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- I'm using the MWR swap header with the B1S1 O2 sensor only reading CYL1. I've been wondering about the effectiveness of this configuration with a 2ZZ ecu. I don't know that it would cause my misfires, but it seems like the ECU is working from incomplete data regarding its fueling strategy. Should I consider having a bung welded in after the collector?
Thanks for your very thorough and clear post. You sound like a career technical writer.

The answer to your questions quoted above is an emphatic "Yes!" You are absolutely correct in that the ECU has absolutely no idea what is going on with fueling in the other three cylinders. And from there it is not a leap to conclude that this could lead to misfires. Absolutely you should have a bung welded into the downpipe a few inches (6-8 at least) past the merge collector. This is the stock configuration that you would find in any 2zz engine car and it's the way that Toyota intended for the fueling strategy to function correctly.

For mystery misfires, it can be useful to have access to the individual cylinder misfire count. This info is not available in any generic OBD2 scantool app. But it is available in Techstream. Techstream is the official Toyota diagnostic software. If you own a Toyota, I suggest that you get Techstream. Anyway with the individual cylinder misfire count, you can either rule out possibilities, or zero in on possibilities that may be the cause of the misfire.
 

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When you relocate the sensor, you will either need to purchase a sensor with a longer wire (the stock downstream sensor may work for this purpose), or, you may need to purchase a O2 sensor harness extension. Do not buy a cheap extension on eBay. Buy one from Summit Racing. They are available in different lengths from Caspers Electronics. I have tested this product and it works reliably.

 
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