This isn't going well
my 97XJ starts well from cold, runs fine until the engine temp stabilises and then it starts a bad misfire/rough running/no power unless you're on full throttle. Hot idle is poor too, with the occasional cut at junctions etc when coasting down to stop.

It restarts OK but the misfires continue until you leave the engine to cool off. I don't think this is the typical heatsoak issue because it's a 97 with only the one cat. Also, it starts fine when hot even though the idle is a bit off when hot.

It did give a CEL for MAP, which I changed, and then another one for MAP, O2 and Invalid key from Immobiliser, which I just cancelled. It hasn't thrown any codes since but the behaviour continues.

Just before this started, I'd had the power steering pump changed and it was running perfectly until I switched the aircon back on. Then the misfire started within seconds and hasn't gone away despite switching the aircon back off etc.

The belt is new, but squealing. After the PAS pump was changed it also popped a core plug so the whole thing (under the bonnet) was covered in Coolant due to the fan throwing it round the place - but it was already misfiring before that happened.

So.. weird! Does anyone have a theory cos I'm fresh out!

 

so it runs fine until it enters closed loop? how much have you investigated the o2 and map codes?

 

Well, I thought it got to closed loop a bit earlier than full temp. I'll get onto testing the 02 and MAP wiring.

 

It was dodgy wiring to one of the injectors. There's been a lot of messing around with those cables with the LPG/Petrol setup so it's not that surprising. I think it might have been getting a shot of propane as well as petrol which would put it waaaaaay rich on that cylinder.
All sweet now!

 

Correct. Typically, closed loop is entered long before normal operating temperature is reached by the motor. I've seen mine enter closed loop as early as 80f (coolant temperature). Somewhere around 1-2 minutes after starting the vehicle. The sensors have their own heaters.

Because long-term fuel trims adjust gradually, if an oxygen sensor is causing problems, it might take 2-10 minutes after starting the vehicle for problems to present themselves.

The process is something like....1. System goes closed loop. 2. Ox Sensor providing unusual values. 3. Computer reacts by trimming air fuel mixture. 4. Fuel trim gets so improper vehicle starts showing symptoms. 5. Improper fuel trims persist and check engine codes are thrown (if fuel trims are bad enough).