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I was trying to diagnose a 2.4 Grand Am misfire and I noticed how this thing just isn’t really user friendly to diagnose. In trying to confirm spark, I came up with this.
Really, all I did was string some mechanic wire across the plug threads to give it a ground path and then I had an assistant crank. Part of the pic is eaten, but I did wrap the wire around all four plugs. So what do you think? Valid test or did I just qualify myself for my screen name?
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Its not how I would test it, but it may work. Thing is, it doesn’t take a lot of KV to generate a spark in a static environment, such as outside a cylinder. Now take the turbulence from air and fuel rushing into a cylinder and being compressed, that takes some KV to make a spark. So what you need to do is load test the system. Put spark plugs in 3 of 4 boots, and an adjustable spark tester in one. Make sure all coil towers are grounded of course, and set the adjustable spark tester for around 30(it’ll have a number scale). Make the coil work, and see if it can deliver. When replacing them, only use GM parts, aftermarket ones are junk. Also it is helpful to do an amp probe to see if one coil is weaker than another. God I used to hate those quad fours
It should work unless you have high resistance somewhere in the circuit. Then it may give you a false positive result. It may show spark now but, you have a lot of other things you need to take into account inside the cylinder which will raise the amount of kv to overcome the compressed air fuel mixture. While you have them out i would check the porclean and regap them. I also would ohm out that ground wire and see what kind of resistance you have going on.
If the coil is not in between the two. I would do a voltage drop from the center electrode tip coming out of the porclean to the plug on the other side. Check all 4 if they read close to the same then your problem ain’t there.
If the coil is in between the 2 ohm the parts you can aka the boots and the plugs.
I don’t know if this is true for in the field or not but, in my experience fuel injectors seem to be more of the cause of misfires lately then spark plugs. Though you also did not list what you already checked etc just asked about the spark plug test.
Looks good to me, even lets you check all ignition function at once. As stated, spark in the cylinder is a different environment. That being the case, I’ve still never thought a spark looked good outside the engine and it turned out to be a problem. On the other hand I’ve identified weak spark outside the engine that turned out to be the misfire cause many times. It’s just another one of those common sense things, isn’t it? Weak spark looks weak and strong spark looks strong. Never failed me yet.
Problem is, you can have good spark via that test, but you may miss a weak area of the ignition system. When you stress the system, with high resistance in the secondary circuit, you can start seeing things like secondary voltage coming thru plug boots, burning thru coil cases, etc. Ford even has a test for stress test their COP systems. That’s one reason why I wont buy those inline spark tester lights. They tell you there is KV passing thru, but it doesn’t accurately stress the system, so its of no value when diagnosing some running issues, or more importantly, finding something before it’s an issue yet.
i had done those inline spark tester but, one time, coil jumped over 20kv but, two days later crap out…still sitting in our parking lot….i would use scope to get some graph going….
anyway, people like start part shotgun…anyway…most of young guns would…
picture above…almost only way to test when even dealer wouldn’t have proper testing wires anyway.
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