This will probably be more information than you want but...This is my son's Blazer. He asked me to check on an oil leak he had, oil all over front left of engine compartment. He also had a check engine light for his O2 sensor (more on this later) Turns out it wasn't oil but power steering fluid from a bad high pressue hose. Changed out the hoses and p/s pump and while I was at it changed out the oil supply/return lines to the oil filter
(fender wall mounted filter). While doing this noticed that the rubber in the motor mounts was virtually non-existent. Replaced the motor mounts. I also went through all of his vacuum lines from his front wheel drive differential vacuum actuator to his transmission(and all points in between). While replacing the vacuum lines I noticed that the O2 sensor lead wires (that run down by the distributor and across the bell housing to the sensor) had been severed. I don't know if they were cut(by someone?, why?) or if with the bad motor mounts the engine had flexed back against the firewall, cutting the wires (is that even possible?). Anyway, I purchased a new sensor lead harness from Chevy and a sensor and wired it back up. No more check engine light. Now, my question is, if I wired it up incorectly (I don't think I did), would I get a check engine light or could it be sending some weird signals to the ECM and still not be showing a check engine light? As far as the catalytic converter
is concerned, I ran the motor at idle until at temperature and checked the inlet/ oultet temps and the outlet temperature was about 30 degrees cooler then the inlet (using a handheld IR thermometer). My understanding is that the outlet temperature should be considerably higher if the catalytic conversion "kicks off". I didn't check it at a higher RPM (2500 RPM?).
In answer to your other questions, I don't have gas in my oil, the air filter is new, and I did change out the temp. sending unit and the thermostat (195F). It does seem to be indicating a low coolant temp (170-180F). Actually, I bought a new '93 Silverado with the 4.3L and after a few years my coolant temp. reading dropped. I changed out the temp. sending unit and thermostat with no improvement. It didn't seem to affect the performance so I didn't worry about it. I had a friend with a Chevy S10, 4.3L, same thing???? Have you heard of this before? But, back to the Blazer.
My son wasn't in immediate need of the Blazer so I was able to work through these problems over a few months, as time permitted. Between the time that we parked it and when we started driving it again the tags expired, so I'm stuck test driving it in the neighborhood (30 mph). I've had the battery disconnected multiple times and I believe you have to put a vehicle through it's paces for the computer to relearn the optimal settings, but could that cause such terrible gas mileage?
Hope this helps. Thanks.