Removing of catalytic converter = better MPGs? True or False?

[Mickyblue]

Im narrowing down why my last trip in my 96 camry le did barely 28mpg on the highway, about 4 years ago (50k miles ago) it did 32mpg. I have good plugs, wires, air filter, cap, rotor, good oil etc. I did have the cats removed a year or so ago, and since then it makes more power, but still the MPGs havent gone up. Im really confused, im sure the 237k miles on it and the fact i do 70% town driving has something to do with it.

Any ideas i can do short of engine rebuilding? Because engine runs great, infact runs about as good now if not better when it was new. Just the MPGs are down since a few years ago.

But a mechanic told me since my cats are gone (drilled out, removed) that is why my MPGs are down, but it makes no sense

[kingnba6]

never heard of the cat delete making more/better mpgs

[carsrus]

It can decrease MPG. Plugged cat will also decrease MPG.

Its probally the mileage and age. Lower compression, older fuel system etc. I wouldn't rebuild an engine to get 4mpg.....

If your driving 70 % city and getting 28mpg that pretty good. Really good actually.

[Mickyblue]

Well if i replaced the front manifold cat (california spec) would that improve efficiency? Or not make a difference?

[Mickyblue]

Ive actually had the compression checked and it all checked out like new

[mspringer]

With that many miles, you have wear. A stratic compression test does not measure wear throughout the entire stroke, or measure bearing wear.

As others said - 28mpg is very good.

Also simple things like tires can make a difference (heavier tires, or grippier rubber, etc).

Wheels make an even bigger difference - When I went from 14" wheels to 18" wheels, I lost 4 mpg on average.

Not only that, quality of gasoline has gone down over the years. 10% ethanol fuel will reduce mileage by a significant percentage - something like 5% or better off the top of my head (not doing any research).

Also, with that many miles (or far less) you could have a minor crack in a hose causing a vacuum leak that would reduce mileage.

Personally, I would not worry - I am getting 22-26 around town, depending on driving, with my stupid 18's.

[gr91]

i thought the cat was there to burn unburned fuel. therefore increasing mileage, either way 28 is still damn good, my 91 Corolla used to do AT THE BEST 22mpg! AT THE BEST! so i wouldnt even worry about it, age definitely creates wear. so still great mileage.

[toovira]

When was the last time you change the timing belt and did timing with timing light? And also that you bypass ECU retard/advance by shorting TE1 and E1?

My car did 28 MPG forever....but she sounded little clanky. Then it went to 18 MPG...then to about 12. Then one day, she died on the road. As it turned out, the timing belt broke. I put a new one on, re-do the timing with timing light. Now I get 31-32. Moral of the story is that you can easily have stretched belt causing camshaft/crankshaft timing to be off.....and no matter how well you do the 10 degree advance with the light, you'll never get MPG back.

My is a 95 with 340000 miles....yes, that's the correct amount of zeros.

[Silent Runner]

I pulled my cat-con out too. Didn't see any change in milage/fuel economy. Got slightly more interior noise from the exhaust, and the the exhaust smells a bit more than before. Slight improvement in high rev. performance, most noticable during passing manouvers and top end of gears. I'd put it back now if I had that choise. (I loved the electric motor smoothness and quietness of the 3VZ-FE, in it's original exhaust config).

[Mickyblue]

I actually had the timing belt changed recently actually. Im just narrowing down by power of deduction of what caused 4mpgs to dissapear on my freeway commutes. I did read online somewhere, cars 1996 and new, thanks to the new OBD2, that without the cat it makes it burn more fuel, which if thats true thats such crap and shouldnt be. I pulled the spark plugs, and found the outside cylinder closest to the passenger side had some white powder on it, but the rest of the plugs were clean with the slight hint of red from chevron fuel i use all the time.

so what else could it be, i may get the front cat replaced sometime again because im not a fan of the way it smells, it runs incredibly well i dont know why it goes from 32 to 28 so quickly. But the past 2-3 years i went from driving on the freeway all the time to doing ALOT of town and city stop and go driving. Id say the last 30-40k miles was pure stop and go.

[reluctantmechanic]

Cat removal will give you a modest increase in hp.

Not sure about the laws in your area, but I believe most states have strict laws about removing the cat, regardless of age (or whether the car must still pass a smog) I've heard 1,000 fine.

Aside from the fine, your engine was designed to have the backpressure from the cat. without it, exhaust gases dont "flow" well, which causes a build up of heat in your exhaust valves...damaging the engine.

the 5 or 10 hp gain isn't worth it.

[JungleSuperlord]

Originally Posted by gr91

i thought the cat was there to burn unburned fuel. therefore increasing mileage, either way 28 is still damn good, my 91 Corolla used to do AT THE BEST 22mpg! AT THE BEST! so i wouldnt even worry about it, age definitely creates wear. so still great mileage.

A catalytic converter is there to convert hydrocarbons (fuel and oil), carbon monoxide, and nitrogen oxides into carbon dioxide and water in order to reduce emissions. Having unburned fuel light off inside the cat is actually very bad for the catalyst material and can lead to clogging and catalyst efficiency losses.

Without the cat an OBDII car will not get correct readings from the oxygen sensors and will command extra fuel into the mix in order to help cool the cylinders and avoid a lean condition (and thus avoid detonation, a.k.a. pinging, due to excessive heat). When the car's computer does this it does it to save itself at the expense of fuel mileage and hydrocarbon/carbon monoxide emissions.