ThermoKing APU doesnt always start

One of two possible issues. If the unit does not have a good ground it will be rough starting, or may not start. Sometimes the ground connections get corroded. I had a TriPac that I had to run new cables from the engine to the truck frame in order for it to start easily. I also have a Evolution model that the fuel pump went bad, and starting roughly was one of the first symptoms.

 

Fuel pump. The injection pump is capable of pulling enough fuel to get it started and maybe idle it, but it'll sputter and want to die with a heavy alternator or compressor load.

I'd recommend a Facet fuel pump mounted outside the APU enclosure.

Amazon.com: EMPI 41-2010-8 VW Bug, Buggy, Sand Rail FACET 12V FUEL PUMP w/FITTINGS: Automotive

 

And if you have an older unit, get one if the primary screen filters like they use on the newer Evolution models. It has a clear screw-on bowl and screen that's easy to clean. Mount that outside the enclosure before the fuel pump.

 

P553890 Donaldson FUEL FILTER, IN-LINE
This is the round fuel screen. $35 + tax at Thermo King

 

www.aftermark.net for the fuel pump.
The 60203 is the one for the Tripac. It's ROUGHLY $200 @ Thermo King. $39 @ aftermark.

If you need any parts for the Tripac, try www.thermobyproducts.com Their prices are excellent.