Dead Cylinder / Bad Spark Plug

[ChrisMinelli]

On an older car, how do you know if you have a dead cylinder or a spark plug that isn't firing?  On a modern car the computer tells me (yeah I know...)  I ask because it occurred to me I may be missing something important and not even know it.  I assume it would run rough enough for me to know something is amiss?

[JBNeal]

The quick way is to pull spark plug wires off (1 at a time) of the spark plugs while the engine is running...if the engine has no change in how it runs on a cylinder, then that cylinder is not getting any fire.  To verify, turn off the engine, remove the suspected spark plug, and with the plug wire attached to the spark plug, touch the spark plug to the head to ground the spark circuit while cranking and watch for a bright white spark at the electrode.  This method I used recently on a JD 425 to find one of the 2 older spark plugs had a yellowish spark, so I replaced both and the new plugs had the bright white spark...engine startups were faster and the engine could pull a steep hill with ease, something that it had labored to do as mowing season neared an end.

 

But be sure ya have good insulation on the plug wires or you'll be lighting up like them spark plugs...a coworker bought a used Grand Prix that I told him wasn't running right, so we went out in the parking lot at lunch for me to show him this trick, but by pulling wires off of the individual ignition coils as they were much easier to reach...and that's how I found #5 coil was grounding to the mounting bracket, as I got lit up in a humorous way that my coworker thought was so funny until he realized that I could not let go of the wire and had to slap myself away to disengage...it was a shocking way to earn a bacon chzbrgr + a DP, but we both got a funny story to tell ppl ??

Edited December 31, 2020 by JBNeal
inserted information that fell off of the page

[keithb7]

Do you have a tachometer you can hook up? Watch RPMs. Pull one spark plug sure. RPM should drop slightly.  If no drop in RPM, good chance that cylinder is not firing.

 

If you want to know if you have a dead cylinder due to no compression, a compression gauge would tell you pretty quickly.

[Tooljunkie]

15 hours ago, JBNeal said:

The quick way is to pull spark plug wires off (1 at a time) of the spark plugs while the engine is running...if the engine has no change in how it runs on a cylinder, then that cylinder is not getting any fire.  To verify, turn off the engine, remove the suspected spark plug, and with the plug wire attached to the spark plug, touch the spark plug to the head to ground the spark circuit while cranking and watch for a bright white spark at the electrode.  This method I used recently on a JD 425 to find one of the 2 older spark plugs had a yellowish spark, so I replaced both and the new plugs had the bright white spark...engine startups were faster and the engine could pull a steep hill with ease, something that it had labored to do as mowing season neared an end.

 

But be sure ya have good insulation on the plug wires or you'll be lighting up like them spark plugs...a coworker bought a used Grand Prix that I told him wasn't running right, so we went out in the parking lot at lunch for me to show him this trick, but by pulling wires off of the individual ignition coils as they were much easier to reach...and that's how I found #5 coil was grounding to the mounting bracket, as I got lit up in a humorous way that my coworker thought was so funny until he realized that I could not let go of the wire and had to slap myself away to disengage...it was a shocking way to earn a bacon chzbrgr + a DP, but we both got a funny story to tell ppl ??

Those dual wire coils fire twice and with a pretty good whack. 
 

moving suspect components to another cylinder is another way to troublesoot a bad part. 
spark plugs may fire in open air,yet may not fire under pressure. Champion plugs have gone in the toilet now too. Have had two where centre electrode floats around and closes gap. It will get you. How do you look at a spark plug when diagnosing? Me too, it looks good until you point the electrode down. 

[Sniper]

You know, this is where one of those old school engine analyzers with a cylinder drop out test would come in handy. 

[kencombs]

17 hours ago, ChrisMinelli said:

On an older car, how do you know if you have a dead cylinder or a spark plug that isn't firing?  On a modern car the computer tells me (yeah I know...)  I ask because it occurred to me I may be missing something important and not even know it.  I assume it would run rough enough for me to know something is amiss?

Red part, cylinder isn't functioning so you want to isolate to ignition, compression or fuel?  I'd do the following, first compression test, then swap spark plugs with a working cylinder, then spark plug wires.  In most cases you will have a valid answer after thoses steps.

 

Blue part, I have several  OBDII equipped cars of my own or that I work on for family, none of which will identify a cause of a misfire.  They will all identify the failing cylinder, but a lot more work has to be done to pinpoint the cause of 'misfire cylinder xx'.   The most recent that I've dealt with involved a bad plug, a EGR valve, and a failed injector different cars.   That doesn't count a failed turbo, but at least that one said 'under boost'.  the EGR valves had some clues in other codes but not definitive cause.  The other two just knew nothing.

[greg g]

Do a compression leak down test. Swap the plug for one from a good cylinder. Assure your sparkplug leads are well  seated and in good condition.

[dpollo]

A dead plug will be sooty black  compared to the others.

[Tooljunkie]

14 hours ago, kencombs said:

Red part, cylinder isn't functioning so you want to isolate to ignition, compression or fuel?  I'd do the following, first compression test, then swap spark plugs with a working cylinder, then spark plug wires.  In most cases you will have a valid answer after thoses steps.

 

Blue part, I have several  OBDII equipped cars of my own or that I work on for family, none of which will identify a cause of a misfire.  They will all identify the failing cylinder, but a lot more work has to be done to pinpoint the cause of 'misfire cylinder xx'.   The most recent that I've dealt with involved a bad plug, a EGR valve, and a failed injector different cars.   That doesn't count a failed turbo, but at least that one said 'under boost'.  the EGR valves had some clues in other codes but not definitive cause.  The other two just knew nothing.

I make a living repairing vehicles,my scan tool is as current as snap on can provide and i pay a good buck to keep it updated. Im not going to get into the complexities of modern diagnostics, as the point is moot. 
 

i can say, a test light and a compression tester is about all you need to figure out what goes on with a miss on this sweet old iron. Inspect plug, do a compression test. If its like my 218,a valve tends to stick from time to time. Penetrating oil down the carb while its idling has got it moving,and seems like it happens less often now. 

[Tooljunkie]

A leak down tester can be as simple as an air regulator and a modified spark plug. Smash out the porcelain, weld an air fiiting to it and away you go. All you need to know is where the compression is going. 

[Dave72dt]

Timing lights work well for locating a missing cylinder. Just move the pickup lead around to each individual cylinder and check for consistent flashes.  Inconsistent flashes on all cylinders may indicate a problem in the primary ignition circuit.  Ohm meters to check plug wire resistance, coils and condensers.