Another 2.5l running rich - SOLVED! - interesting cause

[Doc Payne]

Hi all. I’m working on a new to me 1989 Comanche, 2.5L TBI, AX5, 2WD. 
 

It starts right up every time, runs fairly smooth after replacing all vacuum lines, but is running very rich. Strong fuel fumes will burn your eyes with it running. Black sooty spark plugs. My situation seems to be the same as this previous post here. 
 

Oxygen sensor heater has battery voltage on yellow wire, 6 ohms between heater supply and ground, 0 ohms to battery to connector ground, and 0V on signal wire with sensor unplugged. My understanding is there should be 5V. The gray sensor wire has 0 ohms to ECU connector pin 35. When running, the oxygen sensor signal starts about 0.70v and increases to 0.94v and stays there (digital multimeter). 

Things I’ve checked per the 1989 FSM (have the full set)

- grounds per cruiser’s tips

- EVERY engine bay connector cleaned

- Thermostat is new Motorad 195: previous had failed and wasn’t allowing engine to reach operating temp 

- MAP sensor function appropriate

- CTS function appropriate 

- MAT: ~1600 ohms at operating temp when running and sensor unplugged. Pulled and used a heat gun directly on sensor element and it immediately responded by dropping below 100 ohm. So I believe the sensor works but it’s getting cooled by the excess fuel.
- TPS: this one also might be a problem, adjusted WOT voltage to 3.65v but at idle it’s 1.2v which seems high. 

Am I correct in my thoughts on the ECU oxygen sensor circuit being faulty? I don’t have a spare to swap in, I’ll have to purchase one. 

[Doc Payne]

Wanted to update this as I’ve found the issue, and have yet to come across a similar post with it. 
 

Here is the injector I pulled out. Someone had unscrewed the center post screw and this was allowing excess fuel flow. I screwed it back in and the rich smell was gone, and the O2 sensor started fluctuating between 0-1v as it should. 
 

 

I ended up replacing the injector since I wasn’t sure on the calibration anymore. I went with a GM TBI injector from a 1990 s10 Blazer 4.3L V6. All seems to be well now!

 

I did also have a faulty MAT sensor. The wires had started to fail where they enter the sensor and when I wiggled them a bit, the resistance went to open. So I drilled out the intake manifold hole to 9/16” and tapped with a 3/8 NPT. Used a GM air charge temp sensor (Standard motor products AX1) and it functions perfectly.

 

Other notes for the next guy to come through.  The 2.5l does not have a 5v reference at the O2 sensor connector output wire (gray on the 1989) like the 4.0 does. The 2.5 sensor only puts out a 0-1v signal when running.