1995 Toyota Corolla Idle Air Control Valve and Gas Mileage Problem

I have a 1995 Toyota Corolla that normally averages 38 mpg. I usually keep track of the mileage at every fill up and have noticed the mileage going down slightly each week. I didn’t really think anything at first, I thought it might be the weather or possibly the gas itself. But the problem persisted until the car was getting only 30 mpg. I knew something must be wrong so I took it to Toyota to have it checked out. They found nothing wrong either electronically or mechanically. The car only has 80,000 miles on it so I figured it couldn’t be wearing out causing any problems at least not yet. When I was in the car business, it was not uncommon to see a Toyota Corolla with 200,000 miles on it and still running strong.

Upon closer observation of the engine, I noticed that it ran fast as if it were cold after having driven it for several miles and the temp gauge was in the normal range. Under the same conditions, a few miles down the highway, the idle was normal, then went back to fast idle. I went to the dealer again and asked what it could be and they said they have never heard of that happening. I decided to investigate the problem myself to avoid expensive diagnostic costs. I checked a maintenance manual for the car and found that there was an Idle Air Control (IAC) valve that allowed air to bypass the throttle body when the engine was cold to allow for a faster idle. Once the engine warmed up, the valve would close and idle would return to normal. I checked and changed the easy stuff first, PCV valve, air filter, etc. in vain.

I decided that I was going to have to remove the idle air control valve and check its performance. It resides directly under the throttle body and has cooling lines running through its housing to provide water from the cooling system to the thermally activated engine that turns the idle air control valve on and off. I was working on the engine when it was cold so it wouldn’t leak too much water. When I got to the valve stem I noticed some corrosion and cleaned it up; otherwise it worked normally.

After putting the IAC valve back together I drove the car to see if there was any change and no. It was still erratically idle. However, as the engine warmed up, I noticed that the temp gauge dropped a lot as the thermostat opened and gradually returned to its normal range. It seemed strange so I took a temperature probe and plunged it into the coolant to see exactly where it was and found that it was about ten degrees colder than it should have been. At that point I just left the engine idling for fifteen minutes and the idle went back to normal and the coolant temp reached its operating range. That told me the problem was the thermostat. I replaced the thermostat with a genuine Toyota part, although it’s more expensive, it’s worth it because it’s exactly the same as the old one and the gasket pattern is the same. Replacing it with an aftermarket thermostat would require a different type of gasket and I didn’t want to risk causing another problem just to save five bucks on a thermostat.

Gas mileage is back to its normal 38 mpg and idle slows down as soon as the engine runs for about ten minutes and the temperature gauge is steady. It’s as good as new or maybe even better.