Detective a cylinder with low compression

[AmadeusCharger500]

Car started running rough. No warning that I could remember. So I started pulling plugs and wires, one at a time. Found out number 3 cylinder is not working right, makes no difference with wire on or off. However the plug itself is firing.

Where does a person go from here to figure this out?

[A383Wing]

run compression test...my guess would be a vacuum leak or burned valve

[AmadeusCharger500]

how will the compression test help if I already know # 3 is bad? Just trying to understand

[ws23rt]

Could be a stuck valve. Pull the valve cover and look for a loose push rod.
Once you know why it is bad you will know what has to be done to fix it. These things seldom fix themselves. (although a stuck valve could).
Make sure the plug is not shorted/or otherwise bad. Simple things first.
You said the plug is firing? Or is the plug wire supplying fire?
If the compression is not low on that cylinder than it must be an issue with the plug firing.
#1 compression
#2 fuel
#3 spark ---If you have all of these you will get an explosion in there.
We know you have fuel because the rest of the engine runs.

[AmadeusCharger500]

Plug wire is firing, I got a nice shock from it, but I also checked to see the spark at the plug is good. Will pull valve cover like you suggest, tomorrow since the light is fading.

[A383Wing]

wait...you said plug is firing because you got a shock from wire??

that tells me you got fire to the plug, not that the plug is working...pull the plug, either post a pic of it, swap it for one that works, or just put a new one in that cylinder....you could have a fouled plug, don't take off valve cover before you check the obvious

Bryan

[ws23rt]

wait...you said plug is firing because you got a shock from wire??

that tells me you got fire to the plug, not that the plug is working...pull the plug, either post a pic of it, swap it for one that works, or just put a new one in that cylinder....you could have a fouled plug, don't take off valve cover before you check the obvious

Bryan

I was just going to say that

Bryan we should get paid for this stuff. Shops charge for it
Just kidding. Shops have a different bottom line than we do.
Bruce

[ottawamerc]

Is the plug fouled , wet any indication that something is wrong with it? I would next do a compression test before pulling anything apart IMHO

Scott

[A383Wing]

Bryan we should get paid for this stuff. Shops charge for it
Just kidding. Shops have a different bottom line than we do.
Bruce

not a bad idea....I got too much overhead and not enough money coming in right now... 

[heyoldguy]

how will the compression test help if I already know # 3 is bad? Just trying to understand

All you know that it doesn't make a difference if you pull the plug wire off #3, you don't know why it doesn't make a difference. By doing a compression test, you can determine if the mechanical portion of that cylinder is okay. Then a leak down test will tell you if the rings, intake or exhaust valves are good or bad. If the compression in #3 is okay, then you have an electrical or fuel problem.

[Dmichels]

A plug may fire out in the open but when it is under compression it may not. A bad carbon core wires may still show a spark to ground but will not have enough kick to fire a plug under load. As everyone said above check your compression. You will find out much info. Is the cylinder low? is a valve stuck open? are 2 adjacent cylinders showing low? (bad head gasket) Change out the plug and wire for a know good one. Also check the dizzy cap, you could have a carbon track not too common but I have seen it. You need to rule out the easy stuff first.
Dave