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Old 12-08-2013, 12:28 AM
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intermittent misfire shows on oscilloscope

I'm working on an '89 Chevy 2.8L with an intermittent misfire. A *very* rough drawing is attached. All cylinders have approximately the same ignition pattern, except #6 shows an intermittent spike (red line on drawing) during the firing section when the engine misfires. It's not the plug or the wire, and after closing the gap, it may be less frequent at idle but the engine still misses when car is driven. Seems to be worse after car warms up. Any ideas of what this could be? I'm guessing ignition module or coil pack (hopefully not the ECM).

This is the first time I've used a scope to troubleshoot a misfire, and was surprised to learn it can show ignition as well as fuel problems. I'd suspect an injector, but from what I've read, it seems lean/rich (clogged/leaking injector) affects the slope of the firing section instead of causing a spike. I hooked the number #6 plug wire to a grounded (uninstalled) plug and the firing section showed a solid straight horizontal (instead of a somewhat changing slope depending on fuel injection), and I thought I saw the spike (which would eliminate the injector) but couldn't repeat it after driving the car again. It was GOOD to get a consistant misfire after pulling the plug wire (I don't do this with HEI when the engine's running...) as it confirmed the problem was with cyl #6.
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Old 12-08-2013, 01:32 AM
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I'm glad to see someone is using a proper ignition oscilloscope to diagnose a misfire. I use one almost daily, but I am one of a few.

The area in your drawing you marked the anomaly is the burn line. This is the period of time during which the spark is arcing across the gap. Any changes in voltage indicate changes in conditions in the combustion chamber at the gap of the spark plug. My initial reaction is to think the plug is fouled, but this is usually accompanied with a low firing line.

Anything that could cause the mixture to be wrong could be the culprit, as can a mechanical problem. A vacuum leak at that intake port could cause this. A bad fuel injector could too.

Do you have a gas analyzer? That would make diagnosis of your misfire much easier.
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Old 12-08-2013, 09:16 AM
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Switch the plug from # 6 to another cylinder. Check again. Is #6 still mis-firing, or is it the other cylinder you put that plug in mis-firing ?
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Old 12-08-2013, 05:15 PM
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I'm assuming all new plugs and wires etc?
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Old 12-09-2013, 09:00 AM
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Quote:
Originally Posted by 4 Jaw Chuck View Post
I'm assuming all new plugs and wires etc?
Yes. About 10K ago I put in new wires. New plugs were installed attempting to fix this (no change). I ohmed wires (good), tried the old set (no change), then put in one of the old plugs. The spark looks consistent (on plug outside/grounded on engine) but I need to look at it at night. [next time I get back to this car will be Dec 24].

With the old plug installed (a "taper tipped" platinum with about 10K miles on it), the scope pattern changed drastically (much oscillating on the burn line). I didn't install/compare an old platinum in a good cylinder. I had about 3-4 hours on this car after I saw the firing anomaly.
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Old 12-09-2013, 09:20 AM
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Quote:
Originally Posted by Denny View Post
Switch the plug from # 6 to another cylinder. Check again. Is #6 still mis-firing, or is it the other cylinder you put that plug in mis-firing ?
Will do this (GOOD simple test that I didn't think of at the time...) when I get back to the car. A different plug also misfired in #6 but firing characteristics changed much. I'll post some diagrams to 'hpengineprep' reply later.
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Old 12-09-2013, 09:40 AM
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I have taken plugs out before. Bought and installed new ones right out of the box, and had a miss. Some times, I have had several that just didn't fire right. New doesn't always mean new so to speak.
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Old 12-09-2013, 05:44 PM
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I've never had good luck with champions in a HP motor although I use them for jetting chops. Those Platinum just plain don't work well, I've tried them in various cars and ended up taking them out. Palladium seemed OK (just try and fine them now?!) but by far NGK has been the ones I went back to.

Don't get me started on those multi tipped gimick plugs...talk about junk. The only thing that's worked for me a far as gimmicks has been cutting off the side electrode so it only extended halfway across the center.

Have you tried regular plug yet? If your burning a small amount of oil it will glaze those platinum plugs almost immediately, they don't run hot enough on the center electrode to burn off any contaminants in a worn engine...fine in a new car...garbage in a motor with 100K miles.

If your engine has high miles and low compression from wear nothing your going to do will solve those issues, an oscilloscope is fine for finding issues that are intermittent but a continuous issue may be more basic like a bad ground, corroded wire, low voltage etc.

I had a 1984 Bronco II once and the entire harness was full of cracked wires, that was a tough one to diagnose until I started pulling the harness apart looking for shorts, basically had to rewire the entire engine compartment. Ran great after but high mileage still created issues with plugs like your having until I ran plain old NGKs in a one higher heat range. Never blew any blue smoke while it ran but the crank breathers smoked like a chimney even at idle.
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Old 12-09-2013, 06:46 PM
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I agree with the above. You can always use a regular plug. An ignition system will always run on a regular plug, but not always on platinum or other special plugs. And forget the multiprong, they will likely shroud the spark in that engine. Stick with stock unless it's modified. Is it modified?
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Old 12-09-2013, 07:32 PM
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Lean

Lean mixture. put some fuel in there. Many have lean misfire in order to achieve max mpg. Highly noticeable in some applications. Put some fuel cleaner to the injectors to get it back to the fine line set up by the oem. If its using a bit of oil, its gonna be hard. Had a '95 305 in a pickup that was just calibrated too low on air/fuel ratio.
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Old 12-10-2013, 02:00 PM
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Judging by looks of the pics. It may be an intermittent ignition #6 wire.
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Old 12-11-2013, 05:25 AM
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Quote:
Originally Posted by OLNOLAN View Post
Lean mixture. put some fuel in there. Many have lean misfire in order to achieve max mpg. Highly noticeable in some applications. Put some fuel cleaner to the injectors to get it back to the fine line set up by the oem. If its using a bit of oil, its gonna be hard. Had a '95 305 in a pickup that was just calibrated too low on air/fuel ratio.
I ran STP FI cleaner through it and not much happened. Ran Lucas through one tank and it ran good (intermittently) for the first time, but never "fixed" it. Then ran a tank of Chevron (w/Techron???) through it and it seemed to fade back to as before. Ran more Lucas through it (double dose), and it intermittently started hitting on all cylinders again, and seems to run fine when cold, but misses at operating temp.

Car uses no oil - has new (~10K miles) Chevy long block in 92K mile car (leaked/ran low on water once, noticed gauge at hot, let it set not long enough, started it, heard a pop..., cracked alum head at top in front, ran it leak free ~3 years sealed with hi-temp epoxy until I got a new block).

I'll post more details, but this started after I let car sit outside (in CA) about 3 years/no start without putting fuel stabilizer in it. The car always ran good with this 2.8L V6 (89 Cavalier RS station wagon).
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Old 12-11-2013, 05:36 AM
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Quote:
Originally Posted by OLNOLAN View Post
Lean mixture. put some fuel in there.
Sounds like a good test. I'll monitor the misfire/scope and spray some starting fluid in to see if it stops (I don't like to use that stuff, but don't have a way to spray in gasoline [idea to do this is welcome]. If it's lean (intermittently blocked injector), the anomaly should stop - if injector sticks open, I should see little/no difference (???).
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Old 12-11-2013, 05:38 AM
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Originally Posted by lg1969 View Post
Judging by looks of the pics. It may be an intermittent ignition #6 wire.
As was suggested, I'll swap the plug/wire with another cylinder and see if anomaly moves from #6 (when I get back to the car/CA in a couple weeks). I've installed a different plug/wire but I want to see if the problem moves. The wires ohmed good, but "intermittent" is the key word...
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Old 12-11-2013, 05:55 AM
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Quote:
Originally Posted by 4 Jaw Chuck View Post
I've never had good luck with champions in a HP motor although I use them for jetting chops. Those Platinum just plain don't work well, I've tried them in various cars and ended up taking them out. Palladium seemed OK (just try and fine them now?!) but by far NGK has been the ones I went back to.

Don't get me started on those multi tipped gimick plugs...talk about junk. The only thing that's worked for me a far as gimmicks has been cutting off the side electrode so it only extended halfway across the center.

Have you tried regular plug yet? If your burning a small amount of oil it will glaze those platinum plugs almost immediately, they don't run hot enough on the center electrode to burn off any contaminants in a worn engine...fine in a new car...garbage in a motor with 100K miles.

If your engine has high miles and low compression from wear nothing your going to do will solve those issues, an oscilloscope is fine for finding issues that are intermittent but a continuous issue may be more basic like a bad ground, corroded wire, low voltage etc.
Interesting about platinum plugs. I've never noticed a problem, though don't have cars that use oil (except my GMC pickup which needs valve stem seals [not sure if I've changed plugs, but it's got a slight miss - will check it].

I had Bosch platinums in for about 10K miles - they looked good when I pulled them, but I noticed (with plug grounded on outside of engine) they have a TINY spark compared to more standard plugs (exposed center electrode). The Bosch platinums are old - I don't know if they still sell that kind - they have a tapered ceramic C.E. insulator but no visible Center Electrode.

I've got Autolite Platinums in now. I installed them as is with what looked like a narrow gap, misaligned electrodes, etc. Then I gapped them to .045, lined up electrodes, and filed the ground electrode to halfway across center (as I read in an old book). No noticeable difference in performance - still had intermittent misfire.

I was concerned about wiring/low voltage until I saw all other cylinders are hitting fine (though signals vary/jump a bit - which I attribute to computer jacking with fuel mixture when the engine's missing). I thought maybe the ECM wire to primary coil is bad on #6, but it looked like (in schematic) there is a single wire going to the DIS to fire all 3 coil packs.
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