Note: Allpar does not take responsibility for the veracity of any information or opinions here, does not claim expertise, and is not responsible for any consequences. Please proceed at your own risk.
by Bob O’Neill
A drivability issue is defined as high or low idle issue, which includes the idle speed dropping when coming to a stop or slowing, erratic idle, codes presenting such as those related to injectors, O2, TPS, AIS, or coolant/air charge. These codes may present with or without the power limited light showing.
To troubleshoot drivability issues like those above or low fuel economy take the following steps first. Do this before replacing parts.
First pull the codes. For many of the cars built from the early ’80s through the mid ’90s, do the “key dance.” This means that you turn the key on then off in the following sequence. Don’t start the car, turn it only to the on position. The sequence is on - off - on - off - on. Then watch the power limited (or check engine) light. It will start to flash. The codes will be two digits.
For example, if the Power Limited Light (or engine light) flashes once and then a short pause then again, that’s a 2. Then if after a long pause there are three more flashes with short pauses between put the two sets together. This would be a code 23. Check out Allpar’s code ‘decoder’ for an explanation of what each code means. http://www.allpar.com/fix/codes/
Once you have identified which code has been presented, it’s time to start the diagnosis. This process begins with a basic step. Since these cars are all computer controlled the voltage at the computer from the battery is critical. Low voltage can be attributed to several things. First, suspect the battery itself. Be sure it is good and able to give the system the proper voltage. If the battery is good that doesn’t mean that the proper voltage is getting to the battery.
If the wiring between the battery and the computer has aged it may have lost some of its ability to transfer that voltage. So, check the battery terminals for corrosion. Remove the terminals and clean the post as well as the terminals where they touch post. Then on the other end of the ground side it bolts to the engine. Check that point to be sure that the wire is solidly attached to the terminal and solidly attached to the engine.
Check the wiring of the harness under the hood. As our cars age the wire’s insulation also ages. The heat of the engine tends to harden the insulation. Then with the vibrations this insulation can flake off. If the insulation does flake off it exposes the bare wire which can connect to other bare wires or short to the engine and chassis. If you find this condition you need to replace the area or length of wire that is bare. This can be done by splicing a new wire to replace the section of wire that has no insulation.
Next check EVERY connection starting with the ones closest to the battery and continuing to each connector under the hood. Disconnect each connector and inspect it for corrosion and to be sure that each connection can make solidly. Before reconnecting these clean them thoroughly and use some dielectric grease to prevent moisture from getting to the contacts in the connector.
Next check the vacuum hoses and tubes.
The heat affects the vacuum hoses and tubes the same way as the wires. Over time, the heat of the engine hardens vacuum lines, which can then crack. Check every hose and tube for cracks, spits and loose connections. If you find a problem correct it. If you do locate one hard rubber hose it may be time to replace all the rubber hoses. Be sure to check the connectors too. These are made of plastic and also can crack. Even if they don’t break the cracks can still cause leaks. These should be replaced but be careful. Some of them have what is referred to as an “orifice.” An orifice is a very small passage for vacuum which is in line with a larger tube or hose. For the exact size of the orifices check the Factory Service Manual (FSM) for your engine. But usually they are about .020” to .030” inches in size.
With the battery, wiring, connectors and vacuum lines to all be known to be good there is one last step. The connectors at the computer should be inspected and cleaned. Remove one at a time from the computer and clean the connectors. It is a good idea to use some contact cleaner made for electronic connections.
Once all the diagnosis has been completed then determine if the problem still presents. If so then you can be confident that replacing a part should solve the problem. Check the codes again to help identify the component which is faulty.
Other repair tips
| More idle speed / stalling diagnostics
| Codes | Sensors and such
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