Mysterious Problem

It all started after an oil change at Jiffy Lube and the special fuel injector cleaning service they offer. I noticed the car ran a little rough and I wasn’t getting as good gas mileage as usual, but I thought that was due to the extremely high temperatures during August. The problem seemed to resolve when the weather cooled down, but then several weeks later I noticed a soft “clacking” noise when I accelerated quickly. I pointed this out to my husband. We drove the car around and really laid on the gas, but didn’t hear the noise. Thought everything was fine. He looked under the hood and found the air filter had not been properly seated after the oil change and air was being sucked into the car without being filtered. He fixed it. Then several days later the check engine light came on. No problems, just the light. Checked the codes. Basically came up with fuel mix lean. Brought the car to dealership. Said a vacuum hose came off and needed new belt tensioner. Aforementioned things done. Drove car home heard noise again, but no problems. Drove car to work next day. After 5 min it was running really rough, didn’t want to accelerate, engine light on, and seemed like it wanted to stall when stopped. Checked codes, same ones, but nothing that cited the source of the problem (none of them have). My car is a 2003 Mazda 6, just out of warranty. My husband thinks mass air sensor or maybe fuel injector cleaner ruined fuel injectors? Anyone have any ideas? Dealership wants to replace O2 sensor “cause it’s completely dead”, but husband says they all check out fine. Anyone have a clue about what this could be? My husband is great with cars but says he has never come across a problem like this before.

Fuel injector cleaner won’t hurt the injectors.

He may be right about the mass airflow sensor. And there are some other possibilities. Your engine’s computer figures out how much gas to spray into the incoming airstream (how long to keep the injectors open at each pulse) by taking signals from the mass airflow sensor, the manifold absolute pressure sensor, the upstream oxygen sensor, the engine speed sensor, the temp sensor, and the throttle position sensor and running those signals through a program, the output of which controls the injectors. The downstream oxygen sensor simply checks its signal against the upstream oxygen sensor to see if the cat converter is working.

Any of the above sensors being bad could cause lean running. Some, like the temp sensor and throttle position sensor, have typical symptoms not quite the same as yours, so they can be initially “set aside” as suspects. Others, like the MAP sensor, the MAF sensor, and the oxygen sensor, typically produce your symptoms and codes.

How did hubby check the oxygen sensor? If he did so by actually reading the output, then it sounds like he’s headed down the right path. He may also want to remember that a weak fuel pump, bad pressure regulator, or even clogged fuel filter can cause your problems and not forget to check the fuel pressure.

Post back.

Whoops, I missed the Jiffy Lube comment (thanks Grampy). While injector cleaner won’t hurt injectors, and the timing may have been a coincidence, they may have done something in their process to do so. Without knowing exactly what they did it’s hard to tell. After having this opinted out to me I’d be inclined to first check to see if the injectors are firing properly.

… And stay away from J.L. for obvious reasons

Bypassing the air filter will definitely get the mass flow sensor dirty. I think CRC makes a cleaner for it.

A jiffy is an actual measurement of time. It is a small fraction of a second. How small? I forgot, but it’s small enough.