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Compression tester should come with several adapters. Screw on 10mm one that will match threads on spark plug. Also compression test is done at WOT for no restrictions. Crank engine for 6-10 revolutions. Don’t know what numbers from manual says, I think you want around 180psi. Desired to have no more than 10% difference between cylinders.
Clue here is it runs fine until warmed up. This pinpoints 2 possible sensors as most likely culprit:
ECT - when indicating cold-start, ECU injects extra petrol for cold-start cycle. But if ECT doesn’t properly report warmed-up engine, ECU may continue to inject extra fuel. Which will be too rich and cause stumbling and poor running.
O2 sensor - is most likely culprit if you have quick change from good running when cold to suddenly falling on its face. When cold, ECU ignores O2-sensor completely (open loop) and blindly injects pre-mapped fuel amounts (+ cold-start enrichment). After 2-5 minutes when engine is warmed up, ECU uses O2-sensor to adjust fuel mixtures (closed loop). Bad O2-sensor will indicate lean mixtures all the time and ECU will continue adding more and more fuel. This will cause poor running and stumbling engine.
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