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intermittent misfire shows on oscilloscope

15K views 35 replies 10 participants last post by  4 Jaw Chuck 
#1 · (Edited)
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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#2 ·
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.
 
#17 ·
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.

Do you have a gas analyzer? That would make diagnosis of your misfire much easier.
I've attached a drawing showing what happened when I put in the platinum plug (the burn line oscillations during misfire). It's hard to see the Bosch plug spark during the day - amazing the engine ran well [until this misfired started]. I didn't try the Bosch on a non-misfire cylinder to see if there were no burn line oscillations (but I don't think they were there without the misfire [all this is from memory]).

The plugs weren't fouled but as pointed out, one (new plug) could be bad. (as said elsewhere, I'll swap the plug/wire with another cylinder to see if I can get the misfire to move).

NO... I have no gas analyzer and they appear to be expensive. Curious what it might tell me. I use my nose (bend down and smell exhaust for a few seconds) and seem to smell raw gas at times - I'm not sure if this was when it was cold (likely if running rich at startup) or if it's (also) when the engine was missing badly.

What might a gas analyzer show? (I could take it to a shop [after trying to find a good one] but want to do this myself). I took it in for CA smog check, but it wouldn't hold 25mph steady enough to pass that test - because of this failure, the smog tester gave no emission numbers.
 

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#4 ·
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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#16 ·
That'd get you going alright. I've not had bad new plugs, though have gotten them with gap way off. I cracked an insulator once while installing plugs and engine missed bad - could hear/see the arc at night (old Chevy with points).

I generally look for a change in performance and if it "seems" the same, I assume parts are good, but this could bite me... On this car, I pulled the DIS/coils, checked for cracks, etc (saw none), put it back together and car ran worse. That "mild crack" I heard when tightening a coil pack must've messed one up - the car ran worse... found the secondaries on the 2/5 coil were open - replacing the coil got me back to the "normal" intermittent misfire.
 
#6 ·
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.
 
#13 ·
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.
 
#7 ·
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?
 
#8 ·
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.
 
#10 ·
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).
 
#12 ·
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...
 
#15 ·
Lifter!!!!!!!!!

One other thing to check might be a lifter that is hanging up from time to
time.

I have a 350 Chevrolet engine with an intermittent miss.

I had this problem and it drove me crazy trying to figure out what was happening.

I changed plugs, wires, vacuum lines, distributor and a few other parts before trying one very simple thing I should have tried first.

One quart of Marvel Mystery Oil solved the problem for me.

A lifter that hangs up feels just like an electrical problem and that is what I looked for first which was a mistake.

Hope it works for you. Jimbo
 
#19 ·
One other thing to check might be a lifter that is hanging up from time to
time.

I have a 350 Chevrolet engine with an intermittent miss.

I had this problem and it drove me crazy trying to figure out what was happening.

I changed plugs, wires, vacuum lines, distributor and a few other parts before trying one very simple thing I should have tried first.

One quart of Marvel Mystery Oil solved the problem for me.

A lifter that hangs up feels just like an electrical problem and that is what I looked for first which was a mistake.

Hope it works for you. Jimbo
Good point - a sticking lifter could happen after letting a car sit 3 years. I may have gotten side-tracked/fooled by improvement after using Lucas FI cleaner. It could also free up/get worse if the engine is cold/at op temp.

I haven't had a lifter stick, but when adjusting valves on a Pontiac, tightening too much held the valve open and created a constant misfire (a bit more tightening and I could hear the piston hitting the valve - only did that once and luckily no damage). Would be interesting to see what this does to the ignition signal on the 'scope.

I'll put this on my list of things to try. I changed oil after pulling it from storage but used no additive.
 
#20 ·
Sounds like you answered your own question. You need to send those injectors out for service and probable replacement, they use ultrasound to clean them and then flow test them to verify they are not restricted.

Pretty important part of this puzzle.
 
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#22 ·
I guess you mean about me finding sloppy injectors (after my cleaning them, they worked on/off with a switch test, but could get stuck while getting pulsed at op temp). If I prove it's not ignition (by swapping plug/wire with other cylinder and seeing problem move), I'll probably shotgun it with a new injector (weighing cost/time of cleaning vs $60 for one new injector) after trying Marvel Mystery oil (won't hurt). I'm leaning toward injector problem because it first ran better with Lucas FI cleaner.

The car ran GOOD for a period/runs good when cold, but this could still be a lifter sticking/valve open/intermittent no or low compression. (Compression measured between about 140 and 148 psi on all cylinders/warm engine)

I'm also curious what will happen to the ignition signal if I spray gas/ether when it's missing (both on good cylinders and bad #6).

Point of troubleshooting this "good" problem is to see what's causing the anomaly on the scope and to be able to ID such problems (intermittent misfires) in the future.
 
#24 ·
Just saying...your analyzing burn lines on an occiloscope to the Nth degree when you obviously had blocked injectors and from your description didn't clean them properly or flow test them on a duty cycle.

Not trying to deter you but ignition inside a combustion chamber is totally dependant on mixture strength, dispersion, and velocity. You may be underestimating how important this factor is in your equation.

Only trying to save you from chasing your tail by micro examination of a simple ignition system event when you obviously have an injector issue that you haven't properly resolved.

Any person who has cleaned injectors for a living would be shaking their head, not all injectors can be cleaned...ever. I sent a set of injectors out for testing and cleaning off my 1991 200HP Merc V6 outboard, maybe 500hrs of actual usage...always had fuel additives run through it and nothing but clean fresh double filtered premium. Always stored with storage spray and fuel rail drained before winter.

I was expecting perfection but I seen the flow data and three of the six were way lean and droplet sizes were wrong. I was lucky, most people never bother to check these things unless they fail but a two stroke is unforgiving about mixture. They were not cleanable and had to be replaced, I had him replace all six to be on the safe side and flow test them to each other so they were all within 1%...he went through 18 injectors to get six that flowed and patterned the same

Not only did it idle smoother after but I picked up 3mph on the top end, best $80 I ever spent, probably saved my motor too. Most outboards with EFI at this age die from running lean and stick a piston.

I think your seeing the same thing here but your stuck in diagnostics and not verifying ancillary systems that affect the whole.

Good luck.
 
#35 ·
Just saying...your analyzing burn lines on an occiloscope to the Nth degree when you obviously had blocked injectors and from your description didn't clean them properly or flow test them on a duty cycle.

Any person who has cleaned injectors for a living would be shaking their head, not all injectors can be cleaned...ever. I sent a set of injectors out for testing and cleaning off my 1991 200HP Merc V6 outboard, maybe 500hrs of actual usage...always had fuel additives run through it and nothing but clean fresh double filtered premium. Always stored with storage spray and fuel rail drained before winter.

I was expecting perfection but I seen the flow data and three of the six were way lean and droplet sizes were wrong. I was lucky, most people never bother to check these things unless they fail but a two stroke is unforgiving about mixture. They were not cleanable and had to be replaced, I had him replace all six to be on the safe side and flow test them to each other so they were all within 1%...he went through 18 injectors to get six that flowed and patterned the same

Not only did it idle smoother after but I picked up 3mph on the top end, best $80 I ever spent, probably saved my motor too. Most outboards with EFI at this age die from running lean and stick a piston.

Good luck.
THANKS much for this info. The main problem is fixed (bad coil pack) but now has a "more typical" rough idle in drive. I changed cyl 5 injector because of plug deposits, but car still idles a bit rough (got to check everything again). I used the switch test and all injectors looked decent (though cyl 1 stuck open/leaked a bit until I hit the switch a couple times - plug looked okay so I swapped #5 since I only had one injector).

I'll go through the injectors later after I get another vehicle to drive. It seems you got a lot of testing done for $80... all testing sites I saw quote $20-25 per injector. One site says 5%-7% flow rate variance is okay, so you got yours dialed in at 1%. Impressive to note the difference (+3 mph on water), but my car is a daily driver so I'd just like a smoother idle. Whenever my FI cars idled rough, Gumout FI cleaner always smoothed them out. This car was sitting a while so the situation is different - and much has been said about this type injector (1989 MFI) having problems.

The link notes problems with injectors and ethanol gas - esp on marine engines. It says testing/cleaning is typically $25-40 so it would almost be the same cost to replace my injectors (~$40-50 from Rock Auto) though sounds like a good idea to get new ones flow tested.
Servicing Fuel Injectors: On-Car versus Off-Car | Fuel Injectors IncFuel Injectors Inc

At some point, I'll make a simple duty cycle injector tester, but for now I'm just glad the car is running okay. Taking it in this morning for a smog test - it always passed easily before.
 
#25 ·
I certainly sounds like that cylinder is lean. It most likely runs better when cold due to the richer mixture. The lucas may not be cleaning better but may be changing the fuel properties enough to mask the issue. You can always introduce a little propane into the intake air stream to richen up the mixture a little to see if the issue goes away.

As far as plugs, you cant go wrong if you put in what the OEM did originally. I have replace more new plugs that didnt work. Bosh seem to be the biggest offenders. They dont seem to work right in anything.
 
#29 ·
I certainly sounds like that cylinder is lean. It most likely runs better when cold due to the richer mixture. The lucas may not be cleaning better but may be changing the fuel properties enough to mask the issue. You can always introduce a little propane into the intake air stream to richen up the mixture a little to see if the issue goes away.

As far as plugs, you cant go wrong if you put in what the OEM did originally. I have replace more new plugs that didnt work. Bosh seem to be the biggest offenders. They dont seem to work right in anything.
The problem seemed to go away when cold, so maybe due to being richer, running open loop, injector sticks/ignition messes up when hot. Seems if the Lucas changed fuel property enough it would be more consistant, but it's still intermittent, but who knows... THANKS for the idea of introducing propane - will be easy to do while monitoring ignition signal.

I've not had spark plug problems that I've noticed (aside from when I cracked an insulator), but I'll be getting standard OEM recommended plugs from now on. For now I'll swap the problem cylinder's plug/wire with another to see if anomaly moves.

I'll be "loaded for bear" when I go back to CA. Got several tests (maybe an hour/two - first/easiest will be the propane), and an injector, coil and DIS module (to be replaced in that order) if the tests don't point to something.
 
#28 ·
I got a Hantek HT-25 lead on ebay for $21 (incl shipping). There are other/cheaper sources/google for $17 (one includes shipping).

Here is a link to a good Toyota oscope intro:
http://www.testroete.com/car/Toyota... - Advanced Emissions and Driveability/13.pdf

Ignition signals can show ignition or fuel problems. All I see says if a cylinder is lean the firing line voltage will be high, and if rich it'll be low (the Toyota doc says this is true with fuel injected engines). I've seen nothing that shows a spike in the middle of a burn line.

On my car, I couldn't trigger on/see the firing line (with Tek analog scope), but maybe that cyliner is lean (injector intermittently sticking closed) and the burn line spike results from a lean mixture/bad spray pattern. From what I understand, actual combustion takes place slightly after the spark fires (which is why timing is set advanced) - I'm still learning about this and appreciate input from guys on here and LOADS of info on inet.
 
#30 ·
Lots of the early EFI systems had injectors that would get dirty and no amount of cleaning would help.
I had a 3.8 gen 1 buick that had a dirty injector.It would only misfire when in top gear,torque converter lockup,light engine load.
The problem was accentuated in that when these conditions were met the airflow volume was low and the injector couldn't atomize correctly and would misfire.
That would also create a firing line like your seeing to due low cylinder pressure . Low pressure from a lean mixture not firing as a lean mixture wont make the pressure/power a full on cylinder with 14.7 to 1 stoich will make
 
#32 ·
I had a 3.8 gen 1 buick that had a dirty injector.It would only misfire when in top gear,torque converter lockup,light engine load.
The problem was accentuated in that when these conditions were met the airflow volume was low and the injector couldn't atomize correctly and would misfire.
That would also create a firing line like your seeing to due low cylinder pressure . Low pressure from a lean mixture not firing as a lean mixture wont make the pressure/power a full on cylinder with 14.7 to 1 stoich will make
You describe when the misfire is most noticeable - TCC locked in 3rd/top gear with light load/on flat road. In 2nd gear it's hard to notice, other than lack of power.

I've read that the old MFI injectors weren't so great. I've got another and will swap it out when I get back to CA. I'll check the spray pattern quickly (on/off switch) when I pull the fuel rail, but it get real bad with pulses/when hot.

I don't understand why low cylinder pressure causes the burn line spike (why the voltage jumps), but it makes sense if the mixture ignites shortly after the plug fires.
 
#33 ·
Back in CA. Just took it out once, but the car's running pretty good (I don't like this - now harder to check...). Maybe that double dose of Lucas loosened up the injector after sitting a few weeks. I'll be swapping out that #5 injector when I get a chance - it still runs a bit rough/misses, but hasn't gotten a steady misfire yet - I looked at it quickly with my scope but saw nothing.

I got one of those Hantek 1008 USB scopes and with two different laptops (XP and Win7) I can't get it to work right. Works good on a 2v square wave (the units self-generated test signal), but bluescreened with both units when hooked to plug wires (with Hantek leads). It gathers data inconsistantly for a few seconds, then the signal saturates (square wave/noise from top to bottom of display) and locks up the computer - could be hardware, but I'm going to send it back. Never was able to store an ignition signal. Too bad as it has good features (nice stored setups for gathering ignition/automotive waveforms).
 
#34 ·
Car started misfiring after a couple days - bad signal showed on cyl 6 again (good!).

Installed new Delco copper plugs (yielded flatter/more consistent burn line than Autolite platinums but car ran the same), swapped #6 wire with #4 (problem stayed on #6), and sprayed propane and starting fluid while monitoring signal (no difference).

Noticed cyls 6 and 3 burn lines would change duration ~0.3ms (photo of cyl 6 when 'normal' and during misfire); 6 showed the anomaly/spike - 3 just changed duration. Both of these run from the same coil pack (I learned DIS systems like this will invert one cylinder's ignition pulse - wondered about that for a while...). So I replaced the coil pack and problem is gone.

The burn time of these cylinders is still shorter than other cylinders (about 0.8ms vs. 1.0ms) which is borderline according to info in the link, but car runs okay so I'm done with it for now (need to get a smog check to get license tags then start working on the 95 Taurus transmission... [suspect broken pump shaft]).
Spark Burn Time

The photos were taken on a Tek 2246A scope set at 0.2v and 0.5ms per division using Hantek HT-25 pickup leads. The Hantek 1008 finally worked with the win7 laptop (let it run after lockup) but I could never record/trigger on the anomaly [proving what hpengineprep said about USB vs analog scopes].

Cyl 5 spark plug had light gray deposits (seem to remember this means running lean or rich - any ideas???). This happened twice after about 800 miles (cleaned/moved plugs around). I replaced cylinder 5's injector so will see what happens (the spray pattern didn't look bad with on/off switch). [thanks to 4 Jaw Chuck about injectors/testing]. I couldn't see anything obvious on the scope, though the burn line looked a little low.
 

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#36 ·
I had those injectors done years ago, pricing is likely more now.

Brucato is who did mine, Tony Brucato is a really fair guy and honest...you can trust his diagnosis. Outboard two strokes is his specialty but I'm sure an automotive injector is no problem, just be prepared for bad news. As you would expect two strokes are picky about fuel injection and he will stress test them to the max and if they are garbage he will fail them if they don't clean up. He will supply test data logged from the tests for each injector so you can make a decision on where to put them on rich cylinders etc.

After my experience having them flow tested and fooling around with old ones I would just buy a set of flow tested units from him and sell the old ones or junk them. They wear out just like anything else and when you consider how many cycles these things go through I consider them throw away devices after so many cycles.

One thing is for sure, picking up the speed I did on a boat was the cheapest speed I ever gained on it...amazing what a good set of injectors will do for performance!

Here's the website link:

Brucato High Performance Products and Service. Brucato SVS (air intake system) and PCU (adjustable ECU) for Mercury EFI Outboards.
 
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