Hey everyone, this week I finally sorted out the idle issue and wanted to post in case anyone else is wrestling with something like this.

It's taken a long time to chase this down, and I spent a lot of time scrutinizing the schematics for the EFI system on this truck. I purchased the old OEM service manuals and scrutinized just about everything. Tried replacing more parts than I'd care to admit as well. You name it, I've probably looked at it, pulled it apart, tested it, cleaned it, replaced it, etc. Two mechanics took a look at this truck and shrugged their shoulders.

The biggest clue came when I realized that disconnecting the AC coolant temp sensor would return the truck to mostly normal without a check engine light. MPGs were normal too, so I figure it wasn't running the "dummy" ECU map. That was good enough to get the truck through its last NC state inspection (it turns 30 next year and will be exempt). So, I ran it over the winter with the AC electricals disconnected but of course warmer weather is approaching so I've been back on this over the past couple of weeks.

I spent a ton of time on the EFI schematic and chasing anything that looked suspect at all. The big change was when I started focusing on the AC system schematic. I realized there's a small electrical controller module for the air conditioning system that pipes two wires directly into the ECU. The schematic showed that module has two grounding wires in the pigtail that are bonded together and have one discrete ground. I located that module, it's on the left side of the black AC box under the dashboard behind the glove box. (Mine is a LHD US-spec truck) Pulled that connector and tested the black leads. They had continuity to each other but not to ground. They both terminate to a o-ring terminal that fits under a screw at the top of the AC unit. That screw is in a small L-shaped bit of metal that bolts on one end to the AC box and the other to the firewall. I started the truck with the AC electricals connected and of course it ran poorly. I connected a jumper wire from that L-bracket to ground and... ta-da everything was immediately normal. Disconnect it... broken. Reconnect it... normal. I finally found the smoking gun after 6 months of chasing my tail. That ungrounded module must've been sending some wild signals to the ECU causing it to idle super high and the timing was all over the place.

It also explains why early on the truck would sometimes idle up if you shut the passenger door with some force. That ground is located only about 18 or so inches from the door and it was iffy. The impact would sometimes break continuity and the truck would start to act strangely. After a while, the door thing didn't make a difference anymore because the ground point had lost continuity entirely.

Anyhow, cleaned and tightened the relevant points and everything is back to normal. Running like an absolute champ and the AC works great just in time for warmer weather. An obscure little ground point for the AC system located way up under the dash had me banging my head against the wall for months. I suppose the silver lining is that I now have a big box full of good spares for the EFI system if I ever need them. If you're wrestling with something similar please don't hesitate to reach out. I'd be glad to help however I can.

Thank you to all who offered help both in the thread and in private messages and even on the phone. So glad ol' Frank is back to normal!