Recent purchase of a 1994 Honda Civic LX. Generally runs fine, but idle is erratic at times. Usually idles around 2,100 rpm and then at times does the 400-2,500 rpm swing. Saw some posts on here recommending to clean out the IACV, but the previous owner installed a new one before I bought it. What's next?

Second, the car starts great in the morning, but if I shut it down for a few minutes I can't restart it. I have to wait about 10 min or more and then it starts slowly--catches a few times and then finally starts. Related? Thanks!

 

The cooling system needs to be full so that the ECU properly detects the engine temperature. It is designed to idle fast when first started cold then slow down as the engine warms up. This also means that if the thermostat is stuck open and the engine never reaches proper operating temperature, it will continue to idle fast.

Check that the throttle cable goes slack and the throttle plate is against the stop when no one is pressing the gas.

Take off the big hose from the air cleaner at the throttle body, and note that there are 3 holes in the side of the throttle body before the throttle plate. They are there to bypass air into the engine and keep it idling. The bottom one goes to a device on the bottom of the throttle body called the Fast Idle Thermal Valve. The valve is supposed to be closed when the engine is warm, and open only for the first start in cold weather.

The engine will run with the hose disconnected. Start it up and then block each of the three holes with your finger. As noted above, if the engine is warm, the FITV should be closed and the bottom hole should not be drawing any air. Stuck open FITV would need to be replaced, though in warm climates it is not really necessary-- some people just remove it and fabricate a metal plate to block off the air holes.

The middle hole goes to the IACV. If you unplug the wires from the IACV it is supposed to spring shut, which will cause the engine to slow down and the middle hole to stop drawing air. If the middle hole is drawing air with the IACV unplugged, the IACV is stuck open.

The top hole goes to a screw valve that is manually adjustable to set up the idle range (though the exact idle speed is controlled through feedback in the ECU that is not adjustable). With the IACV unplugged and the engine warm, adjust the screw valve so the engine keeps running, but very slowly (450 rpm or so). Normal idling should then resume when the IACV is plugged back in. The screw valve looks like a big aluminum screw in the top of the throttle body that can be turned with a flat screwdriver.

Do not adjust the throttle plate stop screw attempting to set idle speed. It needs to be set so the throttle plate closes completely but does not slam against the sides of the throttle body. This is normally done once at the factory and that screw never needs further adjustment unless someone has messed with it. Idle operation is controlled with the air bypass system, not the throttle plate.


Not starting when warm is often the main relay. It was a lemon part that often fails in that way, but the problem can be repaired. See the "electrical DIY" section of this site for an excellent writeup of how to repair the relay. Everyone who has a 1988-1995 Civic, the years which which had the problem relays, should do this as a preventative measure even though it may not solve your problem.

 

iacv would be my guess maybe it was not installed properly or they fiddled with adjustments.

 

Thanks for the tips. I checked the air inlets and the high idle was due to the throttle not returning 100% to the closed position--I just had to give a little more slack on the cable.

However, I removed the main relay and checked the soldering. It looked fine. I guess the internal parts may be bad and the whole relay should be replaced. Looks like about $35 on eBay for a used one.

As far as the oscillating idle goes, I checked the new IACV and it looks good. Circuity checked out when I unplugged it, and the car died. So pretty sure it's working properly. Do you recommend warming up the car more and checking the FITV?

 

If it stalls when you unplug the IACV, you may need to open the screw valve some. But this test should be done with the engine fully warmed up.

Also check that the cooling system is full. With the engine cold take the radiator cap off and make sure the radiator is completely full to the top.