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, 2016 at 16:31

3 Answers 3


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. Apr 26, 2016 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, 2016 at 22:30
  • This was my concern as well, My self living in Africa where it's not that cold, we are losing performance due to heat being put into the air? But that said at this stage its already in the system? Will this heat not maybe help the air better ignite? Jun 17, 2021 at 1:29
  • @Zapnologica - No. What happens is when the air is heated, it gets thinner, which means a less dense air charge into the engine. Less air (due to the less dense air charge) means less power. Jun 17, 2021 at 1:43
  • @Pᴀᴜʟsᴛᴇʀ2 but my thinking is that air is already in the system inside the pipes. how can the amount of air change just due to the heat? Jun 18, 2021 at 15:02

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.


I reviewed answers and i don't think they were enough, its a matter of heat exchange more than heating up freezing blade of throttle valve , how it done? I will tell you , now think as physics conception , coolant temperature is always stabilized at 80 to 110 Celsius , so when weather is cold it heats up the room around throttle body actuator but when car goes in crowded area in some hot countries you will have a higher rate of engine heating soak specially when A/C is always on which is harmful to actuator in your very expensive throttle body , in this case coolant is carrying the extra amounts of thermal energy back to expansion tank to recycle it later into radiator , it works as the same concept of transmission oil cooler ,( in high speeds it keeps recirculation with radiator to avoid freezing coolant using the heat of transmission fluid .

  • Welcome to Motor Vehicle Maintenance & Repair! I hate to tell you, the answer I put on this question is why it's there. If your intake air temperatures are reaching above 100°C, you've got some serious issues in your environment. Most vehicles (especially stock ones) pull air from outside of the vehicle. Outside air will never get to the temperatures the engine coolant is running at, so the only heat transfer which is going to happen in the TB is from the coolant into the air stream. Robert doesn't need this feature in Israel, so bypassing is a really good option. Nov 28, 2019 at 18:52

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