Hey gang, I'm having some unexplainable problems with my 92 4x4 5spd. The digital FSM I have doesn't give info on open and closed loop operation.

I'm looking for a link or whatnot to learn more about what the ECM needs to run in closed loop since mine won't always enter closed loop. Randomly it will run closed either for an entire trip or part of the trip. It also sets a 41 for TPS immediately after clearing codes but it is a new OEM TPS adjusted and tested good. Could this be keeping it in open loop?

I began the investigation since it smelled like the truck was running rich and confirmed it by reading steady high o2 voltages. The sensor is a year old denso 234-4052 and responds rapidly to a lean condition and fuel cut when coasting. And the random times when the truck enters closed loop it oscillates about 2x per second at highway speeds. The heater side ohms out normal also. So I think the sensor is good. I've been trying troubleshooting steps in the FSM and it says to check o2 output at the diagnostic port, voltage between Vf and E1, while doing some things to change A/F ratio. All I get is a steady 0.08V while testing no matter what I push, pull, or plug, and it flow charts me to replace o2 sens. I do the same tests backprobing the ECM instead of the diag port and get correct o2 feedback.. from slightly negative voltage if very lean to 0.98V rich, all responding correctly to induced A/F changes. So it seems the signal is getting lost somewhere.

In order to preserve what's left of the cat and fuel econ, I have leaned my VAFM 12 clicks!!!!!!! to get close to 0.5V feedback during normal driving if it's going to be stubborn and not enter closed loop. The truck runs great! Starts and Idles smooth, is responsive, and doesn't smell like an overworked cat. What the hell is going on here?

 

Q: "What the hell is going on here?"

A: You've got a code 41 and you are worried about the Ox signal instead of fixing your bad connection..

You've got a broken wire, a loose connector, or very unlikely a bad ECM.


Start by backprobing the VTA wire at the ecm. If you have correct voltage that moves from <1 to ~=5v , Unplug the ECM, locate the VTA wire and unseat it from the connector and pinch the female lug down so it gets a good bite on the male pin on the ECU. Replace the lug in its connector and all done.

If you don't see the VTA signal you need to work your way back towards the TPS wiggling wires and connectors as you go until you find where it's cutting off the signal. Repair and replace as needed.

...
Yes you need VTA to run in closed loop! You need air temp THa , load (VS), throttle (VTa) and engine coolant temp (ECT) all with in the closed loop range to enter the closed loop feed back mode.

Under high load it drops out of closed loop and goes to power enrichment.
Throttle smashed to the floor, same thing power enrichment.
Coolant cold, warm up enrichment comes into effect.

With any one of these signals or OX not getting to the ecm it goes into the fixed run safe table.

 

Right on, that seems to have done the trick! By now I had triple checked the wiring from TPS to ECM harnesses but couldn't confirm that last connection so that was good advice. Tweaked all 4 TPS terminals and took the cover off the ECM to measure the pins going into the board confirming the terminals were snug. All measurements spec'd out and CEL shows normal mode when keying on. Two trips so far and both entered closed loop within a minute or so of driving.

Now for measuring o2 feedback at Vf in the diagnostic block.. it's an amplified or processed signal which is why the FSM shows 0-5V? In theory it would mirror the lower voltage the sensor is putting out?