Lukewarm is about all you can hope for from the factory heater. 400w and a huge chunk of iron and low temps and moving air, that's doing pretty good.

Keep in mind the factory heater is a starting aid, not a comfort item. It's not supposed to heat the engine to operating temp, it's not supposed heat the cabin. It's meant to heat the engine enough to start, and the engine does not need to be warm to the touch for that. For that, it excels, and you can help with blankets over the hood, cardboard or whatever you have handy to block drafts, etc.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap