I'm not sure on the 4ZD. There are coolant passages in the intake manifold on the 4ZE cause that's where the temp sensor for the ECM is screwed in. I think part of the reason for the coolant circulation is to temper the air to keep the NOx down. And like I said there are also two coolant tubes on the underside of the throttle body which appear to prevent frost build-up in the throat under certain atmospheric conditions (like freezing fog). Happened to me once with a VW. Strangest thing to pull the air filter and see a plug of rime ice in your carb.