Taking an idle from wild to mild

Last week a customer brought in her 2002 Mazda Protege5. The car came in for basic maintenance (85,000-mile service), but the customer noted that the vehicle suffered a rough idle at stoplights when warm.

I told my technician to perform the regular maintenance and test-drive the car to check the idle. After driving it for approximately 15 minutes, we could not confirm the customer complaint.

A common problem on this Mazda engine is that the main air intake boot from the mass airflow sensor to the throttle bodies may crack. When the car comes to a stop, the extra air from the boot may cause the idle to drop.

But in this case, the boot was in great shape. We also checked the throttle plate while the air boot was off. There was a little bit of carbon buildup, but nothing that would affect the idle. We cleaned the throttle and put the boot back on. We connected the car to the scanner to check for engine codes – there weren’t any. We checked engine data, but everything was in specification.

REPLACING THE MOUNTS

The one thing we did find: The rear motor mount was broken. We started to think that the customer was feeling a vibration at idle and may have mistaken that as a rough idle.

Most cars today have automatic transmissions so that when the car comes to a stop, the engine and transmission remain partially engaged. If a motor mount is broken, the powertrain (engine and transmission) will vibrate a little more than normal. But this Mazda has a manual transmission, and every time we came to a stop, the car went into neutral. It is difficult to feel powertrain vibration when the car is in neutral.

This Mazda had three motor mounts and one transmission mount. The front and rear motor mounts dampen forward and backward torquing action. The left and right (side) mounts hold the weight of the engine. If a front or rear motor mount breaks or wears, the engine tends to move a little more than normal. If the side mounts break or wear, the engine tends to vibrate.

We then took a closer look at the other three mounts. On inspection we found them worn and sagging. The side mounts seemed especially bottomed out. We called the customer and suggested that we replace all four mounts.

It is difficult to see cracks in the side mounts because the weight of the engine presses down on them. As we removed each mount, we found that each had sagged at least three-quarters of an inch or was broken. Once we installed the four new mounts, the powertrain was seated higher in the engine bay. The average life for motor mounts is approximately 80,000-100,000 miles, yet adverse driving conditions and age can wear them out much faster.

One important side note: Use only factory original equipment supplied mounts. Aftermarket mounts don’t offer the same quality of rubber and don’t dampen the way the cars were designed.

We concluded that this was more of a vibration problem – not an idle problem. We performed a longer-than-normal test-drive to make sure that everything was OK, and the Mazda idled smoothly.