Using Oscilloscopes on Vehicles

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Misfire Under Load

WisdomAugust

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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Fuel Pump or Filter

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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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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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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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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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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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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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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.