General Information

The idle quality test allows more in-depth diagnosis of such engine complaints such as rough idle, missing when idling, or delayed starting time.

The test scope is divided into:
- Idle quality, engine speed per cylinder.
- Idle quality, burn time per cylinder.
- Idle quality, while starting.

It is necessary in all three test sequences to evaluate the tabular examples and/or the graphic examples. The two should be compared and checked for agreement.

When evaluating the idle quality measurement, the following should be noted according to the number of cylinders the engine has:


4-Cylinder Engine:

Figure 1:

a) Speed drop from previously firing cylinder
b) Defective cylinder

On 4-cylinder engines, the 2nd engine speed drop, or the following drop in the case of multiple failures, should be evaluated.

With multiple failures, the oscilloscope pattern should read from the bottom up.


6-Cylinder Engine

Figure 2:

b) Defective cylinder

The first engine speed drop should be noted.


8-Cylinder Engine:

Figure 3:

The first engine speed drop should be noted. The engine speed drop occurring after the defective cylinder can be ignored.


12-Cylinder Engine:

Figure 4:

Figure 5:

Retrieving the idle measurement value is difficult due to the large number of cylinders and the simultaneous firing of both ignition circuits. During a smooth engine idle, a small engine speed difference may occur between the cylinder banks in the engine speed table and engine speed graphic. As can be seen in the table, the average engine speed change value of one cylinder bank may exhibit an engine speed decrease when compared to the previously ignited cylinder of the other bank (See Figure 5). For this reason, idle quality interpretation via the table (arrows) and the engine speed graphic is more difficult.

Figure 6:

In certain cases, the problem may present itself differently from engine to engine. Therefore, it is almost impossible to determine fault assignment from the engine speed table.

Figure 7:

By tracking the engine speed of the individual cylinders (starting from the left column), it is shown in the examples given that cylinder 4 (See Figure 7) and/or cylinder 10 (See Figure 9) is the defective cylinder resulting in a engine speed decrease and engine running complaint.

Figure 8:

One method to identity the correct cylinder is to print out the engine speed graphic and draw in two lines. The lines should intersect the trailing edge of individual engine speed segment for each cylinder bank (Figures 6 and 8).

Figure 9:

The subsequent, and somewhat larger engine speed decline should not be observed for evaluation. In order simplify evaluation, the dotted course in the given example represents the engine speed sequence without a running complaint (Figure 4).