I have talked about one of many areas of the modern automobile I am very passionate about: the reprogramming of vehicle computers or modules. I have also discussed some of the possible causes of an illuminated check engine light.
Today, I will talk about the other side of computer programming... a fault of a mechanical kind.
When your vehicle comes limping into a shop and the check engine light is on, you are no doubt worried of what may be the cause of the problem.
Of the many vehicles I test and diagnose during the week, one vehicle usually crosses my path with this type of problem. Once the basic stuff is completed and ruled out, like a visual inspection, technical service bulletin updates and battery voltage checks, the fun begins.
I hook up an enhanced scan tool to extract the codes and other pertinent freeze frame data, which is a recording of engine and transmission data information as to when the check engine light came on.
Please keep in mind that there are literary hundreds of diagnostic trouble codes (DTCs) that could pop up for any number of reasons. This is just one example:
The vehicle was 2003 Buick Century with a 3.1L engine. The customer complained of rough running condition, misfires and (of course) a check engine with a DTC P0303 code. This indicates the powertrain control module sees a misfire of some sort on Cylinder No. 3. I decided to do a relative compression test using a Snap-On MODIS automotive lab scope. A relative compression test is an electrical current load test and is another way to test for engine mechanical compression. Why this test and not a regulator mechanical test? For me, it’s a much quicker way to determine the integrity of the engine condition. It was determined that there was a mechanical problem, rather than an injector, ignition or wiring problem of No. 3 cylinder misfire. Once the customer was informed and agreed to go ahead and remove the valve cover to check the valve train for any broken, damaged or missing pieces, I found the No.3 intake rocker arm mounting stud had stripped the threads out of the aluminum head. This was not allowing the intake valve to open and close, causing the misfire and rough running condition.
During any misfire event, it does not matter what make, model or year vehicle you drive, the first step is to determine if it is a mechanical fault. If that's not verified first, then it could be long and costly day for you.
Always confirm the problem.
