There are a few thoughts on why the engine struggles to crank when you try and start a hot engine:
1. The engine itself ‘tightens’ up when it is warm, making it difficult to turn over.
2. Advice on the forum is that the absence of a decent earth strap from engine to chassis can cause a voltage drop.
3. If the ignition advance is too much it would make it difficult to turn over.
There is nothing I can about (1), and (3) is controlled by the ECU, so all I can do is fit a proper earth strap and hope it helps.
Here it is:
I also decided to add a voltmeter into the circuit so I could see if the battery was struggling. I didn't want it to be a permanent distraction so mounted it on the auxiliary electrical panel where it can easily be viewed when troubleshooting.
As you can see, at 12.5V it is quite healthy. When the ignition is switched on it drops to 12.3V, which is fair. If you turn on a high-power system like the demisters or the headlights then it drops to 11.8V, not unexpected. Once the engine is running it shows 14.1V which means the alternator is charging it well.
The only slight concern is that while cranking it initially drops to 9V before recovering to 9.8V. That is marginal.
Something to keep an eye on.
Update: March 2014 - Now had a chance to try a hot start and everything was fine. See "Sunday Run with Hot Start"