Won’t go into closed loop

[‘89Jimmy]

Hello again,

1989 350 won’t go into a closed loop condition. I’ve just replaced the water temp sensor and O2 sensor as easy and inexpensive attempts at fixing this. I have noted that the O2 sensor reading only swings from about .435 V to .450 V, even on the new O2 sensor (the old one was the same). It would appear the ECM is not getting a good signal. Any typical issues I can look at quickly? Any ideas?

 

[Wes Harden]

The ECT, and ECM ground wires at the tstat are suspect, they get mistreated a lot. Check you ecm b fuse, truck will run with that fuse blown.
Have you confirmed the coolant temp with an IR gun or similar ?

How long had truck been running when O2 voltage was observed ? If no closed loop the ecm will not look at o2. If you give the engine a vacuum leak the o2 voltage should go down.

 

[‘89Jimmy]

ECT is brand new (just replaced this morning), though the old one read fine as well. I would assume the wiring is good too? Don't have a temp gun, is the harbor freight $20 one worth anything?

I observed the O2 readings from cold all the way to operating temp. Probably 15-20 minutes while idling.