Unlocking the Hood: An Engine Troubleshooting Guide

Higher gas prices, increasing repair costs and required emissions testing are important reasons for today's vehicle owners to test and tune their cars to keep them running smoothly. Computerized cars may seem scary to first-time D-I-Y'ers, but with a little basic knowledge you can learn that troubleshooting and tune-up is easy and fun! This article is designed to teach you about the new engine technology and the new test equipment for troubleshooting computer-controlled cars. Armed with this knowledge, you can help yourself "unlock the hood" of newer cars and save money on vehicle repair costs. This article is divided into 6 sections:

 

 

 

Over the past 15-20 years mechanical engine controls have been replaced by vacuum, electronic and most importantly, computer controls. It is normal to be uncertain about working on a computer-controlled engine at first, because it is new and different. But with just a little time, education and experience, "high tech" becomes easy and safe. Remember how quickly we learned the new technology of VCRs, microwave ovens and personal computers? 

 

 

Computer-controlled engines and older engines have one thing in common: they both experience breakdowns that can be tested and repaired with simple troubleshooting steps. 

In the early '80's, car manufacturers added computer controls to vehicle engines in order to meet Federal Government economy and emission standards. A computer-controlled engine is basically the same as earlier types. It is still an internal combustion engine with pistons, plugs, valves and cams along with associated ignition, charging, starting and exhaust systems. Only the engine control systems have changed, and they have become computer-controlled. If the vehicle problems do not relate to the computer-controlled areas previously mentioned, you troubleshoot and fix them the same way you always have! 

 

Problems in the computer-controlled system are usually due to bad sensors (which tell the computer what is happening to the engine), bad actuators (the computer-controlled devices that affect engine operation, such as fuel injector) or bad wiring/poor connectors. The computer itself rarely goes bad. Actron's owner manuals are a good reference source, as they are fully illustrated and easy to understand. They clearly show how to test and tune today's cars using Actron test equipment. You can refer to these manuals for specific applications and test and replace many of these components yourself with the proper Actron equipment -- saving yourself time and money!

 

 

If your car was manufactured after 1981, chances are good it has an on-board computer and requires one or more of the Actron tools for computer-controlled vehicles.

 

Some of the benefits of do-it-yourself maintenance: 

 

 

The engine computer can do special tests to diagnose vehicle problems prior to making DIY repairs or taking the car into a professional shop. Test results are reported as numerical "trouble codes". Use a Code Scanner or Scan Tool to obtain trouble codes quickly and easily. The owner’s manual for the manufacturer specific code scanners (GM, Ford and imports) includes detailed test instructions and over 300 trouble code definitions. The Autoscanners include a CD-ROM with over 5,000 code definitions.

 

 

 

Computer-controlled engines require good, working sensors. Severe performance problems can result when sensors fail. Check sensors for proper operation. 

 

Critical MAP (Manifold Absolute Pressure), MAF (Mass AirFlow) and VAF (Vane AirFlow) sensors must be kept in reliable working condition.

 

Faulty ignition modules can cause no-start problems. Check modules for proper operation.

 

 

Before we describe the tests for computer sensors, it is vital to remember that many important checks can still be made the old familiar way. The basic engine has not changed and the computer does not control everything! 

 

Use a compression gauge to test for proper compression in each cylinder. Improper compression can cause rough idling or running, backfiring and poor fuel economy. Can also be used to test valve problems, piston rings and timing chain.

 

Use a vacuum gauge to test for proper intake manifold vacuum at idle. Improper vacuum can mean problems with ignition timing, piston rings, valves, valve timing, valve springs, valve guides, manifold leaks and some carburetor adjustments. Even small vacuum leaks can cause hard starting, stalls, and poor engine performance. 

 

Correct timing is essential to maintain top performance. Timing refers to when the engine tells each spark plug to fire. A chart under the hood of most cars shows what the timing should be at various RPM levels. Use a timing light to set the timing on any engine (computer-controlled or not) that has a conventional distributor ignition system. Timing cannot be manually adjusted on cars with distributorless ignition systems, but it is important to check computer timing to see if it might be the cause of engine problems. Use an advance timing light such as the Timing Advance Analyzer CP7519 to check whether the computer is "advancing the spark" according to spec as engine speed increases. 

 

Leaky vacuum lines can cause problems for any engine. A vacuum pump can be used to check vacuum lines as well as vacuum-operated solenoids and switches. 

 

Start, crank or "bump" engine while under hood. Useful for compression tests, checking camshaft or crankshaft sensors and adjustment work on valves, distributor or camshaft. 

 

Low batteries can cause intermittent problems well before the battery completely dies. Battery voltage should be at least 12.4 volts. Use a voltmeter or battery load tester to check. 

 

The cranking system requires at least 8-1/2 to 9 volts to start the engine. Use a voltmeter to test cranking system voltage. 

 

Use a voltmeter to test lighting systems. 

 

 

F) Unlocking the Hood: An Engine Troubleshooting Guide