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Author: WisdomAugust

Using Oscilloscopes on Vehicles

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 Author| Post time 2017-9-4 08:13:10 | Show all posts
Worn Cam Lobe:
This problem is not a daily occurrence, but we're exploring how to interpret what the scope
is trying to tell us in the waveforms. Following picture looks almost similar to the lean injector
in picture 4 or picture 7, but the difference is in the kilovolt demand and the level where the firing
line starts. We need to compare this with a good cylinder (see No. 5). This is a case of reduced
volume of HC and not a lean air/fuel ratio.

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 Author| Post time 2017-9-5 15:52:25 | Show all posts
Edited by WisdomAugust at 2017-9-6 15:34

Worn Cam Lobe



When there is less volume of both air and fuel, there is going to be less compression and
therefore a lower kilovolt demand. Misfire depends on how severe the valve lift is affected.
With a 20 percent lower cam, the misfire might occur beyond 2,000 rpm, while a 40 percent
reduction in valve lift might begin misfiring at 1,600 rpm. No lift at all obviously will be dead
at any rpm.

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 Author| Post time 2017-9-6 15:33:20 | Show all posts
Inadequate fuel supply:
Whether the reduced fuel source is caused by the fuel pump, pinched fuel line or
dirty fuel filter, the scope does not know the difference. When you take a close look
at picture 6, you will notice that the fuel starvation could be anywhere on the firing line.
The next step is a simple current test as illustrated. Under load, the engine might start
to buck and hesitate. But long before the intermittent misfire happens, indicators are
present as flashes, moving arbitrarily from cylinder to cylinder progressively getting
worse with increased load. This is where preventive maintenance pays off.

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 Author| Post time 2017-9-7 09:03:54 | Show all posts
Fuel Pump or Filter



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 Author| Post time 2017-9-8 13:06:34 | Show all posts
Misfire under load:
When a misfire code tells you which cylinder is at fault, it is like diagnosing blindfolded,
because you still don't know why, the condition when or how often it did repeat, and how
many other cylinders are close to the same ordeal. Picture 7-1 indicates that the misfire
was fuel related with only about 25 percent active combustion. We also know that it is not
an intake leak, because it appeared only under load.

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 Author| Post time 2017-9-11 09:36:04 | Show all posts

Misfire Under Load



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 Author| Post time 2017-9-12 09:14:42 | Show all posts
We can tell that it was not lean at the start, because kilovolt demand is normal and
the firing line is not elevated. We can readily see it is not a dirty screen or an odd spray
pattern. If this pattern remains permanent and does not vary, it is not cleanable and is
likely a high electrical resistance problem getting a late start. A simple replacement of
the injector would suffice. The fact that a scope can provide all that information in detail
eliminates trial and error repair methods.

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 Author| Post time 2017-9-15 10:09:56 | Show all posts
Valve seating:
This is a common but often neglected problem. When a valve is not seated,
it does not cool, which might lead to a variety of possible failures. It is easily
spotted with a scope at idle, preferably in drive. A non-seating valve, whatever
the cause, allows compressed gases to escape. Everyone knows that a burned
valve is very noticeable at idle, but at 60 mph, that small crack does not make
much difference, because there is not enough time to escape. For that reason
testing at low speed is essential.

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 Author| Post time 2017-9-16 09:33:27 | Show all posts
The other reason for analyzing at idle in drive is because the spark occurs near TDC
when compressed gas movement is at its peak. The effect of these escaping gasses
can be seen on the scope as a hash at the beginning of the firing line.

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 Author| Post time 2017-9-18 08:09:22 | Show all posts
We will see a low kilovolt because of low compression, directly followed by hash
caused by air flow distorting the spark. Depending on frequency, the problem could
be traced to worn valve guides, carbon deposits, weak valve spring or poor lubrication.
Conclusive diagnosis depends on how many cylinders are affected and if the problem
is intermittent or permanent.
The value of a scope is apparent only when the potential is fully understood. Let us
assume that every cylinder performed as it was designed. The air inhaled is determined
by bore and stroke of the piston. This air is matched with a precise amount of fuel and the
process of total combustion matches the coil output energy with very little residual energy
to spare. All cylinders are created equal and should follow that precise pattern of combustion
efficiency. Any deviation will affect the firing time and alter the residual coil energy. Because
the lab scope is time based, there is no better tool to measure, compare and display combustion
efficiency per cylinder.

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