The question I would have is what is causing the unburnt fuel... is it because the computer is enriching the fuel mixture when the engine is cold? And if that is the case, what is the computer using to determine when it should enrich the fuel and when it can stop enriching the fuel. Is it based on the intake air temp or is it based on the coolant temp... or is it a combination? Although I don't have the answer, I suspect it has to do with the coolant temp more than the intake temp... so the faster you can get the coolant up to temperature the sooner the enrichment is stopped. Yes, a higher intake air temp will help warm up the engine faster, but I suspect the difference in time is going to be negligible. Getting the block up to temp using a block heater will have a much greater effect.
Also, if you are powering the heater off your own engine, aren't you loosing some of the efficiency you are trying to gain? Unless you are able to do an amazing insulation job around the heating element so that 100% of the heat generated is being transferred to the intake air, you'll be losing some of that heat that you're generating into the atmosphere (well, to the engine bay which is open to the atmosphere, in effect)... so the 280W heating element may raise the temp of the incoming air by some amount, but not necessarily 20 degrees... the element has to heat the air plus the surrounding ducting which is being constantly cooled by the incoming air and the cooler air in the engine compartment.
Personal opinion (gut feel, no data, so take it for what it's worth) is that you'll get the most bang for your buck out of a block heater that you plug in at home.
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Originally Posted by JRMichler
Don't some diesel engine have electric WAI? If so, find one of those in a junkyard. It's designed for under hood use.
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In the case of my TDI, there are glow plugs in the coolant line... the purpose being to accelerate the heating of the coolant. Not sure if other diesels use electric WAI.