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FYI/Thorley Header-O2 Sensor

2801 Views 18 Replies 9 Participants Last post by  KesekiSupra
When I first got my car, I used a Bosch O2 sensor, and it passed emissions . I recently failed emissions, and broke down and bought a factory sensor $98. Between tests I had put on a Thorley header. Comparing both O2 sensors side by side, I see why I failed. The Bosch is so short at the tip, that it wasn't even in the exhaust stream. The Toyota sensor is much longer at the tip, and was designed to work with the header, that was, at the time an OEM accessory part. This was an expensive mistake trying to save a few bucks. Live and learn.
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I went through the diagnostic today, and again it's bad news. I failed emissions again, but the numbers are much better. It seems that the O2 sensor is working, but it is only registering from 4/10 of a volt to a 6/10 of a volt variance, and doing that slowly. If you rev the engine, it spikes the one volt change it is capable of, so the theory is that it works but is not in the exhaust stream enough to make the necessary voltage changes. I'm going to measure the stock manifold, and then call Thorley, and see what they say. The mechanic was also concerned that possibly the ceramic coating on the header was not letting the sensor ground properly, but we were unable to get the proper changes by clipping a ground on the sensor body. I'll let you know what I find out. Back when these headers were OEM, emissions were not run on a dyno, so maybe they never really worked very well.
HC failed the 25MPH test 249ppm, limit 118ppm. NO failed both the 15 and 25MPH test. 2656ppm at 15MPH limit 937ppm. 2977ppm at 25MPH limit 853ppm. The sensor on the stock manifold is at least 3/4" into the exhaust flow, and less than 1/4" on the header if that.
CO is .01 and .12 for both tests respectively. You are most likely right about the CAT, but temp readings indicated it was working. They weren't sure if it could do it's job because of the lazy operation of the O2 sensor (only fluctuates between 2 and 6 tenths of a volt, and slowly at that). Tech at Thorley agrees that the position of the sensor in the header is not in the exhaust stream enough for it to work right. I will most likely relocate the sensor to the collector, so the sensor can monitor O2 on all six cylinders instead of only 1, 2, and 3 and position it so it is in the pipe as deep as it is in the stock manifold. EGR is working. The valve and modulator is less than a year old, I cleaned the blocked passages in the plenum ( the lower pipe is clean), the BVSV is spot on, and the VSV vacuum switch has been replaced. The readings I gave you were before the timing was corrected, it was at 22 degrees for the test ( please don't ask how that happened), so the numbers were much lower when it was corrected, but still too high to pass.
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Pulled the header off today and had the O2 sensor flange welded directly to the pipe. The sensor was not in the exhaust flow at all with the Thorley setup, so even using the longer Toyota part, there is no way that it could have ever worked properly. Had to weld studs to the flange with this setup, but it just makes installing the sensor that much easier. I'll let you know how it runs with a working O2 sensor in the header. The Cat. looked good, so I'm hoping it will go through emissions this time. There was not enough room to relocate the sensor to the collector.
Put everything back together this morning, and this car has never run better. It has much more power, especially at the top end, and it easily pulls 7,000RPM before the power flattens out. This is a must do modification of the Thorley header. From the face of the flange to the inside of the pipe shouldn't be much over 5/16". This thing rocks now, it unlocks the true potential of the header.
It passed emissions, but my cat. was also gone and the motor is a little tired with 197,000 miles on it, so NOx was still a little high. It won't pass in two years from now when they drop the numbers again. The biggest benefit was the power increase. It is a different motor altogether. Most noticeable is drive-ability, low end power and it will run itself into the rev limiter in a heartbeat now. I got a good 7,000RPM out of it once before I posted, and then the limiter started kicking in every time, at just under 7,000 RPM. I had a marginal automatic transmission that is being changed now. It was real fun for a couple of days. With cleaned and balanced injectors, I believe this car could be a little beast. If you have a ceramic coated header, find a way to ground the O2 sensor directly. I bolted a large braided ground strap to one of the sensor's studs, and then attached the ground wire to that so it wouldn't burn up, but there may be a better way.
All my systems are clean and working, hoses, belts, including timing have been replaced, EGR passages cleaned, valve and modulator replaced, VSV replaced and all other components test correctly. Valve seals and guides are a bit of a problem, and I could very well have injector problems, that I haven't gotten around to checking yet. There could be a bad connection in a connector somewhere for all I know. I failed emissions, with the new cat (just barely), so Toyota replaced the new O2 sensor I had just purchased. It passed the next time. I was somewhat near the limit on NOx on the 25MPH test, so if I don't find anything else wrong, I'll either rebuild or put in a rebuilt 6M before the next test.
I'll try to get pictures when I go under to replace the transmission.
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