I'm working on a motor that uses a vacuum and electrical harness from a 1986 Mercury Cougar 3.8L with EEC-IV. I have an unused vacuum line from a solenoid and I'm not sure where it goes. It's labeled as "Intake heat control valve (31)". In the description, it says it warms the heat riser in the intake manifold.

The motor does have a preheater, but it is controlled by a bimetal valve (23) attached to the main vacuum line. So I'm guessing it's a completely different system than that.

What exactly is this valve for, and what do they normally attach to? How is this system different than the other vacuum controlled preheater?

co hcv

2 Answers 2


It's a vacuum solenoid controlled "flap" that uses exhaust heat to keep the intake manifold warm in extreme conditions. I beleive it has a special mounting area on one of the exhaust manifolds. Should look something like this:

Mercury 3.8 Riser Valve

The EEC-IV commands the solenoid (based on what, I'm not sure, but probably coolant temperature and/or intake air temperature), and vacuum is applied to the "motor" (circular chamber with control rod). A diaphargm in the vacuum "motor" moves the control rod, opens the butterfly valve, and redirects exhaust heat to the intake manifold.

(There's a possibility I have the operation reversed: vacuum apply may close the valve, so that spring "open" is the failsafe starting position.)

This prevents an ice-cold manifold from doing the very thing that @dlu mentioned in his answer; namely, condensing out the atomization of the fuel and providing terrible air/fuel mixing.

  • So essentially, it serves the same purpose as the vacuum controlled preheater? Sep 21, 2016 at 19:59

Well if it is anything like an aircraft engine it may be there to prevent intake icing. Evaporating fuel soaks up a lot of heat and ice can form in the intake even on relatively warm days.

It might also help to keep fuel vaporized in very cold weather when the fuel vapors might condense on the manifold before the engine heats up.

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