Rough engine idle

A rough engine idle or stalling is a common complaint but in some cases can be difficult to resolve. Let me use an idle complaint that had me on the ropes as an example. This is a good example of how basic engine diagnostic tests can work to pinpoint the root cause of the cars problem.

As explained on previous pages in this section, sometimes mechanics will overlook basic engine hard parts and systems as a possible cause of the trouble. We as technicians often gravitate to high tech computer systems and sensors when we are troubleshooting engine performance complaints. I find myself doing the same thing at times.

Here is a car repair that brought me back to basics. A customer with a Mazda 626 and a four-cylinder engine had a very rough engine idle. The complaint from the driver was a lack of power. They didn't seem concerned with the engine idling problem. But the two issues where linked together because this engine was running on three cylinders.

Rough engine idle diagnosis

This type of reading is an indication of a serious engine problem. Also, the rhythm of the dropping needle matched the rhythm of the engine idle. I decided to use a cylinder balance test to isolate which cylinder that was not firing.

To my surprise it looked normal or at least like the other ones. So next I decided to do a quick engine compression test on the number four cylinder. And again to my surprise, this number four cylinder held compression as good as the other cylinders.

Engine diagnostic tests result

I performed a cylinder leakage test, and this was kind of overlapping with the compression test. So I was not surprised that the cylinder held air pressure.

After I put some thought into my test results, I realized that the rough engine idle was because something was not happening on cylinder number four. And the Spark plug I pulled out was a clue because it was dry.

I removed the valve cover and found out exactly what was not happening. The intake valve on cylinder number four was not opening. An inspection of the vehicle's camshaft yielded that the intake lobe on the camshaft for the number four cylinder was completely wiped out.

The worn lobe only affects the opening of the valve and does not prevented it from sealing, which is why the compression and the cylinder leakage engine diagnostic tests appeared to be normal.

In order to fix this rough engine idle, replacement of the camshaft and lifters would be required. In the end the owner of the vehicle decided to trade this car against another used Mazda. I hope you found this page helpful for your engine diagnosis problems.

You can see how knowledge of the most basic systems is sometimes required to interpret the data revealed by each individual engine diagnostic test. If your interested in learning more about how the insides work this next page is about automobile engines

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