'97 Vortec 5.7 K2500 Suburban 'Open Loop' issue

Looking for some assistance with a problem I have...

I've got a 1997 K2500 Suburban with a 5.7 Vortec and it won't go into closed loop mode, I have the 'Torque Pro' app on my phone, and a Bluetooth OBDII dongle plugged into the car.

The o2 sensors seem to be sending the correct signals back, voltage jumps around between 0.1v and 0.8v as it should (o2 sensors are brand new)
ECT sender is working fine, the app shows the correct temp. Just to satisfy my own curiousity, I unplugged the other temp sender, just to make sure that they weren't crossed over or anything. As expected, no temp readout on the dash, but the app still read correctly.

The app keeps reading 'Fuel System Status: Open Loop - Insufficient Temperature'. I've run it up to operating temp in my garage (86C / 185F or so) and it just sits there with that message. The check engine light doesn't come on, and there are no stored error codes in the ECM

The truck runs fine, however it doesn't pass the annual emissions/smog test here in the UK, so I'm off the road until I can sort it... Any ideas what I can look at next, as all the usual suspects check out fine? Obviously, being in England, parts are harder to come by than in the States, so I can't just throw parts at it to try stuff (not that I I work like that anyway...)

It should hit closed loop around 120F, that is strange. Also strange it would tell you that but not produce a code.

Yup, that's why it's got me stumped...

Did you just complete work and clear codes...may need some drive cycles to make computer happy again

Ahh, true. Takes a few hits to pop a code.

I haven't cleared any codes, but I did disconnect the battery for a few days while I was doing some recent work... I'll run it a few times, see if that helps. Thanks!

Well I've driven it around a bit, and been back to the garage (to check that my app was reporting correctly) - it's still not going into closed loop and the emissions are high.

Not sure what to try next, any ideas? Are there different parts of the ECM that require signal from the ECT sensor? My thinking is that the app is reporting the temp correctly, but maybe another part of the ECM isn't getting the info? Or is that a load of rubbish? lol

Another thing I noticed, is that on pin 23 of the blue connector I have a blk/wht wire connected. I've looked at all the schematics and they all say that pin isn't used.

I'll trace it back later to see where it goes, but does anyone have any ideas what it might be for?

Ok, so the wire on pin23 on the blue connector to the ECU is a 'known' (just not by me until now...) mod that can be done, as documented here:

https://chevroletforum.com/forum/si...walkthrough-vcm-ecu-ground-wire-update-39893/

Out of curiousity, I pulled the ECT connector off, it immediately changed the ECT to -40 (and threw a check engine light and logged an error code). I then grounded the wire, which changed ECT to 150 Celsius (302F) and threw up another error code, so I'm confident that the ECM is reading that sensor correctly.

Another thing I've noticed in all of this is that my temp doesn't get above 183F or so. I know that shouldn't affect this, but I believe the thermostat should be 195f? So I'm gonna replace that first, as it seems to be a problem, however I'm still stumped as to the original open loop issue.

I did notice in the Torque app that I was getting the below error (idling, in my garage with coolant at 180ish)

TID:$02 CID:$04
lean to rich sensor threshold voltage(constant)
Test incomplete or dependent test failed

The first line (TID...) varies, but the results are the same all the way down.

Not sure if anyone can offer any help, but I'm going to document everything I try and the results as it may help someone in the future!

Quick update: New ECT sensor and thermostat now installed. No change, the old ones were OK. That said, it does get up to 194F now, as I wanted it to - so it's not all bad!

Could the 'insufficient temperature' be related to anything else that I'm missing?