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As long as we're on the topic of engine mechanicals, let’s talk about the effects of a restricted exhaust. Imagine being able to breathe in but not out. It wouldn't be long before you pass out cold. The same thing can happen to your engine. If you have a restricted exhaust due to a catalytic converter failure, damage to the pipes themselves, or a restricted muffler, your engine will be sluggish and underpowered. Surprisingly, testing for this isn't that difficult. All you need is a trusty vacuum gauge. To use a vacuum gauge to find an exhaust restriction, you first need to find a place on your intake where you can hook the gauge up. You're looking for an intake vacuum source. This would be a source that has high vacuum at idle. Look for a line connecting somewhere after the throttle body. Pull it off while the engine is running. If you feel vacuum there, that's where you want to hook up. On a good-running engine, intake vacuum should be between 17 to 21 INHG. (That's inches of mercury for those of you who are curious.) Vacuum is also measured in Bar, but for this article we're going to use INHG.

The needle on the gauges should also hold steady and not bounce around. If it does, this might indicate a mechanical issue with the engine. Also, intake vacuum is highest at idle and drops off to zero the closer you get to wide open throttle (WOT). With the engine running, take your first reading. Raise the RPM to about 2000 or 2500 and watch the gauge. If you have an exhaust restriction, you'll begin to notice that vacuum will slowly drop off over time, and it might be harder to maintain the RPM. If the vacuum remains the same, then you don't have a restriction and you can move on to other tests. Here's a video showing the test in action.


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