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Engine Tune-Up 2003
Another common problem is the Coolant Temperature Sensor (CTS). With the engine cold use the scan tool to see what temperature it is reporting. It should be within four or five degrees of ambient air temperature. Now start the engine and watch the temperature reading. It should climb smoothly and steadily up to normal engine operating temperature, usually in the 190° to 230° range.


The Manifold Absolute Pressure (MAP) sensor would be the next thing to look at. Disconnect the vacuum line from the MAP sensor and hook up a hand vacuum pump. Now slowly apply vacuum to the MAP sensor.

? When manifold pressure is low (high vacuum), sensor output voltage is low, under 1 volt.
? When manifold pressure is high (low vacuum), sensor output voltage is high, 4.5 volts.

Again, it should show a smooth, steady increase in the voltage readings.

Okay, now you've checked all these things and you still can't find a problem. The next thing to do is to locate and expose the PCM. In some vehicles it's located under the dashboard behind the glove compartment. In others it's under the hood. But most often it is located behind the right hand kick panel.

Once you have it out, and with someone slowly increasing engine rpm, tap the PCM and see if there is any effect on the engine. Also move the wires going into the PCM back and forth. If there is an effect on the engine, you have a bad wire and/or connection at the PCM.

Replacing Spark Plugs
Not too long ago the spark plugs were out where you could see them, even if they weren't easy to get to, you could still see them. Now they are recessed into cylinder heads or under covers. In some cases you could trace the ignition wires to the spark plug but many cars have "Coil On Plug" COP ignition coils and are almost always under a cover.

Regardless of the design, it is important to remember to note wire connections and routing. If not you may cross a wire or connection and cause more problems than you solve. If your vehicle has ignition wires, check them for heat cracks or oil soaking. If it is, it needs to be replaced. If it is a COP, it will be fairly expensive.

Never pull an ignition wire off by grabbing the wire itself. Grab the boot and twist it back and forth while pulling it off. This will prevent damaging the carbon core of most ignition wires.

With the spark plugs out, look at the porcelain insulator around the center electrode. If they have a coating of black carbon on them, it is an indication the air/fuel mixture is too rich and you may have a problem with a HO2S that is just good enough not to set a DTC. You can use your scan tool to determine this.

If it is only one plug that has this carbon coating, it is a good indication you have a leaking fuel injector.

When you go to install your new spark plugs, a thin coat of anti-seize will make taking them out next time a whole lot easier. And don't forget to torque the spark plugs to specifications and no tighter. Otherwise you may wind up stripping an aluminum cylinder head.

Exhaust Gas Recirculation
Exhaust Gas Recirculation (EGR) valves can be the cause of many driveability problems. They can stick open, not close completely or operate in a jerky manner. If this happens the engine may stall. It will also run rough; your gas mileage will decrease and have an adverse affect on emissions. And the chances of passing mandatory emissions inspection will be pretty small.

What the EGR valve does is channel a measured amount of exhaust gas back into the combustion chamber to lower combustion temperatures. Like hosing down a fire. What this accomplishes is to lower Nitrogen Oxide (NOx) emissions and can often increase fuel mileage.

As the throttle is applied and the engine speeds up, ported vacuum is applied to a vacuum diaphragm in the EGR valve through a connecting tube. When the vacuum reaches approximately three inches, the diaphragm moves upward against spring tension and is in the full-up position at approximately seven to eight inches of vacuum.

This diaphragm is connected by a shaft to a valve that closes off the exhaust gas port. As the diaphragm moves up, it opens the valve in the exhaust gas port and allows exhaust gas to be pulled into the intake manifold and enter the cylinders. The exhaust gas port must be closed during idle, as the mixing of exhaust gases with the fuel air mixture at this point would cause rough running.