Rick's Free Auto Repair Advice

Surging idle on Honda vehicles

How to fix a surging idle on Honda vehicles

If you have a Surging idle on Honda, read this post. Most computer controlled vehicles rely on the coolant temperature sensor to tell the computer whether the engine is cold and needs a rich mixture. The computer then commands the idle air control valve to open to the correct setting. However, any vacuum leak in the system can screw this up. The extra air leans out the mixture and the engine starts to die. So the computer tries to compensate by adding fuel. That increases the idle speed. Knowing that the accelerator isn’t being applied (by reading the throttle position sensor) the computer sees a high idle as a problem and commands the idle air control valve to close. That makes the engine run worse. So the computer either opens the idle air control valve again or adds fuel. What you have is a computer that’s literally “chasing” a good idle speed. Techs call this a surge.

Honda uses most of those same components to determine idle in their vehicle. However, they use a fast idle valve that has engine coolant running through it. Inside the valve is a wax pellet (just like a thermostat) that opens and closes a vacuum line that allows additional air to bypass the closed throttle plate at cold idle. The fast idle valve has a reasonably high failure rate. When the valve fails, it causes idle surge.

Here’s how to check it out.

1) Start by making sure the cooling system is filled with coolant and there’s no air in the system. If it’s low on coolant, that can trick the fast idle valve.

2) Remove the air duct from the throttle body. Then start the engine and cover the throttle body with a solid object (something that won’t get sucked into the engine but will block airflow). The engine should die. If it doesn’t, you’ve got a vacuum leak that must be addressed before you proceed with the rest of the tests.

3) Once the engine coolant is above 90 degrees F, you can proceed to test the fast idle valve. Start the engine and remove the vacuum line on the lower portion of the throttle body. Check for vacuum. There should be NONE. If there’s vacuum on that line, check the coolant hoses at the fast idle valve. They should be HOT. If the lines are hot and you ARE getting vacuum on that removed vacuum line, you can suspect a bad fast idle valve. But continue with the tests before you replace it.

4) Make sure the base idle is set properly. Refer to the shop manual and set base idle according to the procedure.

5) Finally, check the idle air control valve. There’s a port in front of the throttle plate. That port feeds the idle air control valve and the base idle screw. If the electrical connector to the idle air control valve is removed and the base idle screw is turned all the way in (count the turns if you’ve already adjusted base idle so you can return the screw to the proper position) and there is STILL airflow through the upper port, then the idle air control valve is stuck OPEN.

If the idle air control valve checks out and the fast idle valve doesn’t, then replace the fast idle valve. It’s a little hard to get to, but replacing it is just a matter of loosening the screws, removing the coolant hoses, and replacing the valve. The Honda dealer will know right away what you’re asking for. They keep these in stock due to the high failure rate.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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