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We own a Mazda Demio 2004 automatic, early this year when driving up hill after a code start the engine revs would suddenly go up and down result in the car move-stop move-stop, but if you lift the foot peddle and push it down again it would work as normal, and normal for the rest of the day. Talked with our regular mechanic, he suggested to keep driving as it's difficult to troubleshoot.

And then it started getting worse when winter started as it get colder, the problem start getting worse, right after a cold start in the morning the rev of the engine would go up and down up and down(500 rev to 1500 and then back and forth) for around half minutes or so, the worst time when the rev drops the engine could stall completely. However, after the car warmed up it drives normal for the whole day.

As it started stalling, I figure it's better to get it fixed. I sent it to the garage then left it there, it's about two weeks now as the problem only show up in early morning, and they have very limited time to do diagnostics , they found nothing with scanner, cleared the throttle and eventually replaced the throttle body and mass air sensor, but the problem still exists, they are suspecting fuel pressure but can't find a pressure gauge at the moment and still working on it, and I was told they are running out of ideas.

This is quite frustrating, not only worried about how much it would eventually cost us to fix the car, but also worried if it would ever get fixed at all. I don't know how uncommon such a problem is, but surely it has been seen before. I'm hoping AA expect could shed some light around it, what do you think the problem could be?

Thanks,

Hi there,
The symptoms you are explaining would seem to be the most common problem experienced and as most mechanics would conclude that it would be in the throttle body/cold start system. However as they have tested and replaced the unit (do you know if the unit is new or used?). A used part when chasing a potential issue will make it hard to trust. However these kind of problems really need concrete diagnosing before anything is replaced as it could turn into an expensive goose chase.

Hi Cade, not sure if the part is new or used, I will give them a ring today, just wanted to see if they missed anything obvious. Thank you for your help!