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Author: WisdomAugust

Using Oscilloscopes on Vehicles

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 Author| Post time 2017-8-7 13:21:32 | Show all posts
What About Fouled Plugs?
Scope analysis has a lot to do with common sense. Let us imagine what a fouled plug looks like on the scope. No picture illustration to memorize, just use your imagination. There is no spark gap and therefore no nose or residual energy left. Because carbon resistance replaces the spark gap, the coil energy is drained off, as indicated by a curved slope from the kilovolt spike to the dwell line. Resistance of carbon foul could be as high as 2 Megaohms demanding more kilovolts than, for instance, a wet plug caused by a dripping injector. How do we know the difference? Very simple!

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 Author| Post time 2017-8-8 10:04:28 | Show all posts
Edited by WisdomAugust at 2017-8-8 10:07

Leaky Injector
At low RPM, it might look like a fouled plug, but at higher RPM the dripper might start firing
because there are fewer drips between firing events. But when this cylinder fires, the fuel
mixture is still rich and the scope shows a lower kilovolt plus a longer firing time with hardly
any nose. The computer looks at that dripper as too rich and subtracts fuel, driving the other
cylinders extremely lean.

There is one exception: the cylinder next in firing order, which benefits from the overflow of
the dripper. With computer controlled engines, total engine analysis of all cylinders tells us
the complete story and verifies diagnosis, and that is important to save valuable time.


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 Author| Post time 2017-8-9 08:20:31 | Show all posts

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 Author| Post time 2017-8-10 08:23:50 | Show all posts
Exploring Firing Time
The coil output is designed to maintain the firing event for as long as there is controlled fuel in the combustion chamber.
Because a collapsing magnetic field determines coil energy, engine RPM or load doesn't affect it. The firing time indicates
how well this energy is used in burning the fuel mixture. After establishing the validity of the electrical components at
idle, firing time is our ruler to measure conductance.
Because all cylinders and injectors on that engine are created equal, comparing the firing pattern is the easiest method
to seek out the cylinder that is different.

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 Author| Post time 2017-8-14 09:20:10 | Show all posts
Lean injector: In following picture, we noticed that cylinder B has a shorter firing time.
The absence of hydrocarbon is particularly visible at the end of the firing time, where it
terminates with a high nose. It is as if the coil is saying, "It's not my fault, there is plenty
of energy left." However, there is something else to observe in cylinder B. The evidence
of a lean condition already starts at the beginning of the firing time with a higher kilovolt
demand and higher firing line starting point in comparison.

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 Author| Post time 2017-8-15 11:43:54 | Show all posts

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 Author| Post time 2017-8-16 08:14:05 | Show all posts
Comparing with a good performing cylinder at the same speed or load is essential.
How do we know that "A" is OK? First, note that there is no coil energy wasted in high
kilovolt demand. Second, consider the smooth conductivity of the firing line of total
combustion as long as there is coil energy available. Finally, the nose tells us to what
extent coil energy is used up to burn all the fuel. This is a perfect match of air, fuel
and spark duration.
We do want to know the where, when and how before drawing a conclusion. Besides
location, it helps to know how many cylinders are affected and what the RPM or load
was when the problem occurred. It does not take any skills of scope pattern interpretation
to identify a shorter than normal or greater than average firing time to pinpoint the oddball.
Any short spark duration indicates a higher resistance either due to high kilovolt or reduced
HC. A longer firing time indicates a lower than average resistance.

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 Author| Post time 2017-8-17 08:26:42 | Show all posts
Helpful Hints in Scope Pattern Interpretation
Anyone who knows Ohms Law understands that the effect of resistance is in direct proportion
to current flow. When we have a poor connection at the battery post of as little as 0.01 ohm
with the headlights turned on, it creates only 0.1 voltage drop — not even noticeable in brightness.

However, during cranking, that same resistance at 300 amps cranking load means 3 volts less
at the starter. This certainly will slow down the cranking speed. This also holds true for air flow
or fuel flow. A kink in a garden hose does not make much difference if you want a cup of water,
but it becomes a problem when sprinkling the lawn. A fuel filter restricted at 80 percent will not
make any difference at idle, but the car will not have the horsepower to make it uphill.

The point is simply that testing under load makes all the difference to complete performance
analysis. Because you have the scope hooked up, it might take less than a minute in the bay
or up to 20 minutes on a test drive. There are different options or methods of testing under load,
but they all have two things in common. First, it needs to be done with someone in the driver
seat. Second, the leads need to be long enough to reach the equipment inside the vehicle.


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 Author| Post time 2017-8-18 13:25:36 | Show all posts
Test Results Under Load
Resistance and restriction or obstruction all mean the same thing and will have the greatest
negative effect when the demand for flow is high. Let us explore examples.
Restricted exhaust: When the engine cannot exhale properly, all functions are affected.
The firing line on the scope showing perfect at idle, becomes ragged and shows turbulence
progressively getting worse under load disturbing all cylinders. Plus, EGR is doubled or tripled.
Under normal conditions, the EGR valve re-circulates about 7 percent of the exhaust gasses
into the intake. However, with even a small exhaust restriction this might increase to 30 percent
or more. Momentarily inhibiting the EGR valve from functioning, while watching for improvement
on the scope pattern, is one way to verify restriction. Restriction will cause reduced volume and
is not a lean mixture as it is sometimes called. It is not affecting the air/fuel ratio.

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 Author| Post time 2017-9-2 16:33:40 | Show all posts
Edited by WisdomAugust at 2017-9-2 16:35

Dual exhaust:
This, from separate banks, can play tricks if one bank is restricted. The good (non-restricted) bank
will look extremely lean on the scope pattern with a high nose, while the restricted bank seems to
have a richer fuel mixture with hardly any nose. Let us assume that bank No. 1 is 50 percent restricted.
The MAF sensor will report this reduced flow as 100 plus 50 divided by 2 is 75 percent of total flow to
the combined banks. The computer distributes an equal amount of fuel to both intake manifolds.


The good bank No. 2 inhales 100 percent air and gets 25 percent less fuel allotment. The restricted
bank No. 1 inhales 50 percent air and gets the same amount of fuel. That is 25 percent more fuel than
this bank requires. See if you can find a lean cylinder and a rich cylinder in the illustrations that will
match bank 1 and bank 2.


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