I've made a few threads asking about my horrible gas mileage...none quite helped other than to point me in the right direction. My 93 sc400 v8 has just over 250k miles on it and I get about 10mpg. I'm not sure if the O2 sensors have been changed, but my engine light works and has never gone off or failed emissions. I've replaced the plugs and wires, did a fuel system flush replacing the fuel filter with it. I also did a crankcase flush, more than half the hoses, vacuum lines and wires are replaced. The valve cover gaskets need replacing (about to do that) and once about a month after getting the car I poked a small pinky sized hole in the bottom front of the passenger side cat by pulling in into a friggin' jiffy lube and the bay lip caught the cat, which caused an exhaust leak, I put a dab of JB weld on it and it sealed fine and still is...I'm sure the honeycomb in the cat didn't really suffer either since its not rattling even when I give it a smack.

My conundrum occurs when I do research and find this post on the lexus owners club...
"Cats rely on receiving a proper mix of exhaust gases at a certain temperature. Some 'kill' their cat by over-using certain fuel additives, or lack of proper maintenance; fouled plugs (unburned fuel), bad exhaust valves etc. Many very short trips can also contribute to not fully warming up engine and cat, leaving some unburned residue.

Some warning signs that may point to a clogged cat/exhaust chain are noticeable; a major drop in gas-milage, not getting a proper response when accelerating and more than extreme heat from the cat." -IntakerGS

I am also told by a master certified technician that O2 sensors can 'gunk' up and perform sluggishly within a certain threshhold so as not to set the engine light off...and I have 4 sensors, 2 pre and 2 post cat...so technically the cat wouldnt be bad since the O2 sensors post would notice and set off the engine light...UNLESS they are performing at less than their best in the first place so as not to notice a slightly clogged cat...and there are 2 cats so either or could be with issue...lol see how confused I am? It all seems to be logical but this is where someone should come in and tell me there's a way to check all of these things before spending hundreds (roughly 500-600$ with O2 sensors and cats if I perform the labor) So any advice my responsive and knowledgeable peers?

 

It is very surprising that you are not throwing any codes - as if you had cat/O2 issues then you would be looking at a code.

If anything, I would look at your O2 sensors before your cat. Does your car smell like it is running rich? Go to a dyno shop and see if they will stick a wideband sensor up your tailpipe (LOL that sounded funny) to see if you are runnig rich.

Alternatively, you could invest $200 on an SAFC and a couple of runs on the dyno. If you are running rich then they could pull a lot of fuel from your map, saving you gas money and freeing up some HP

Hope this helps,
Jonny

 

would the dyno shop be able to tell me what might be throwing my fuel ratio off? cause for a while I kinda thought my car smelled like it was running rich but the engine light never came on and I kinda brushed off that notion. If the cat was "clogging" it would retain excess fuel residue which could burn off while its running making it smell like that...but you say lean towards the o2 sensors which could also throw off fuel ratio if theyre bad...I guess Ill try out the dyno guys and see if they can find out anything--thanks ^^

 

No, they couldn't tell you that. They would simply look at a wideband and see if you are running overly rich or not...

I would expect it to be the O2 sensors, but I don't understand why you wouldn't have a code...

Jonny