Depends on how much, but:

1. Adversely affects flow and heat exchange.
2. Accelerates internal corrosion.
3. Normally will rise to the highest point of the system but..
4. Can get trapped in the cabin heater - most folks will notice that loss of function first if they live in cold and wet countries like the UK!!

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NormanB
230 TE (W124) 1989 with 153,000 miles on the clock - hoping for at least another 100K