Improvised oscilloscope probe for Automotive ignition analysis.

[Jim-0000]

The coil primary waveform is also very useful, it's how modern ECU's do real-time misfire and ignition fault detection.
Electronic ignition clamps the primary voltage to just under 400V, so max. output voltage is limited compared to points......

Yes, I understand that. When we do receive the portable scope I have coming, we can check the primary.

There's two grounds in a car. One high-current ground is from engine block to battery, and the second smaller ground is from the engine block or battery to the car body/chassis. This is for the in-car loads like fuel pump, ignition, lights etc. A common mistake is forgetting the car body ground which the ignition module may not have been grounded to the block. The firewall is no good unless you have the ground strap from block to body. You can't rely on current flow through motor mounts and U-joints etc.....

I insisted that they fit another ground wire from the back of the block to the firewall. Unfortunately, that made no difference to the fault.

Flipping the carb around, it would mean poor mixing in the intake manifold? I don't see a single carb used on the Holden hi-perf mods.......

Yes, that is what we conclude. This engine was first fitted with triple SU carburettors.They were removed and changed in one of the many attempts at eliminating this fault. 

If possible you can fit an O2 sensor and see each cylinder firing lean/rich to check balance. Maybe joe has tried this.
Otherwise you'll blame the ignition system when nothing is wrong with it.

That would be good. I will see if they can oganise one.

I should add that when they (the owner and his brother) tested the car on the track last weekend, they reported that it was cross firing. That is, it was backfiring out the exhaust every now and then. That suggests to me a major distributor problem.
This engine is running on Methanol currently. Several fuels have been run in it previously.

[Johnny10]

https://www.ebay.com/itm/US-POST-HANTEK-HT25-High-Voltage-OscilloScope-Secondary-Ignition-Automotiv-Probe/302674748580?epid=1061469482&hash=item4678d224a4:g:aHwAAOSwLg5bWELG:rk:1:pf:0

Why improvise?

[Jim-0000]

https://www.ebay.com/itm/US-POST-HANTEK-HT25-High-Voltage-OscilloScope-Secondary-Ignition-Automotiv-Probe/302674748580?epid=1061469482&hash=item4678d224a4:g:aHwAAOSwLg5bWELG:rk:1:pf:0

Why improvise?

That is just what I was thinking while looking for Coax cable. Good point!
Thanks.

[Johnny10]

Which scope did you buy?

I started with the Rigol 1054Z

Worked well because I could roll back the display while learning to find the signals.

A Picoscope or a Fluke 199 oscilloscope would work.
The picoscope will come with automotive software for free.

[Jim-0000]

Which scope did you buy?
............................

I ordered this little one for my low voltage radio control model aircraft application - before I got involved in assisting the owner of the problem stock car and his brother who is also assisting. This has been highly recommended for my uses:

https://www.banggood.com/Original-JYE-Tech-Assembled-DSO-SHELL-DSO150-Digital-Oscilloscope-Module-9V-p-1211151.html?rmmds=myorder&cur_warehouse=CN

I am beginning to realise it will be useless for automotive high tension application. Also, could damage it first time used as such.

[joeqsmith]

If possible you can fit an O2 sensor and see each cylinder firing lean/rich to check balance. Maybe joe has tried this.
Otherwise you'll blame the ignition system when nothing is wrong with it.

Assuming narrow band, I have not used.   The Bosch wideband sensor is slow.  100ms or so. 

Graphs below show about 1.3 seconds of data.  You can see the wideband in the purple trace.   The transmission is an auto for 1-2 but requires kill for the other shifts.  There is a lot of air and unburned fuel pumping through the motor for that 30ms or so.  You can see the wideband does pick it up but its no where near fast enough to see cylinder by cylinder. 

Second graph shows about a minute of data with the engine sitting without a load.  The engine can build a fair bit of pressure on the 2-step. Once off the 2-step, it will tag the upper limiter.  Both events can be seen with the wideband. 

[tautech]

Ah, the two step.  Boom, bang, boom.   

All hell breaks loose. 

Buddies 20B on Meth with 40lb of boost is something that is heard for miles ! 

Oh yes Joe, I left EGT off the list. 

[Jim-0000]

I will be driving up to where the car is (two hours away from here) today to assist one of the brothers work on it further.
My suggestion to throw away the silicone plug leads and replace with copper was met with some ambivalence. (I am puzzled by this!)
I don't want to get too insistent; I am only an onlooker/assistant and not a mechanic.
I might have more to report tonight.

Jim.

[mikerj]

My suggestion to throw away the silicone plug leads and replace with copper was met with some ambivalence. (I am puzzled by this!)

Silicone only refers to the insulation, it's possible to have copper cored HT leads with silicone insulation.  It's the carbon string based cores leads that can break down over time.

[Gregg]

Don’t underestimate possible mechanical problems causing what seems to be an ignition problem.  Things like weak valve springs, worn cam lobes, worn rocker shafts, worn or sticking lifters, worn valve guides, sticking valves, timing chain or belt tension / harmonics, worn distributor drive or partially blocked cooling passages to the cylinder head(s).
Under load problems are often problems that simply are not noticed under light or no load.  For instance the crankcase pressure is much higher under load because of increased blow by and if an intake valve is loose in its guide the mixture to that cylinder can be leaner.  Under load exhaust valves get very hot and can grow considerably longer and if the clearance isn’t adequate, the exhaust valve can be held open too long. 
You stated that it is carbureted; it may be that the power valve isn’t functioning properly making it lean under load; a lean mixture can seem like an ignition problem partly because a lean mix is harder to ignite.  A vacuum gauge may help here, a lot of old timers (like myself) swear by them for carbureted engine diagnosis in certain cases.

[tpowell1830]

Sight unseen is not a good position to do troubleshooting for an engine, however, there are a multitude of causes of an intermittent miss on a cylinder. Theses problems could be of either electrical or mechanical issues. Here are only a few that could be checked fairly easily.

1. Check the plug or plugs in question for arc tracks, especially if they have a boot type spark plug wire.
2. If this is a carburated engine, is that cylinder starved at higher RPMs due to a vortex in the air flow, i.e. in the intake?
3. As mentioned earlier, valve float at high RPMs due to weak springs.
4. Is the cam new or is the cam lobe worn on that cylinder more than on others? Is that lobe higher than others, was this checked out before installing?
5. The distributor parts turn and this can cause runout at higher RPMs if the rotor bearing is worn, which can cause misfires.
6. The rotor itself can move up or down if rotor is worn, causing collisions on distributor cap.
7. The distributor cap can have cracks or problems seating on the rotor, yes, even if they are new, which can cause problems with firing.
8. The plug wires could be the wrong type for the distro/plugs causing unmatched firing characteristics.
9. If a condensor is used on the distro, it could be causing this behavior.
10. The spark plug itself could be the problem, try a different brand or type.

As far as checking the firing for this cylinder, a current probe around the primary of the coil would work at low voltages and when a misfire occurs, the shape of the pulse can tell you a lot about what the problem might be. The voltage on that pulse will rise if there was no spark due to an open and will fall if there was a short. You can use a portable scope or one of the modern automotive computers with a scope on it to do this, such as the Autel or OTC, or a portable o'scope such as the µScope, using a current probe.

There are many more reasons that this could happen, hope this helps...

[joeqsmith]

Ah, the two step.  Boom, bang, boom.   

All hell breaks loose. 

Buddies 20B on Meth with 40lb of boost is something that is heard for miles ! 

Oh yes Joe, I left EGT off the list. 

Mine are pretty tame.   My street bike is injected and the low speed limiter on it is so smooth you hardly notice it. 

[Jim-0000]

............................ Autel or OTC, or a portable o'scope such as the µScope, using a current probe.

Thanks for the reply.
I would like some more details of these scopes. Can you provide full names and any links please?
We are considering purchasing one specially for working on the two cars owned by the brothers. I will order one of the auto probes from Aliexpress linked to somewhere in a post above.

Jim.

[tpowell1830]

............................ Autel or OTC, or a portable o'scope such as the µScope, using a current probe.

Thanks for the reply.
I would like some more details of these scopes. Can you provide full names and any links please?
We are considering purchasing one specially for working on the two cars owned by the brothers. I will order one of the auto probes from Aliexpress linked to somewhere in a post above.

Jim.

I will do you one better and give you a link to an awesome mechanic who uses these tools and the µScope for troubleshooting, link in comments for scope.



Hope this helps...

[Jim-0000]

After another long day working on the problematic Holden grey motor (circa 1950,s), the two brothers, one of whom owns the vehicle have reluctantly agreed to change the plug leads over to solid cored ones. I first mentioned this many weeks ago. Their first response was: "we don't like copper cored leads"!
New spiral wound, stainless cored leads have been ordered on ebay and we will await delivery. (It appears that copper cored leads are no longer available)  Local suppliers could not get them, which seems astonishing to me! The also ordered some Platinum spark plugs.
Meanwhile, I found out that a fellow member of the local Mens Shed does have an oscilloscope along with two suitable probes for ignition analysis.
If the new plugs and leads does not fix the miss fire, we will be borrowing that and using it on the engine. I will ask the owner of the scope to come out and assist, as he is an electronics engineer.

So, we will have to find the problem some day soon, I figure.

[Edison]

So, we will have to find the problem some day soon, I figure.

Is the fault really in the electricity?
Our racing car was a faulty fuel pump - idling OK, irregular running under load, random performance drop - replacing the fuel filter = improvements, replacement of pump = fault has been rectified 

[Jim-0000]

So, we will have to find the problem some day soon, I figure.

Is the fault really in the electricity?
......................................

I am not at all sure and will not be until and if we put an oscilloscope on the engine plug leads etc.
However, we think it is the electrical system. The observed behavior that supports this is:

1. The miss - fire is worst at idle and mid range steady RPM
2. When revved up hard, it disappears, or almost does so.

My assumption is if the problem was lack of fuel, the above would be opposite.

The two mechanics assure me that there is plenty of fuel pressure. I have asked that question enough times to exhaust their patience.
Thanks for the tip though.
There is a fuel pressure gauge somewhere on the car I think? I will look next time we are working on the car. Will not be doing so for a week or so. It is here next door to me, but all covered up with tarps etc.

[Edison]

Checking the ignition is the first!
Minus control - battery / chassis - chassis / engine
Is the injection system fully electronic?
Do you have? speed sensor, engine vibration sensor, air weight, lambda .....
Is the carburettor or ecotronic system?
Hall probe in distributor, burned distributor cap,  reading in chambers (diaphragms, hoses, pressure sensors) jets, float, leaky chambers ........

   there's a lot of it

[Edison]

 little out of far away I would have stopped 

[tautech]

So, we will have to find the problem some day soon, I figure.

Is the fault really in the electricity?
......................................

I am not at all sure and will not be until and if we put an oscilloscope on the engine plug leads etc.
However, we think it is the electrical system. The observed behavior that supports this is:

1. The miss - fire is worst at idle and mid range steady RPM
2. When revved up hard, it disappears, or almost does so.

My assumption is if the problem was lack of fuel, the above would be opposite.

The two mechanics assure me that there is plenty of fuel pressure. I have asked that question enough times to exhaust their patience.
Thanks for the tip though.
There is a fuel pressure gauge somewhere on the car I think? I will look next time we are working on the car. Will not be doing so for a week or so. It is here next door to me, but all covered up with tarps etc.

Check all the A/R system.....hoses, fittings, points plate free, flyweights and springs OK and sometimes old dizzy shafts need rebushing so that points gaps and dwell angles remain consistent throughout the rev range.

[maginnovision]

Two 'career mechanics' should not need so long to figure something like this out. Especially when you consider the symptoms stayed when the ignition components were swapped. I'd recommend taking it elsewhere no matter what the actual issue ends up being.

[Jim-0000]

Two 'career mechanics' should not need so long to figure something like this out. ...

I could not agree more!

Especially when you consider the symptoms stayed when the ignition components were swapped. .....

Also, the missfire was there with the first (or original) engine in the car when it was first acquired. So, much more than ignition components have been swapped. I might have already mentioned this, but the triple 11/2 SU carbs were swapped for a single (forgot the brand) carb and a new manifold fabricated out of two, all in an attempt to fix this miss fire.
The engine was swapped as this one was being modified extensively for performance and the original one was being used meanwhile.

I'd recommend taking it elsewhere no matter what the actual issue ends up being.

You have lost me here. If the "actual issue" is found, there will be no need to take it "elsewhere"?

[Jim-0000]

...............................
Minus control - battery / chassis - chassis / engine.................

The ignition system has been jumpered wtih a separate battery and jumper leads, all independent from the cars wiring. Same result.

Is the injection system fully electronic?..............

No injection system. Please read other posts - naturally aspirated. Two carb systems tried. Same result.

Do you have? speed sensor, engine vibration sensor, air weight, lambda .....
Is the carburettor or ecotronic system?...........No. All analogue. Old world systems. 1950 vintage motor.

Hall probe in distributor, burned distributor cap,  reading in chambers (diaphragms, hoses, pressure sensors) jets, float, leaky chambers ........

Both hall sensor triggered ignitor system and kettering (points and capacitor) system tried. Symptoms worst using electronic ignition alternative.
At least two new distributor caps tried.
Thanks,

Jim.

[maginnovision]

Two 'career mechanics' should not need so long to figure something like this out. ...

I could not agree more!

Especially when you consider the symptoms stayed when the ignition components were swapped. .....

Also, the missfire was there with the first (or original) engine in the car when it was first acquired. So, much more than ignition components have been swapped. I might have already mentioned this, but the triple 11/2 SU carbs were swapped for a single (forgot the brand) carb and a new manifold fabricated out of two, all in an attempt to fix this miss fire.
The engine was swapped as this one was being modified extensively for performance and the original one was being used meanwhile.

I'd recommend taking it elsewhere no matter what the actual issue ends up being.

You have lost me here. If the "actual issue" is found, there will be no need to take it "elsewhere"?

What I mean by the last bit is that even if the actual issue(misfire is a symptom) has/had anything to do with the ignition system it still would be/would've been the right thing to take it elsewhere. Unless of course you don't mind the time lost and the work is very cheap.

[Jim-0000]

What I mean by the last bit is that even if the actual issue(misfire is a symptom) has/had anything to do with the ignition system it still would be/would've been the right thing to take it elsewhere. Unless of course you don't mind the time lost and the work is very cheap.

I would not have a clue where "elsewhere" would be to take it, even it if was mine!