So since I have true duals on my truck with no cats, my o2 sensor isn't getting a reading from the cat, instead it is seeing the extra flow and richening the mixture to compansate. Are these O2 simms any good? Or should I throw on some high flow cats. I'm not getting any codes but my fuel mileage is TERRIBLE! Can't get past 12mpg no matter what and I do 90% highway driving.

Any help is greatly appreciated.

oops should've posted this in the exhaust section. Sorry.

Your rear 02's play no role in fuel mileage or affecting the performance of your truck they are simply there to bust your balls and for emission reasons only. The front 02 sensors do all the work and relay info to the PCM. The Sims that Semo sells work and you should go ahead and get them because you will eventually throw a Cat inefficiency code sooner or later.

Cooo man thanks for thr reply. But I thought the 03's only had one behind cat sensor that read gases and mixtures through the cat. Since it's seeing more airflow and no backpressure from the cat it's telling tha omputer to richen out the mixture. Maybe the sensors are getting bad and need to be replaced. Anyway regardless, went on a 900km round trip this weekend and got at best 13mpg all highway.

The way it's set-up is there's a sensor on the left pipe, and a sensor on the right pipe. I'm thinkin they just relocated the front sensor over to the left and called it good. If that's the ase I should just have to throw a sensor on the left.

The '03 trucks have 1 pre-cat sensor and 1 post-cat sensor. If you have a cross-over pipe, the middle would be the best for the factory pre-cat sensor. This is the sensor that is actually running your engine and needs to sense BOTH cylinder banks. The post-cat sensor is only for emissions testing and will need replacement with a simulator of some sort. Installing the post-cat sensor on the other header will definitely cause a code and is the worst possible location.

Well looked again today, one sensor on the left pipe and one in the pipe on the right side. Don't know how they pulled it off cause I have no codes, and it's definitly runing on 8 cylinders. I have no idea.

exactly what wade said, those are called DOWNSTREAM oxygen sensors, and ours actually are Downstreah HO2Sensors, only the front UPSTREAM HO2sensors affect fuel trim, and are simply a feedback sensor, they fluctuate from below 200mv and above 800mv for a healthy calculation. I removed my downstream HO2sensors and zip tied them to the tranny line pressure harness and the input speed harness, apparently my pcm thinks my CATs are working very very well, i only set a downstream code the first week after installing the longtubes, havent had a code since, but its not a constant DTC, it decides if its inefficient out of every so many exhaust cycles, sims are great, but i havent had any problems yet with leaving them exposed to oxygen.

This is exactly the issue I ran into. The problem is when you buy a header kit with a Y-pipe, they come with 2 cats already, in my case high-flows. It'll work perfectly for an '04+ cause it has sensors on both banks. But it's impossible with 2 cats coming right off the headers to get the first 02 sensor at pre-cat coming from both banks.

I was getting the exact same mileage you were noodelz, and granted I didn't have an A/F guage, it was running rich. After the install, I went from a consistant 15-16's mileage, to any day seeing a 13 being very rare, and always throwing the P0420 code.

After putting BOTH sims on the rear sensor, I have been able to achieve back into the 14's, and creeping into the 15's on the highway. I can still tell it's running a little rich though.

This is one of the downfalls of the '03's, and one of the reasons I've been seriously considering upgrading from this truck, or redoing the fuel system in a way that I can at least have the computer pull readings from both banks. I don't like that it's only getting it from the passenger side. Frankly, I don't even want to touch a cam, let alone anything else until I fix this problem, if at all. Call it piece of mind, but I'd rather the ECM be getting an accurate reading from BOTH banks like '04+.

It might not be as bad in my situation, once I put my headers on I'm gonna have to make a new front end to match existing exhaust to them. I'm gonna make a X over and stick the upstream sensor smack dab in the middle of it. and the down stream after it with a simm. Should work, an suggestions?

I know yo uwant the sensor as close to the collectors as possible to get an a ccurate reading. But I dont see any harm in that if all th e02 sensor does is read fuel mixture. Unless the extra flow form two sides now will affect it.

With a system like mine, it won't work that way. If yer not running cats, I guess it doesn't matter where you put the upstream, best at whatever cross-over like you said. With high-flow cats, I don't know if it makes a difference. Either way, you want that upstream to be as close to the heads as possible for the best reading.

I do have 3 bungs on this header system. One upstrem of passenger cat, one down stream of passenger cat, and one at the Y. Where would you put the 2 sensors? Jay or anyone else wanna chime in?

so the 03s only have one upstream H02sensor?, thats kind of like the 03 f150s ive worked on, the best way would just be to get an X pipe straight to the headers, put your bung in the center of the X pipe put your 2 cats after the X and your downstream HO2sensors after the cats, i cant really say, lol post up a picture of your system and then i can help you.

Go check out my "OBX Header Install" thread, and you can see what I'm talking about. On the stock exhaust pic, I don't think you can see the 02 sensor, so just keep in mind the upstream is ahead of the cat, and down is behind.

You'll also be able to see where I have the 02 sensors right now. I went ahead and put the downstream at the Y, as opposed to right behind the cat. I don't think this does or helps anything, I just did it that way.

But even if I didn't have cats, or with these high-flow cats, I think having the upstream where the Y- is located is to far back from the heads to get an accurate reading from either bank, let alone both. And also as you can see in the pics, I wouldn't be able to put an X RIGHT after the headers because the tranny is right there, and it wouldn't be till behind the cross-member.

quote:

Originally posted by noodelz:
So since I have true duals on my truck with no cats, my o2 sensor isn't getting a reading from the cat, instead it is seeing the extra flow and richening the mixture to compansate. Are these O2 simms any good? Or should I throw on some high flow cats. I'm not getting any codes but my fuel mileage is TERRIBLE! Can't get past 12mpg no matter what and I do 90% highway driving.

Any help is greatly appreciated.

when i hollowed out my cats i just unplugged the after-cat o2 sensors. all i got was a light and a dtc, both of which i couldn't care less about. performance and mileage were completely unaffected. so if you can live with your check engine light, save the money on sims and just unplug the sensors.

so unplug them, reset your pcm so it forgets about seeing cat inefficiency codes, and you are good to go.

This message has been edited. Last edited by: harleyc16,