Rough engine idle

Let's use a rough engine idle complaint as an example of exactly how basic engine diagnostic tests work to pinpoint your root cause.

As I have explained on the previous pages in this section Sometimes mechanics will overlook basic engine hard parts as a possible cause of the problem.

I also find myself doing the same exact thing and the following is a real car repair that brought me back to the basics.

I had a customer with a Mazda 626 and a four-cylinder engine with a very rough engine idle. The driver's complaint was actually a lack of power, and they did not seem concerned with the way he the engine was idling.

But obviously, the two complaints were linked together, because this engine was running on three out of four cylinders.

Rough engine idle diagnosis

Since there is an endless list of possible causes for these types of car problems, I decided to break out my vacuum gauge and run a engine vacuum test.

I connected the gauge to manifold vacuum and let the engine idle.

The needle on the gauge was constantly dropping from a normal reading to less than 10 inches of mercury.

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.

When I shorted out cylinders one through three there was a drop in RPMs of about 125. When I did the same to cylinder, number four there was absolutely no decrease in the idling RPMs.

Since cylinder number four is now the focus of my diagnostics, I decided to pull a spark plug out and see what it looked like.

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 in to my test results.

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 were normal.

In order to fix this rough engine idle replacement of the camshaft and lifters would be required. I hope you found this page helpful for your engine diagnosis problems.

You can see how knowledge of the most basic systems is required to interpret the data revealed by each individual engine diagnostic test.

Learn more about how the insides work on my page about automobile engines



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