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A friend told me when buying a car compression tests are not necessary since if there were problems you would hear the engine misfire so just listening to engine is enough.

I'm currently investigating why my car is feeling a bit sluggish and I asked the mechanic if we should do a compression test and he said 'no, if you had problems you wouldn't really be able to drive the car and we'd hear it'.

So this leaves me wondering are compression tests necessary, if there are problems with compression wouldn't we just be able to hear it or wouldn't the car just be showing displaying symptoms?

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Compression tests certainly have their uses - I wouldn't go so far as to say they should be a routine part of the car buying process, unless you have reasons to suspect an issue or you're buying an RX-8.

A small loss of compression could easily be present without causing a misfire and could produce a feeling of sluggishness or being down on power.

The obvious limitation is that they don't tell you why a particular cylinder is down on power - although it speeds up the diagnostic process somewhat by narrowing down the options and in the case where it is just the one cylinder that has lost compression you can focus all your investigations on that one.

They are simple enough to do and the required kits aren't exactly expensive. If you're still concerned you could buy a tester kit and carry it out yourself.

I imagine buying your own kit would save you money vs. getting the mechanic to do one for you even if you only use the kit once!

  • they used to have vacuum, well depression, gauges really fitted to the inlet manifold - gave lots of useful information - as long as you knew what the readings meant... – Solar Mike Oct 31 '18 at 17:14
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Both are wrong.

One cylinder can be down on power without causing excessive noise or vibration, which is why, when you asked the question the other day, I pointed you towards a Morse test.

A Morse test enables a comparison between the cylinders in terms of the power each cylinder produces relative to the others. In extreme cases, such as a severe misfire, then removing the plug lead from that cylinder results in no change of rpm so showing which cylinder is not functioning...

There are many questions and answers on here covering compression tests which may well be worth your searching for and reading.

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A compression test is a useful diagnostic aid. It's definitely wrong that all compression-related issues manifest themselves as something that can be seen or heard. It's very possible to have low compression due to worn components (such as rings) that will reduce the power output of that one cylinder without being obvious during a drive.

By doing a compression test and seeing high, uniform compression you can be fairly sure that the mechanicals of the engine are healthy, and any drivability issues are being caused by something else (such as fuel/air or ignition).

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