Compression

[Andy White]

Not sure if this is the correct area to post this, but here goes.

What are the symptoms of bad compression or if you have bad compression how does this effect the engine?

Topic

[Lorrin Barth]

[Andy White]

The video doesn’t answer the question I asked.

[jonathan wiseman]

Well honestly a way to find out is the hotter you vehicle gets the harder it is to start back up. Other then that the only thing i can tell you is to use a compression tester and just find out the quicknway if you have low compression or not

[Robert]

There are to many factors to really answer the question correctly….

Is the compression low becuuse bent or leaking valves?

Or maybe the compression is low because of a hole in the piston from running to lean?

I guess you could also have low compresson becasue the head gasket blew from over heating the car…

These are just a few of the causes for low compression each of which would be discovered differently and have different effects on the car. All of which will cause lower preformance, how bad the compression is will also change how noticeable the issue is.

Above poster is correct, throw a $10 gauge on and figure out if it is low then if it is figure out why. But there is no general thing that can tell you that you have bad compression other then actually testing it as the only general sign is lower preformance and well there are about 10,000 other things that can cause that as well.

[Lorrin Barth]

You asked two questions: symptons of bad compression and effect on engine.

In the video I hear Eric saying, “Engine can’t make power.” and “Rough idle” and “Engine doesn’t run correctly” and “Don’t have the power you thought you had.” and “Engine running rough.” So, I’m confused, what is it you want to know?

[Erin]

Bad compression (if bad enough) does cause the engine to run rather rough. Like, shake the whole car rough at any speed. That is, after you struggle to get it to start.
Reason is – if the fuel/air is not compressed, that means a lot of it leaked out during the upstroke of the piston.

Compressions testers are not expensive. They are not a comprehensive test of the engine’s health but give you a quick idea if the engine is mostly holding pressure (anywhere from say 100 to 160 psi, depending on the source you read). Basically a “good enough” test for the home mechanic. If you get something stupid low like 30 or 0, you got real problems.

When you use one though, sometimes there is an adapter that comes with it that you might have to put on the end of the hose. The fun part is that this extra adapter sometimes gets stuck in the spark plug bore so maybe put a drop of oil on the threads before installing into the spark plug bore. Do not oil the inside threads of the adapter though, just the outside ones. AND – put the adapter on the hose BEFORE putting the whole assembly into the bore.

[george gonzalez]

If you have bad compression you can tell– just pull off the PCV valve off the valve cover. If air or smoke comes out the valve cover, more than a hamster’s breath after a run on the hamster wheel, then you have bad compression. If you can floor the gas pedal and the kids on skateboards are passing you, you may have bad compression. If you’re getting 8 MPG, you may have bad compression.

This is written a little bit sarcastically, because often there’s not much you can do about it, short of a complete engine overhaul. If you’re fortunate it’s “just” due to leaking valves and all you need is a $600 head job. But more often it’s due to leaky rings, and that’s like $2,000 and usually not worth doing on an older car.

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