Throttle body heat conundrum

[Rosieuk]

I've recently pulled the pipes heating the throttle body and I've noticed an interesting effect. It seems to improve mpg at low to medium speeds and improve warm up (which would make sense) and is why I did it. The engine seems to run smoother (if a hyundai alpha 1350cc with a mystery idle misfire could ever be considered smooth) but here's the thing. It really doesn't like extended motorway running, mpg goes down the toilet, after a while 70mph went from 11-12l/h to 15, however light I was on the throttle.
When I got back from that trip I checked the modification for leaks and happened to put my hand on the throttle body and the plenum behind it and they were both colder than ambient temperature, normally they're painful to touch. Here is what I think is happening: at low speeds and short bursts of 60-70mph the heat from the engine (direct and radiated) is enough to keep the intake heated but the air cool so economy is better (ie enough to make for better running but not enough to trip the iat). After a while at 70-75mph (say 7 to 10 miles) the cool ambient air overcomes the heating effect of the hot block and components and the plenum & throttle body get cold, thus the iat tells the ecu more petrol and consumption goes up. On this engine the iat sensor is mounted in the centre of the intake plenum.
This isn't so much of a major problem for me because I don't do long distance motorway driving any more, but it would clobber mpg if I did.
Given that this is the case then switching in TB heating at motorway speeds but having it shut down everywhere else would seem the way to go.. Either a manual controlled valve or a VSS automatically controlled valve that cuts in at 65mph+.
My average over the last 3 years has been 38.6mpg, mostly short journeys and I recently started using an octane booster additive (silver hook), 2 refills later my average is 42+mpg, almost an 8% gain. When I manage to use pulse and coast aggressively (it's not easy with the usual 20mph cretins) I've managed to improve that to around 52mpg..
Another useful modification with the LCII 1.3 Accent involves the radiator (or lack of one). The thing is barely bigger than a sheet of A4 paper and any long journey at over 20c ambient it trips the fan in jams or even slow traffic (it's been known to do it at 70mph on very hot days, 97c and climbing is not a happy motor). Where the other half of the radiator should be is a blanking panel of thick plastic right in front of the engine block & cat, my father and I removed it, cut the centre out and replaced it with a sliding aluminium vent (and some reinforced tape) then refitted - it now sits at a much happier 85-87c at speed and triggers the fan over 95% less than before. When the weather gets colder, I'll just slide it shut.. And everything will be nice and toasty. Best of all this mod cost me less than £7 in total.
For anyone who's interested, and lumbered with one, the thermostat on these opens at 92c and keeps opening and closing momentarily until everything is heated up accordingly, the fan cuts in at 96c.

[JockoT]

Wind chill as air rushes through the throttle body will have a considerable chilling effect. Add to that the latent heat of evaporation of petrol, which also chills the inlet manifold and you can see why the throttle body is heated.

[Rosieuk]

True but the point I was trying to make is that it seems to have a benefit at low speeds and rpm and a deficit when the temperature and pressure of the unheated intake air defeats the radiated & conductive heat from the block and components and cools the plenum. This is something I didn't expect and since I've never seen a car or heard of one with speed/rpm controlled TB heating I doubt others have either.
Disconnecting it at urban speeds 25-50mph makes a major improvement. Extended 65-75mph+ is the only time it seems to actually decrease economy.

[JockoT]

Cold intake air improves the power developed but reduces the fuel efficiency. At higher speeds and higher air flows the throttle body will chill and the intake air will cool slightly, leading to poorer economy.
Perhaps at low speeds the extra power available helps to improve the fuel consumption especially as the throttle body won't be chilled as much.
The thing is, if this mod has improved OVERALL fuel economy then good. If not, reconnect the pipes.

[Rosieuk]

this seems to have made a major difference - am going to fill up tomorrow and expect to make over 45mpg when previous average (without octane boost & bypass has been 38.6mpg, octane boost alone was 42mpg).
I'm going to develop this with an adjustable bypass. A brass Y with two valves, one feeding the bypass, one feeding TB heat and an unvalved Y piece to feed back into the cooling system. Three possible modes, heat off, 50/50, heat on. this will allow for all weather conditions here, although a thermostatic bypass would be more efficient me and electrics don't mix and I've not been able to find a suitable thermostatic valve.
I'm surprised that manufacturers aren't using adjustable intake heat, since if I'm right removing this has improved mpg by 3mpg for low/med speeds, but it's proven advantageous at prolonged high speeds.

[oil pan 4]

Those heat lines there are actually to prevent throttle body icing.
Something that I can see being a problem in cooler parts of the UK.

[Daschicken]

I'd say your conclusion is right, the missing coolant line lets the throttle body get too cold. I can't say i've seen lower than ambient temperature with my accord, but it can get around +8 F above ambient if i'm on the throttle. Off the throttle and idling or my sort of P&G on the highway and I see 20-30 F above ambient.