Misfire | 1998 Chevrolet Malibu | 3.1

Have you ever had a car come in with a driveability complaint and only a P0300 misfire code? I am sure you have.This case study demonstrates why it is good to learn what normal scan data pids should be on the vehicles you commonly service.

When this 98 Chevy Malibu came in, it was running very poorly. It was missing on all cylinders,the idle was hunting around, it was a hard start, and just plainly sounded horrible. The check engine light was on with only a P0300 (multiple cylinder misfire) code stored. I always scan through the pid data see see where all the data values are in my diagnostic procedure. By doing this you learn known good values and can easily spot problems. I almost always start with fuel trims since this will point to me if it is a rich or lean issue, in most cases.

This car wasn’t is closed loop yet, so fuel trims weren’t active so I continued thru the rest of the scan data pids. The data pids that caught my eye was for the EGR position. Commanded EGR position was 0% but actual EGR position showed 100%. Problem found, now I just need to verify if it is actually open 100% and if it is, I have to see if it is stuck or if the control wire or driver is shorted to ground. I could also have a faulty EGR position sensor. The EGR on this 3100 is easy to access, so I just pulled the EGR valve and took a look. This valve was stuck wide open from carbon deposits. This is a fairly common issue. The carbon deposits build up on the pintle shaft and then the pintle binds up in the EGR valve. If this was a failure where a piece of carbon held the pintle of its seat instead, I would have just cleaned it, in most cases. A new EGR valve and this car was on it’s way.

Why didn’t this set a code? I don’t know. I see EGR position code quite often but for some reason it did not set one on this car, for this particular failure. This shows you that you can’t always rely on the PCM to tell you what’s wrong by setting trouble codes, but instead you need to use the data that the PCM will give you.