Well, desperate times call for desperate measures. The only way for it to idle too fast is for too much air to be getting into the engine. That sounds simple enough, but we can split it into two systems. The air could be sneaking in someplace that it does not have to go through the Mass Air Flow sensor, or it DOES have to go through the MAF sensor. If it does go through the sensor, it will be measured and the Engine Computer will turn the injectors on long enough to match the right amount of fuel to the air. That will result in the engine having normal power and emissions, just too much of a good thing.
If the air is sneaking in undetected and unmeasured by the MAF sensor, the computer won't know it's there and won't command the extra fuel to go with it. Even though the freewheeling engine speed will be high, there won't be any power there to do work. If you drive it like that, it might perform acceptably on the highway, but not outstanding. If a vacuum leak is near only one cylinder, (think intake manifold gasket), the unburned oxygen will be detected by the oxygen sensor in the exhaust pipe, and it will tell the Engine Computer the entire engine is running too lean. Since it can't differentiate individual cylinders, the computer will increase fuel delivery to all cylinders equally. Three cylinders will be running way too rich but there will still always be that unburned oxygen. You'll have poor fuel mileage and high emissions.
Unmeasured air would be the vacuum leak. Due to the car's age, cracked or brittle vacuum hoses would be the logical first choice, but a broken-out chunk of intake manifold gasket could be the cause too. If it is related to a hose, you can pinch them off at the engine to see if one brings the idle speed down. In particular, look at the large hose going to the power brake booster. Hard plastic hoses will have to be disconnected and the ports on the engine plugged.
Another hint of a vacuum leak is if engine speed goes up even more when you introduce additional fuel to go with the unmeasured air. Use a propane torch to inject the unburned fuel into a vacuum hose that is otherwise sealed from outside air. Some people just spray it in at the air filter, but the MAF sensor will measure that as more air and make the injectors spray more fuel. Of course engine speed will increase then, so that's not an accurate way of doing it. Also, most MAF sensors heat up a temperature-dependent resistor, then watch how much the air flow cools it to determine the mass of the incoming air. Propane gets cold as it vaporizes so that too will confuse the sensor and make its measurements meaningless. If you can inject the propane someplace after the MAF sensor, the test will be more valid.
If adding propane or pinching hoses have no affect, suspect the extra air is going through the MAF sensor correctly. There are two ways for the air to enter the intake manifold; through the throttle blade that you control with the gas pedal, and a bypass passage around the throttle blade. That passage is opened and closed through a valve by the Engine Computer. As it opens the passage, it also pulses the injectors for a few extra milliseconds. That's how it controls idle speed. If the valve is opening the air passage too much, the question is, ... Is it due to being stuck / dirty/ broken, or is it being commanded to open by the computer? If it is stuck on carbon buildup and the computer thinks it commanded it closed, it won't be adding the extra fuel so you'll have the same low performance a vacuum leak would cause. If the computer is opening the valve, it's in response to something and it will command the corresponding amount of fuel to go with the air, so engine performance will be normal.
At this point it will be necessary to see a mechanic with a hand-held computer, called a scanner, that will show sensor inputs and computer outputs. He will be able to look at things that tell the Engine Computer how high to raise idle speed, in particular, the coolant temperature sensor and the MAF sensor. It will also show him how far the computer wants the idle air valve open. If it's very little, the computer is trying to lower engine speed but doesn't have control.
Saturday, March 20th, 2010 AT 7:46 PM