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Old 02-11-2006, 06:18 PM   #1
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Default Anyone know how to set up a multimeter to check a spark plug?

just like the title says people. Anyone?

My plugs look fine, but I changed them, and seem to have gotten rid on my miss. I am thinking a plug is bad internally somehiw and want to see if there is a way to test them to see?
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Old 02-11-2006, 08:49 PM   #2
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prolly be easier to just buy another one and call it a day. The time it takes to test it you could replace it with a known good one
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Old 02-11-2006, 09:06 PM   #3
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This is maybe the easiest test you will ever do in your life.

Put the meter on continuity (cont mode). If it doesnt have that look for the Omega symbol on there. Anything to measure resistance will work. Attach one lead to one side, the other lead to the oppisite side. So easy, right??

If it looks like a low resistance it is good. High it is bad. Some meters will say like 32K ohms when it is bad because that is the max it will show. Some beep when there is a conection. I mean if it is showing like less than 5 ohms you are ok.

This is like a 5 second test. Too bad testing diodes and chips isn't so easy!
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Old 02-11-2006, 09:46 PM   #4
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I'm gonna write this down!
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Old 02-12-2006, 01:08 AM   #5
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thanks.

I already replaced the plugs. The car ran great. I just wanted to narow it down to which cylinder had the bad plug. And be positive it was a bad plug!

Especially now that I have my miss back on plugs that are less than a few hours old!

I am at a loss now. I think I may have a coil or a tps going bad or something. COuld still be O2's possibly, being that I haven't replaced em with new. I used the two rears. I can't belive that they are the problem after trying a different pair, extending the harness the right way. Now the car is pissing me off!

Anyone have any ideas?
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Old 02-12-2006, 09:18 AM   #6
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You can't see all faults with a DMM. Only gross open
circuit, in a plug; however cracks in the insulation
will give you trouble at higher cylinder pressures
where the gap will present more arc standoff voltage
than sneaking out the crack and arcing somewhere
else. Similarly the wires, one pinhole and you can lose
spark under load (or all the time). Looking at the motor
in the dark is one thing to do, misting wires with water
and looking for worsening is another.
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Old 02-12-2006, 11:08 AM   #7
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With apologies to Carroll Smith for adapting his bolt test to this thread, the best way to test a spark plug is to throw it into a lake. If it floats, it's good.

Pulling a plug to check for signs of the engine running rich/lean/hot, etc, is one thing, but if you have any reason to suspect the plugs themselves might be bad, just toss them and move on.
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Old 02-12-2006, 02:07 PM   #8
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Quote:
Originally Posted by jimmyblue
You can't see all faults with a DMM. Only gross open
circuit, in a plug; however cracks in the insulation
will give you trouble at higher cylinder pressures
where the gap will present more arc standoff voltage
than sneaking out the crack and arcing somewhere
else. Similarly the wires, one pinhole and you can lose
spark under load (or all the time). Looking at the motor
in the dark is one thing to do, misting wires with water
and looking for worsening is another.
I agree with Jimmy. You can have a bad plug that still passes the continuity test and I don't know of any entry level DMMs that can withstand being put inline with the plug. If you really want to test them I guess you can get one of those induction driven ignition testers. This will at least see if current is flowing through the plug (meaning it has somewhere to discharge, hopefully the ground strap).

In reality it's probably best to just replace it, plugs are usually $2 at the most.
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Old 02-12-2006, 09:44 PM   #9
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Quote:
Originally Posted by Zeus
I don't know of any entry level DMMs that can withstand being put inline with the plug.
The DMM supplies it the power for the cont test. You are not going to put the DMM inline and test it in amperage mode.
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Old 02-12-2006, 09:45 PM   #10
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Quote:
Originally Posted by 2002_Z28_Six_Speed
This is maybe the easiest test you will ever do in your life.

Put the meter on continuity (cont mode). If it doesnt have that look for the Omega symbol on there. Anything to measure resistance will work. Attach one lead to one side, the other lead to the oppisite side. So easy, right??

If it looks like a low resistance it is good. High it is bad. Some meters will say like 32K ohms when it is bad because that is the max it will show. Some beep when there is a conection. I mean if it is showing like less than 5 ohms you are ok.

This is like a 5 second test. Too bad testing diodes and chips isn't so easy!

acctually if you buy yourself a nice fluke brand dvom they have a diode test built right in em now
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Old 02-12-2006, 10:15 PM   #11
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Quote:
Originally Posted by 2002_Z28_Six_Speed
The DMM supplies it the power for the cont test. You are not going to put the DMM inline and test it in amperage mode.
What I'm trying to say is that a plug can be bad but still pass a continuity test. A slightly more accurate test would be to place the DMM inline during an actual ignition firing and see what sort of current is flowing through the wire. Then you could place the DMM in parallel and find the voltage running through it. The only problem is most coils produce something like 30,000V and I don't recall the specs on my Fluke, but I'm pretty sure it's out of the range.
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Old 02-13-2006, 12:54 AM   #12
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The best way to test the entire ignition system is with automotive O-scope...A DVM isn't going to show you a thing, unless the plug is completely destroyed.
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Old 02-13-2006, 10:56 AM   #13
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I've had about 80% luck showing faults using the DMM.
Set the ohm setting to 20,000 ohms for the test.
Depending on the plug type you may see between 4000 and 5000 ohms on most of them. If one is way out of range then it's suspect.

If the insulator is cracked the ohm test will likely not show a problem.
I've also seen where the resistor inside the plug is cracked, but shows good on the ohmmeter when cold. The problem only showed on the meter after warming the plug.
If the plug has the screw-on tips, the meter will not show a problem with that either.

If the plug is misfiring then you can usually see it as a difference in plug color. It will simply appear to be burning cooler in that cylinder.
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Old 02-13-2006, 10:56 AM
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