I have a 2001 Mazda Protege LX 2.0 with 121,000 miles. All of the maintenance (and then some) has been done in a timely manner - I am extremely anal retentive with this car. Last week, I was driving along at low speeds, and the car was idling smoothly, as usual. All of a sudden, when I pulled up to the next light, it acted like it wanted to stall - the idle started bouncing all over, and eventually my CEL came on. The code was P0300, for a random misfire, but other than the weird idle, the car acted fine - smooth acceleration, high speed driving, no power loss - not what you’d expect with a misfire. I should add that the spark plugs, wires, and ignition coils were all replaced about 5,000 miles ago, as were the EGR valve, vacuum hoses, and intake manifold gasket.
My scan tool showed that everything looked normal, except for the ignition timing, which was bouncing around between 10 BTDC and -2 BTDC (I assumed this was the reason for the erratic idle). I hooked up a vacuum gauge, and while it was completely steady, it was about 4-5 mmHg lower than what it normally is (reading ~17, normal is 21 or 22). I also did a compression test, and was getting ~180 psi across all cylinders, as per normal.
Perplexed, I took the car to a well-respected garage in my town, which has fixed many a problem on my cars quickly and honestly. They spent an entire day with it, and couldn’t figure out what was wrong with it. Their only idea was that maybe I had gotten some bad gas. Since I was on 1/3 tank, I filled up to see if I could dilute what was already in there.
Yesterday, I was driving the car at 55mph on the highway (road construction, ugh!), when I hear a slight squeal/squeak. It is more metallic sounding than a belt, and then all of a sudden I heard a little rattle, almost as if something got spit out the tailpipe. I didn’t see anything in my rear view mirror.
Since the car was still driving smoothly, I waited until the next exit to pull off, and as I came to an idling stop, I noticed that my idle was now again very smooth. It had been getting progressively better over the previous few days (no more CEL), but it seemed now like it was back to normal.
Finally, I hooked my scan tool up to the car again today out of curiosity, and I noticed that while the ignition timing is now steady, it is reading steady at 0 degrees BTDC. I confirmed this with my timing light. The spec from my repair manual is 8-10 degrees BTDC, but since everyother sensor seems to be reading correctly, and the car is driving smoothly, I am at a loss for what to do. I also hooked uo my vacuum gauge again and it is reading about 19mmHg, which still seems slightly low. I know there is something wrong, but I don’t know where to start looking.