In December 2013, my Opel Astra 1,4T was close to its 30 000 km service and still very new. I informed the sales representative about an intermittent noise that is coupled with a loss in power. It was first noticed in June 2013 but has become more regular over time. When the problem occurs, you hear three (not particularly loud) quick metallic “ticks” while feeling a power loss underfoot and then everything is back to normal again. The car does not switch off, nor do any of warning lights illuminate on the dashboard. The problem is most noticeable when the cruise control is activated since one’s foot is not on the accelerator. It generally happens after a good 15-minute drive when in sixth gear and around 90 km/h when reaching an incline.
I have logged over 50 instances (date/time/speed/gear) of this issue and have been able to recreate the problem with two different GMSA representatives from the Bellville branch in the vehicle. I left the vehicle with the branch (on three occasions) for them to test drive at their leisure but without success. In truth, if you don’t know what to listen for, it will probably go unnoticed unless it’s pointed out to you. GM stated that unless the problem can be replicated on demand, they are unable to assist. I would be most grateful for any suggestions.
DUNCAN SHERIDAN
Via email
Answer: We are impressed with your log sheet (emailed to us) detailing the occurrences of the problem and wish the dealership had the same enthusiasm to resolve the issue. Admittedly an intermittent fault is the most difficult type of automotive riddle to solve. It is a shot in the dark but it sounds like a combustion issue related to knock (auto ignition).
With modern-engine-control strategies, the spark-timing of petrol engines will be advanced to just before the onset of knock to deliver more power and result in higher efficiency during the combustion process. This also caters for different fuel octane ratings. When the engine encounters a knock event, the knock sensor (accelerometer) on the engine block will detect this and the ECU will immediately retard the ignition timing for engine protection purposes. This will obviously also result in an instantaneous drop in power. When there are no knock events occurring, the ECU will start to advance the ignition timing again.
It sounds like there is scenario when your vehicle’s engine suddenly runs into a knock event that is responsible for the metallic “ticks” you hear. Knock normally occurs during high in-cylinder pressures which tie up with driving up a hill in sixth gear. The ECU then retards the timing which results in the loss of power you feel. The situation will soon return to normal as the ignition timing is advanced again as explained.
Our proposal will be for the dealer to install a logger on your vehicle that can record main engine parameters including the ignition timing. If you can continue keeping logs then they should hopefully be able tie up the events with a sudden retardation in ignition timing (or there is a button to press on the logger to mark a specific event). If this is the case then the investigation can focus on why the engine suddenly experiences a knock event.
