Coolant loss

Check your holding reservoir, my mack pentacle at btc had the reservoir crack on the passenger side inward of one of the molding seams, you couldn't even see the crack until the tank was removed

 

Wrong cap as in weaker. Probably not stronger though.

If your system builds 15 lbs of pressure. A 10 lb cap won't hold. An 18 lb cap will.

 

I have had coolant loss in both the Volvo's I drove. I put a bottle on the overflow line to see if it was being pushed out the overflow and it was both times. The first one was repaired by Volvo and was the injector cups leaking which the mechanic said was a common problem with the D13. The other time it was the high pressure cap on the side of the reservoir.

 

How many miles on the engine?

 

The oil sample would be first thing I would do. Also check coolant for exhaust pressure

 

I agree with the others. Do an oil sample because you don't want extra wear on the motor if you want your truck to last longer.

 

I don't mean to be an *** about this but the first post should tell us what kind of and year truck it is and what engine. I am thinking the reserve container is cracked which happens a lot or the radiator or BOTH.

I replaced a bunch of radiators on the same year trucks in my fleet because two of them leaked under pressure - plus they were so clogged, causing it to overheat.

Another group had leaking reserve containers which dribbled when the engine was under a load, and they all did it at a bottom seem, so they were defective. It wouldn't do it when idling but under load and the pressure at 16 psi of pressure to cause it to happen. We didn't find it until a driver was moving a heavy load across the lot and there was a dribble on the cement behind the steer.