I've seen a number of tractor engines with air pre-heaters in the intake manifold. I have no idea what they look like, but I would expect a nichrome wire grid that heats the air up pretty good in the 45 sec or so you are supposed to run them.

Must be in the manifold, though -- the air would cool off too much from the air cleaner housing.

Glowplugs are a better system.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!