As techs, we prefer to deal with the car with a driveability issue attached, as opposed to the light being the only problem, but both scenarios should be approached in the same manner.
Attach your scanner and make note of the codes that are recovered as you develop the diagnostic strategy for this job. But don’t be too quick to clear the codes; the next step is to review the freeze-frame data to see what was going on when the code was set. This information will be more helpful with certain codes, but it’s a good habit to get into no matter what the situation.
First, we’ll look at some problems that have driveability complaints attached. The most common of these are misfire codes. If the misfire is obvious, you may be tempted to get to work and start switching coils, checking wires or whatever, looking to diagnose the problem. But before you do, check the codes and see if there isn’t more information available; if nothing else, the P0300 series of codes will identify the cylinder causing the problem.
But there are many other clues that may be available that will help in the diagnostic process. If you have a misfire code along with a system lean code, you should be thinking that the misfire could be a result of a manifold or other vacuum leak.
Even with no code, take a look at the data with an eye toward the fuel trim numbers: Do we have a high positive number on long-term trim, indicating that the system has been adding fuel? Or, is it the less common problem of a negative number that would have you looking at a rich condition, where an extreme case like an injector sticking open or a fuel pressure regulator leaking in the vacuum line could lead to the misfire?
Another common driveability problem that may or may not have an associated code is a stumble or cutting out on throttle tip in. If the check engine light is lit, hook up your scanner and retrieve the code; the most common will be a system lean code (for more information on engine airflow sensors and engine management systems, click here). While you have the scanner installed, take a couple of minutes to look at the other available parameters. Are the temp sensors reading correctly with a cold engine? The coolant and air temps should be close. Taking a look at the throttle position switch, paying particular attention to the off idle area, there have been some reports of problems with the switches that become apparent when the connector is “wiggled.” Don’t overlook mechanical concerns; be sure to check the air intake hoses for cracks that will open up when the engine moves with torque.
In the case of “no start” problems, it pays to know the service history. If the car has suffered a broken timing belt, it’s a good possibility that some debris from the broken belt has found its way to the crank sensor, causing damage to both the sensor and trigger wheel. On the 2.4L engines, the balance shaft belts have been known to fail and go unnoticed until the pieces finally take out the crank sensor, resulting in a “poor run” or “no start.”
REAL-WORLD EXAMPLE — ELANTRA GLS
We recently had a 2002 Elantra GLS in the shop that had assorted problems. Let’s take a look at this job, as it’s a good example of some common problems you might encounter. The car wasn’t in great shape and a persistent check engine light was preventing it from passing state inspection.
As is often the case with this kind of job, the customer was reluctant to make a big investment, but wasn’t in the position to buy a new car. The obvious problem was a code for a solid misfire in the number 3 (P0303) cylinder, as well as a code for a slow response from the front O2 (P0133) and a system lean code. In addition, there was a slight but audible exhaust leak at the flex pipe before the cat. The customer had already installed a new O2 sensor, but the code persisted.
We explained to the customer that there was no way we could give a firm price on the job, but received authorization to get started with the diagnosis and get further authorizations as the job progressed.
The first step was to deal with the misfire. Since we also had the system lean code, we knew the problem was with cylinder 3; we went right to the manifold in that area. A quick shot of intake cleaner at the manifold flange smoothed the engine out, letting us know we were on the right track. To confirm and pinpoint the problem, we used a stethoscope where the sound of the leak was obvious.