made, so it comes sluggish fresh off the showroom floor. I think it's operating nearly as well as my old 95 did.

So what else could cause 10MPG on the highway? Truck is rated for 16, my old truck (gutted of 200 lbs) got 17-18.

Have you checked the cap, rotor,plugs and air filter lately?
A new fuel filter might help too.
Is this a 4 banger or a V6?

If a cat is plugged, then it is unlikely that it will glow. the glowing is caused by it working, i.e. oxidizing (burning) the CO and unburned Hydrocarbons. Many will heat up enough to glow a little even in normal operation, but if your engine is dumping fuel, then it might overheat enough to really glow...

If it is plugged, it will increase the back pressure and cause the engine to run sluggish.

Air filter looks pretty good, checked that on Saturday. I do need to check all those items. It's a V6, new motor and was installed for $5K back in 2007.

something of interest: when removing the gas cap to fill-up, it's pressurized. My 95 didn't do that, but then it had a gas tank leak at the top.

Also, I found what looked like silver antiseize or grey grease, a thin film, all inside the throttle body. I had removed the large rubber boot that goes from the air filter to the TB to reroute an accelerator cable. Noticed it then, just did a quick clean out with a rag, it's been very cold outside.

Shaun, what can happen is, the ceramic center starts new with the aspect ratio of a soup can. It slowly breaks-down until it has the aspect ratio of a tuna can. ..at which point it can flip sideways, and almost completely block the cat exit to the tailpipe.

I had this happen once. So, I took the cat off, set it in a garbage can, and rammed a crowbar down the center, to brreak-up what was left of the ceramic. (re-installed - ran great)

Good idea, thanks Glen.

I've done this upgrade on a number of cats. Performance improves sharply.