Bad Cat?

[Applejack]

I suppose there is no easy way to tell if my catalytic converter is bad but are there some tell tail signs?
Here's what I've got.....1999 4runner with horrible mileage. I'm all the way down to 12mpg in town. I've made no changes to the vehicle to make this happen. I used to get 16-17 in town and then it dropped for a while to 14 mpg and now today I calculate 12. I can't keep driving this thing if it gets any worse.

 

[Owyhee H]

For testing a cat, I went to a local exhaust shop and they measured the pressure in front of and behind the cat to see what the difference in pressure was. I was told within 1 pound it is fine, mine was .5 pound, on my original cat. They did this by poking a small hole in the pipe, which they fixed afterward. Best of all it was FREE.

I think most exhaust shops will do this check free of charge.

Good Luck. Hope you find the culprit.

 

[YukonRob]

Isn't rotten egg smell sometimes attributable to a failing cat? Might be as simple as smelling the exaust if so. Any other sympotoms - CEL or anything?

 

[shortbus4x4]

Check engine lite on at all? Have you done anything to the vehicle recently? When was the last time a tuneup was done? Noticed any difference in performance? I would be looking at other things to explain the mileage drop first before I suspected the CAT. Any good tech with a scan tool should be able to diagnose the problem in an hour or two. Give me some more details.

 

[SC T100]

I know on some of the T100s with the 3.4, the O2 sensors will get funky and along with a dirty MAF can sometimes cause the mileage to drop pretty good...all without throwing a code. You may want to have a scan tool see what those 02 sensors are doing.

 

[corax]

take it for a "spirited drive" and measure the temperature before and after the cat. If it's working the exhaust should be hotter AFTER the cat. I would look for ~100 degree difference. An easy tool to use to measure it is one of these:

A vacuum gauge will also tell you alot about your engine, the gauge is generally less than $20 at most parts stores or Sears. Here's a decent article with graphics to help you get started.

 

[RabidRover]

Sounds like you are where I was a couple of months ago ... take a look at your cats and see if there signs of excessive heat. I also got a CEL with a P 240 code as well as a yellow caking on the front cat. It was really hot even after sitting outside for a few minutes in -25 weather. I never heard of pressure testing a cat before ... I would definitely take it to a muffler shop and do that if it is free. I also have ordered new 02 sensors which I will replace when it is a bit warmer just in case I was running a bit rich ... given the price of the cat, I figure it is worth the extra coin.

 

[Goin]

When. I first bought my used 4runner it had a bad cat and threw a code. I have had pretty poor gas mileage this winter in my 2001. Iwas averaging 14mpg on my drive to work ( even mix city/town). When I changed my plugs last weekend I gained a .5 mile on the last tank.

 

[corax]

I never heard of pressure testing a cat before ... I would definitely take it to a muffler shop and do that if it is free. I also have ordered new 02 sensors which I will replace when it is a bit warmer just in case I was running a bit rich ... given the price of the cat, I figure it is worth the extra coin.

No such thing as pressure testing it, but if it's clogged it will build excessive pressure in the exhaust between itself and the engine. Usually, if you can remove it easily, you can look through it and see if it's clogged. However, remember that most newer engines have a catalyst at the exhaust manifold (warm-up or pre-converter) and another under the car - the preconverters are usually a pain to get to.

#6 would be the preconverters and #8 the main converter (this pic isn't from a 4runner)

 

[RabidRover]

No such thing as pressure testing it, but if it's clogged it will build excessive pressure in the exhaust between itself and the engine. Usually, if you can remove it easily, you can look through it and see if it's clogged.

Good to know ... I visually inspected it and I could see why it was shot. It was clearly clogged.

 

[Owyhee H]

Its not pressure testing the cat, it is testing the pressure in front of and behind the cat in the exhaust, which tests the flow. I didnt know about this test until they did it on my car less than a month ago when they put on a new muffler. I even saw them do it.

 

[OldSven]

I don't remember if you are supercharged like mine but I had the same 12mpg issue and not sure which one of the things took care of it the most but I'm get a lot better now even with winter fuel.

-New o2 sensor
-Cleaned MAF and throttle body
-Dropped down to a 265' tire
-Also found a bad vacuum leak out the back of the charger

The vacuum leak was probably the biggest issue, it was making my fuel trims go nutz, once that was fixed it had lots more pickup and power plus the fuel trims were way down. The computer was having to adjust for the extra air coming in after the MAF, go around with some carb clean to some of your vacuum lines and give it a squirt if the engine boggs down you probably have a leak...

 

[Applejack]

I'm not supercharged and I'm all stock except for a Magnaflow muffler.
Here's all the info I can give; my cat always stinks like sulfur but that may be due to high sulfur content in our fuel here. I have changed the plugs recently, but no the wires (I don't think that would be doing it), changed the fuel filter.
I'm running 265/75's stock gearing, small lift, ARB bumper and that's it. This very same set up used to get me 16/21 minimum unloaded.

No CEL's or anything have popped up, there seems to be normal exhaust flow out my tailpipe at idle, my engine runs butter smooth, no loping or rpm cut outs at any speed. Bad mileage is my the only symptom.
I'll clean my MAF today and see if that does anything.

 

[offthebeatenpath]

I'm watching this one....my Taco has the same symptoms....

 

[Ramont]

I own a muffler shop and had a 99 4runner v6 (that I gave to my brother) and ran a Magnaflow high flow cat and a Magnaflow muffler. Best set up in my opinion for these 4runners. Sounds mean and improved my power and gas mileage

You may have a bad cat but it's hard to diagnose from a computer. If it's heating up real bad (some will glow) its a tell tale sign. You can try smacking it with your hand (wait til it's cooled) and listen for any broken catalyst banging around in there. Most of the time the gas mileage goes first, then the power loss and then its too late. If it's the cat, take care of it before that catalyst gets into your muffler and you have to change it out also.