How to Test a Fast Idle Thermo Valve

by Chris Stevenson
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The fast idle thermo valve (FITV), commonly used on many Honda cars, is a sensor that controls the fast idle or warmup circuit of the fuel system. The FITV contains a thermowax device that controls a plunger. When the engine is cold, the plunger contracts to allow additional air into the throttle body (intake manifold), creating a deliberate vacuum leak. The vacuum leak raises the engine rpm until operating temperature has been reached. When the thermowax heats up, it seals the plunger, cutting off the extra vacuum to the throttle bottle. A process of elimination is required to narrow down a faulty fast idle thermo valve.

Step 1

Place the vehicle in "Park" for an automatic, or "Neutral" for a standard transmission. Apply the emergency brake. Raise the hood of your vehicle and disconnect the snaps to the cold air intake box that sits over the throttle body. Use a screwdriver if the cold air box is fastened with screws. Pull the box and cold air hose away from the engine. to allow access to the throttle body.

Step 2

Locate the fast idle thermo valve near the throttle body or on the side of the intake manifold. Consult your service manual for its location, if you are unsure. The valve will have a cooling hose and a vacuum line running to it. Check the various vacuum lines that enter into the intake manifold, and at the throttle body location.

Step 3

Find the vacuum line routing schematic on your engine cover. It will show all of the lines and their locations. Check each vacuum line for burns, kinks or loose connections. Pull the PCV (positive crankcase ventilation) valve from the valve cover and shake it to make sure the check ball inside floats freely. Ensure that the vacuum line routed from the fast idle thermo valve to the throttle body has a good connection at both ends.

Step 4

Use a socket and wrench to tighten the intake manifold bolts, starting in the middle of the manifold, and working outward toward the ends. Get them good and snug, not overly torqued. Check the tightness of the throttle body base bolts, and tighten them if they seem loose. Look inside the throttle body opening -- the throttle plate should be completely closed. If it's open even slightly, use a socket to loosen the throttle cable bracket and move the cable one notch, to relax the cable and shut the throttle plate.

Step 5

Connect one lead of a tachometer to the negative (-) pole on the distributor coil, or on the coil pack (newer vehicles). Connect the other lead to a good engine ground source. Turn the tachometer knob selector to the appropriate number of cylinders on your engine. Start the engine and look at the rpm numbers. For a cold engine, the fast idle rpm should be anywhere from 1,500 to 2000 rpms, according to your service manual specifications. If it reads significantly below this number, with the choke functioning normally, the problem might be the fast idle thermo valve.

Step 6

Keep the tachometer leads connected and the engine running. Look inside the throttle body opening for two very small holes, or ports. The top port belongs to the idle air control valve (IACV). The small port below it belongs to the fast idle thermo valve. Place your finger over the bottom hole. If the engine is cold, you will feel a definite suction. If no suction occurs, the valve is not opening properly, and this could be due to a clogged-frozen plunger in the valve. The valve must be cleaned or replaced.

Step 7

Run the engine until it has reached normal operating temperature or when the radiator cooling fan activates. Place your finger over the fast idle thermo port again and feel for any vacuum suction. The engine idle should now read from 675 to 750 rpm, according to your tachometer. This means the fast idle thermo valve has closed, cutting off vacuum. If the idle remains very high after complete engine warmup, it indicates the thermo valve has remained stuck open. The valve will have to be replaced or cleaned.

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