My 90 V6 PU had 320K miles, No Check engine light on, at Idle rpm up-down in variation about 100rpm at specs at 800rpm, exhaust fumes not smelled dry, no smoke. Timing set at specs. O2 sensor might be original, I bought the truck used at 110K.
How many O2 sensors does the truck have. When do I have to replace them ? Thanks.

 

Wow, that's a lot of miles, certainly beyond the life expectancy of an O2 sensor. How's your gas mileage? I'm tempted to say "if it ain't broke don't fix it"

You have 1 O2 sensor and it's used by the ECU to help adjust the air/fuel ratio. That's why I'm asking about your gas mileage.

Do you have a multimeter? Here's a test proceedure from a 1993 FSM for the 3VZE (your's may be about the same):
http://personal.utulsa.edu/~nathan-b...01heatedox.pdf

I changed my O2 sensor at about 90,000 miles on my old 3.0 and I didn't even notice a change. So, now I have 2 of those sensors, one with 90,000 miles on it and one with about 25,000 miles on it.

You may (depending on rust rate in Houston) be openning a can of worms if you try to remove nuts on an exhaust system that old.

 

wow DEFINITELY replace it. I had no check light but replaced mine for the first time at 220k, and I noticed an immediate difference in performance. Not much mileage change for me though, but I have other issues right now.
www.sparkplugs.com

 

The mileage about 18, 19 combined city, highway.

 

I replaced mine at 180k with a bosch for like 30 bucks, didnt need it but thought it would be ok. and i didnt really notice an increase in power or mileage