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I have been dealing with an elusive CE light and some shaking of my car for a while now.

For a while it was just shaking occasionally, I thought it was a moisture problem, but that doesn't seem to be the case. The car is a 2005 Mazda M3. I read that these cars sometimes come with a defective engine mount that is too soft, and this looked to be the case to me. The CE light was coming up as intake runner control and also an o2 sensor, but my mechanic didn't think these were the problems.

Recently it started some new behavior. Every morning for the past week or so, my car will stall once, then start fine the second time. Except this A.M. it started fine. The check engine light came on a few days ago, but has since went off. I realized that this started the morning after fueling up, or at least I think. I fueled up at a gas station that I don't usually go to but started going to recently for convenience purposes. My fuel consumption has been horrible, I'm already almost empty again. It's a fast track, which is a fairly large chain, so I thought I could trust their gas. Would bad gas cause the stalling or misfires?

If not, could a problem with the fuel system itself cause stalling or the shaking? The shaking isn't only when idling, it's when the car is warming up. Even when I'm in 4th gear it will happen as it's warming up.

How can I even find out what type of gas I should be using for this car (I got it recently, used)? I have been just using regular, could this cause it?

I can't get it into shop for another week

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    Your car is electronically controlled, and needs to be scanned- read electronically. Post the fault codes from a scan to enable informed help.
    – Allan Osborne
    Sep 25, 2014 at 18:33
  • I agree with Allan. You need to get this code scanned to start a better diagnosis. Another factor to consider is that temperatures are lower first thing in the morning, and some components are sensitive to temperature. I do agree that the fuel system sounds suspect, especially considering your poor fuel economy. Shaking could be due to some cylinders not firing, but once again you should check more things out.
    – James Palmer
    Mar 25, 2015 at 3:26

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I think this could definitely be cause by bad fuel. It would probably do good for you to put a bottle or two of gas treatment into your tank. Put it in and run the fuel down to near empty (not all the way, but run it down to about an 1/8th tank), then fill the tank full with gas from a known good source. You may ask how do you figure out which gas is good? Usually it comes from busy gas stations. When they are turning their fuel over quickly it doesn't have a chance to pick up a lot of water sitting in the tank.

As for what octane of fuel to run, a good rule of thumb is to run the lowest octane you can run without pining occurring. Usually, the only vehicles which require higher octane fuel are those which either turbo- or supercharged (like a lot of VW/Audis), or are specifically high performance vehicles (like a Corvette). I would venture to assume you Mazda3 should only be using 87 octane fuel.

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