On my GQ dual fuel wagon, I had a auto problem I thought may've been related to some long distances I was travelling on consecutive days of 30+ degree heat.

After the final day of this travelling, I noticed that the next time I tried to drive, could not change out of 1st. I'd regularly checked fluid and oil levels for everything during this period of long distance travel, so didn't think problem would be from underfilling. Thought I'd let the engine run to get some heat up to check level anyway. As I was trying to limp in 1st to safer ground, I found it would now change normally, until I realised it wasn't changing into 4th.

Did transmission oil/filter change. Oil had it's colour and did not smell burnt, filter still looked reasonably clean, and nothing but the usual sludge and fine metal filings you'd expect around the magnet.

A few days before, I'd removed the radiator, cleaned and flushed, removed engine block plug and flushed. Put back together, refilled with good coolant. Temp gauge still sat on half or slightly over when up to temp.

Auto was definitely smoother, and after a carby cleanout and some slight adjustments to the idle/mix/accelerator, it's running better than it has in ages. However....

I still have to warm it up for around 10 minutes from a cold start before driving, or auto will not change from 1st. And still no 4th. I've worked out I use about 30% more fuel because of this. Frustrating.

I've gone through the manuals, scoured the forums, and have found so many likely causes/fixes suggested it's hard to know where to start. I'm hoping this description has a familiar ring to it for someone, and that they can narrow the field a bit for me - preferrably with reassuring happy ending.