1995 Toyota Corolla Gas Mileage Problem And The Idle Air Control Valve

1995 Toyota Corolla Gas Mileage Problem And The Idle Air Control Valve

I have a 1995 Toyota Corolla that normally averages 38 miles per gallon. I usually keep track of the mileage at each fill-up and noticed the mileage dropping slightly each week. I really didn’t think anything of it at first I thought it could be the weather or possibly the gas itself. But the problem persisted until the car was getting only 30 miles per gallon. I knew something must be wrong so I took it to Toyota to have them check it out. They found nothing wrong either electronically or mechanically. The car only has 80,000 miles on it so I figured that it couldn’t be wear causing any problems not yet at least. 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.

During closer observation of the engine I noticed that it idled fast as though it was cold after I had been driving it for several miles and the temperature gauge was in the normal range. In the same conditions a few miles down the road the idle was normal then after that it went to fast idle again. I went to the dealer again and asked them what it could be and they said that 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 (IAC) idle air control valve that let air bypass the throttle body when the engine was cold to allow for a faster idle. Once the engine was up to temperature the valve would close and the idle would return to normal. I checked and changed the easy things first, PCV Valve, air cleaner, etc. to no avail.

I decided that I was going to have to remove the Idle Air Control valve and check its performance. It resides directly below the throttle body and has coolant lines running through its housing to provide water from the cooling system to the thermally activated motor that turns the Idle Air Control Valve on and off. I was working on the engine when it was cold so not too much water leaked out. When I got to the valve shaft I noticed some corrosion on it and cleaned that off, otherwise it was operation normally.

After putting the IAC valve back together I drove the car to see if there was any change and there was not. It still idled erratically. As the engine was warming up, however, I noticed that the temperature gauge went way down when the thermostat opened up them gradually returned to its normal range. That seemed odd so I got a temperature probe and dipped it into the coolant to see exactly where it was and found that it was approximately ten degrees cooler than it should have been. At that point I just let the engine idle for fifteen minutes and the idle went to normal and the coolant temperature went to its operating range. That told me that the problem was the thermostat. I replaced the thermostat with a genuine Toyota part even though it is more expensive it is worth the money because it is exactly like the old one and the gasket pattern is the same. To replace it with an after market thermostat would require a different gasket type and I didn’t want to take the chance of causing another problem just to save five bucks on a thermostat.

The gas mileage returned to its normal 38 miles per gallon and the idle slows as soon as the engine runs for around ten minutes and the temperature gauge is steady. It’s as good as new or maybe even better.