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I recently added a cold air intake and cleaned the throttle body and IAC valve (I was having hunting idle issues) on my '01 Accord V6, and I noticed that there were coolant lines running through the IACV. Instead of reconnecting them, I bypassed the flow with a loop of transmission hose and instead routed the breather intake through it which now cools it instead of heats it. I did that because I noticed that the hunting idle only happened when I started the car warm.

Since I did all that, the idle is perfect and steady. I understand that heating the IACV is supposed to prevent icing that might cause the valve to stick, but I haven't had any issues with that. I've even run it on 20-30F mornings and the IACV, throttle body, and even the manifold will stay cool to the touch, but nowhere near cold enough to cause icing.

I guess my question is did I miss something big about the design of the IACV system or is heating it extraneous in my car?

  • It would help to know where you live… – dlu Nov 19 '16 at 16:32
  • @dlu central Pennsylvania. Temps range from as low as -12F in winter to 90F in summer. – tmac1 Nov 19 '16 at 16:36
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The use of the IAC and coolant running through it are just as you speculated: to prevent icing. In most places there really aren't any issues with the freezing issue. Even in cooler climates, there usually aren't any issues. As we all know, when there is a tremendous amount of air moving in/around/over a metal object, there is usually a cooling effect, especially if the metal in question is aluminum.

Diverting or bypassing the IAC's coolant flow can actually give a slight performance boost in some cases. While the coolant flow through the IAC can help prevent icing, it has a bad side effect after the engine is warmed up: it heats the incoming air flow, which degrades performance due to a less dense air mixture. Bypassing the coolant flow allows the incoming air to be closer to incoming air and therefore will be denser going into the engine and make more power. This usually isn't a lot of extra power. In vehicles with larger V8 engines, it can be upwards of 15 wheel horsepower. In a little I4, the gains will not be as noticeable.

For the most part, as long as you diverted the coolant flow and not plugged hoses, you should have no issues. In some applications, the coolant flow is crucial to maintain how the system works. If you plugged the hoses, this can cause a disruption in the normal flow, which may have side effects. As long as the coolant continues to flow through the hoses as it should, there won't be an issue with this.

  • Is there significant air flow through the IAC valve when the engine is developing power? – dlu Nov 19 '16 at 20:35
  • Off idle, air flow through the IAC is minimal (if any). The butterfly has to be closed in order for vacuum to form and air to flow through. – Pᴀᴜʟsᴛᴇʀ2 Nov 19 '16 at 20:43
  • @Pᴀᴜʟsᴛᴇʀ2 It's a 3.0L V6, so there might actually be a noticeable difference. That bad side effect you mentioned is part of why I bypassed it in the first place; the IM and throttle body would be so hot after running that I couldn't safely touch them (and the operating temperature is perfectly normal). – tmac1 Nov 20 '16 at 3:20
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I'd be surprised if the heating is extraneous in general – too much cost and engineering for a component without a purpose. But, you might be able to get away without the heating. It will depend on where you live, how you drive, the weather conditions, etc.

With a fuel injected engine (assuming it isn't throttle body injection) you don't have the problem of evaporative cooling due to fuel, but there will still be cooling due to the air pressure drop across the throttle plate. Icing can occur at temperatures well above freezing – especially when humidity is high.

Is there a problem you're trying to solve by removing the heating?

  • There was a fair amount of carbon in the IACV, but I was only getting the hunting idle issues when I started the car while it was already warm. I might change it somehow as the weather gets colder so it just warms the TB, but before even in cool weather it would heat the TB to the extent that it was too hot to touch, which seems a bit excessive. Also, what's so bad about the plate itself getting some frost on it? The hinges and surrounding TB stay relatively warm, so it shouldn't stick. – tmac1 Nov 19 '16 at 16:57
  • Ice on the plate will affect the size of the air passage, which depending on how the ECU is implemented might change the performance. Also, the whole area will cool and eventually the plate could get stuck. I'd expect it to be worse when closing the throttle at speed, say when coming down a hill. At worst it will shut the engine down or cause you to lose throttle control. – dlu Nov 19 '16 at 17:06

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