First, you must use low silicate heavy duty antifreeze in Cat diesel engines. Automotive coolant has quite high silicate to protect the aluminum components. The silicate will combine with some of the cavitation inhibitors and cause them to drop out of solution. Contaminating the cooling system and reducing the anti-cavitation properties of the coolant additive package.
Now in answer to your questions.
1) Is it OK to mix your coolant w/ tap water prior to filling system?
You should never use tap water in a cooling system of any engine. Too many contaminates.
2) Is it OK to mix your coolant w/ distilled water prior to filling system?
It is best to mix coolant before filling. Use distilled if you must, but you would be far better using premixed Long Life Coolant. Even distilled water has contaminates which can damage the cooling system. The water in premixed coolant is distilled, filtered and treated so as not to damage the system in any way.
3) Is it OK to flush system with tap water prior to filling?
Flush if you have contamination, but if the system is in good shape simply install new coolant.
4) "50/50 Final Charge" ... will my engine use this in its cooling system?
50/50 mix is good for most areas, if i remember 50/50 is good to around -40 degrees F. Never use more than a 60% antifreeze 40% water mixture.
5) Should I be changing my coolant every 2 years? or ???
Using ELC in your cooling system will extend the service of your cooling system. There is a sample/analysis that can be done to keep the coolant at top performance.
6) My shop uses a pre blended mix + additives to fight cavitation + new coolant filter when the flush my system every 2 years... should I have anything to worry about?
What ever you use, make sure you can purchase additional of the EXACT coolant. Coolant's from different Mfg. should not be mixed.
7) My oil samples all check out fine.
A couple of cooling system notes.
No amount of additives can make bad water good. The 'bad' has to be removed before it is put in the system. Use pre-mixed coolant.
The 'pill' in the coolant filter dissolves in the first 30 minutes after installing a new filter. Be sure to use a filter of the same Mfg. as the coolant you use. The additives may not be compatible if brands are mixed.
40% of all engine failures can be traced back to a cooling system problem.
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