My method is turning on the A/C while it is 90 degrees out and then heading up to Mt Bachelor at a high rate of speed.
Warms up the old motor pretty fast making the hill climb.
On my 300 is I hit the 100 mark on the temp guage it had better be running...

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique