FOUND A LARGE MASS IN MY RESERVOIR...Checked my reservoir after a cross country pick up of a recently purchased truck (2005 chevy Silverado 1500 4.3ltr v6 extended bed single cab) and found a large solidified mass in my reservoir as well as very murky coolant. Curious as to the recommended courses of action to prevent further damages and any currently possible damages. My truck currently doesn't overheat at all .I do know i need to flush the system i.e.; coolant system cleaner, distilled water, several flushes. I just want other opinions in order to build some knowledge and become more self sufficient. Attached should be photos and a video.
Things to consider which i think i know of to suspect. The thermostat could get stuck open. and radiator could get clogged.
I think the suspect is mixed coolants or someone tried to build a sand castle in my reservoir.

 

Is it the same consistency as sand?

I’ve never seen accumulation like that before but there’s a few possibilities (assuming it’s not the result of sand castle building ).

1) If the reservoir cap was/is missing, dirt would be collecting in it over time.

2) Coolant system stop leak was added to the reservoir instead of directly into the radiator.

3) Cooling system has simply been ignored for too long and the sludge has slowly accumulated in the reservoir.

Start with flushing the block, radiator and heater core separately just using a garden hose. Watch for sludge as you start water flow to determine if it’s isolated to one part of the system. Flush for at least 2-3 minutes in each direction and hook up to a hot water feed if you can.

Once finished, remove the thermostat and fill the system with straight tap water. Drive it about 5 miles (highway if possible), drain the system and repeat until the water comes out clear.

If it’s really bad, add a bottle of CLR.

Buy 2 thermostats - install one, fill with tap water and drive until the engine reaches operating temperature. Drain the system - if the water comes out clean, remove thermostat, disconnect hoses and use compressed air to remove remaining water from the individual components.

If it’s still not clean, repeat until it is.

If you don’t notice any overheating concerns throughout this process, install the 2nd new thermostat, and fill with proper coolant mixture.