D4-7U Left side radiator water leak only when the pony motor was running

Seems you may have pony compression pressure leaking into the cooling system and displacing the water. Check pony head torque first.

The leak was coming from the left side up by the radiator. The pony engine seems to be running great. Is there a drain valve for the pony motor anywhere? I really hope it's not a compression leak.

Radiator overflow/pressure relief line is on the LH side of the radiator (viewed from seat).

Thank you Old Mag! I will inspect things a bit better the next time I start it to see if I can figure it out. Is there an easy way to see if I have a compression leak and would there be any other symptoms while running the pony motor? Just seems odd that when the diesel motor is running and doing work for another hour there was no leaking. I would assume they are all connected unless there is a check valve or something between the two.

Before you start it next time, check your pony cylinders for coolant seepage.

Pony head gaskets aren't expensive, about $10 last time I bought some from aftermarket vendors, and can sometimes be reused, though I don't think I would attempt it with one that was leaking without a spray coat of gasket sealer or similar product. You don't want to let moisture sit in the cylinder long term because of rust in the bore.

Hi Team,
as others have said the radiator relief drain is at the left of the radiator.
By parking with the nose so far down I would suspect that air from the radiator top tank has found its way rearwards and into the pony motor top cover coolant passages.
There should be an air bleed valve on the top of the Pony top cover--accessible from the left side in behind the pony fuel tap/filter bowl--see scan below.
To check for coolant in the pony cylinders simply open the pony cylinder head drain/compression relief valves and crank the pony over--check for evidence of coolant on the diesel flywheel housing below the valves.
Hope it is as simple as this.
Cheers,
Eddie B.