Hotter heater possible?

Drape the heater hoses over the right fender, garden hose sends water in one out the other until it runs clear. Guess what switch direction of the flow and it runs dirty again. Repeating will eventually get a clear streem both ways.
Other conditions like air flow blocked by mechanical door not opening properly or debrie clogged heater core may exist but the hot water flow is easiest place to start. Good luck