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Check Engine light is on, and mechanic's not helping

  • Article by: PAUL BRAND
  • September 14, 2012 - 4:04 PM
Q I have a problem with my 2005 Mazda 6. My "Check Engine" light came on so I went to the mechanic to have him pull the code. He said it wasn't really reading anything, reset the code and told me to come in if it happens again. The next day it came on again. I stopped at the gas station since I needed gas and filled my tank. When I went to start my car, it wouldn't start. I sat there for 15 minutes before it started. When it did start, I had to give it some gas because it was sputtering. The Check Engine light was still on so I brought the car to the mechanic the next day. He reset the code again and said to let him know if it came on again at the gas station next time.

I filled up about a week later and once again it wouldn't start. I told my mechanic this and he said to try to get gas with the car running next time. So once again I got gas but this time left my car running. This time, as I was pumping gas, the car was sputtering and the tachometer was fluttering the entire time. But when I was done pumping, it seemed to run just fine. My car acts up only when filling gas; otherwise it runs great.

A What's interesting about this question is that the mechanic is trying to pinpoint the problem by asking the car owner to do all the work! Try this, try that, etc. Since the Check Engine light has come on each time, a scan tool should identify the specific DTC fault code associated with this problem. The comment "wasn't really reading anything" doesn't make sense -- the Check Engine light was on!

I suspect the DTC will relate to the evaporative emissions system, which includes the purge valve. If the purge valve is stuck open, raw fuel/vapors can be pumped through the charcoal canister directly into the induction system during refueling, which, of course, would create exactly the symptoms you've described.

Q I have a 2004 5.7-liter Dodge Durango with 190,000 miles. As I come to a complete stop, the tachometer will drop momentarily from around 750 rpm to less than 500. The engine will occasionally stall. I must depress the gas pedal slightly to get it restarted. Initially I had the throttle body thoroughly cleaned, then replaced it this past weekend. I also replaced the oxygen sensors but nothing has helped. I should also mention that the Check Engine light has never come on. Any ideas?

A The 5.7-liter V8 in this vehicle is equipped with a "fly-by-wire" throttle system. Instead of a mechanical cable or linkage connecting the throttle pedal to the throttle body, an accelerator pedal position sensor (APPS) is fitted directly to the throttle pedal and electrically connected to the PCM (powertrain control module). In simple terms, rather than the throttle plate being opened and closed mechanically by movement of the throttle pedal, the position of the throttle pedal is communicated to the PCM, which operates the throttle via signal to the electronic throttle body.

Having both cleaned and replaced the throttle body, which includes the throttle and idle air control valve, I'd suggest plugging in a scan tool and having the APPS swept/checked for dead spots in its potentiometer. Also, check for any type of vacuum or air leak in the induction system that would allow un-metered air into system.

Q I had to replace the engine in my 2004 Chevy Aveo recently. Since I got it running again, the battery light on my dash is on constantly. I have had the alternator tested in and out of the car and still no answer.

A If the alternator is charging properly, look for a short to ground in the indicator light control circuit.

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