On many engines I've seen coolant is circulated through the Idle Air Control valve and the throttle body. One such example is the Mazda 2L 4 cylinder FS used in the 626. The IAC is ECU controlled, and there is a coolant temperature sensor, so what would be the purpose of running coolant through the IAC?

You can see in this diagram for example that it flows in the IAC then out through the Throttle Body ( or maybe the opposite direction, I'm not sure ):

enter image description here

There is a great picture here which shows exactly how the coolant flows through the IAC to the TB.

  • On a 2002 Nissan Maxima I once had the misfortune to own, I think the IAC had coolant flowing through it as a revenue generator for Nissan... A gasket failure on the IAC coolant lines commonly fried the ECU.. a $1,200 part, if you weren't handy with a soldering iron. The only car I ever got rid of because I couldn't fix it... – PeteCon Apr 27 '16 at 16:31

More than likely it's not flowing through the IAC, but rather around the throttle body. In most cases this is done to keep the throttle plates from freezing when it's cold outside. The coolant running through there can provide a lot more heat than is normally available without it, even with a cold vehicle.

They had the same type thing on the 4th Gen Camaro. Owners would typically bypass this as when the coolant is hot, it actually provides heat to the intake system, which brings down HP somewhat. In most cases you'd want to bypass this instead of blocking off both ends so the coolant continues to flow correctly. If you chose to plug this up, coolant isn't going the directions it normally would and might cause cooling issues or coolant backups. While this isn't likely to happen, there is the possibility of it happening, especially for weaker cooling systems.

Where you live, I'd suspect freezing throttle plates are not a worry.

  • Take a look at the diagram I added, it flows in through the IAC, passes from the IAC into the throttle body, and then flows out through the bottom of the TB. Or maybe the opposite direction, but it clearly flows in one, through to the other and then out the other. However, I don't think that would change your explanation. – Robert S. Barnes Apr 26 '16 at 12:52
  • Not more than likely, it IS the reason they do it, to prevent freezing. Before this type of fuel system they used heated intake air systems for carburetors and TBI. – Moab Apr 26 '16 at 22:30

My 94 Acura Integra (should be the same on most Hondas of the time) has a Fast Idle Thermo Valve (FITV) and an Idle Air Control Valve (IACV). The FITV has coolant flowing through it and opens/closes an air valve based on coolant temperature. It is completely mechanical. As the name implies, the FITV sets the high idle when the engine is cold, similar to the choke on carbureted vehicles. The IACV could handle the job of the FITV, so it is redundant to have both.

Having a hot intake allows fuel to vaporize better, which allows fuel to burn more efficiently. Most intakes have coolant running through them.

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