Figure 1
A vacuum gauge is a powerful diagnostic tool. Intake manifold vacuum can expose a number of problems such as a valve leaks, timing adjustment, fuel mixture. The gauge can expose a problem, but cannot pinpoint the problem cylinder. A vacuum transducer and the scope channel function in the PDA Engine Analyzer can help to identify the problem cylinder or cylinders. The vacuum signals in this tech tip, Figure 1, were captured using an EMS Vacuum Transducer. When testing vacuum signals, it is best to use the largest vacuum port that is central in the intake manifold. The vacuum waveforms should have a uniform shape and amplitude for each cylinder. The example above has two signals that have lower amplitude than the others indicating a problem with one or both cylinders.
The vacuum event occurs before the ignition firing. The easy way to identify the vacuum signal is to write down the ignition firing order and advance the vacuum order 50% ahead of the firing order.
Firing order |
1 |
8 |
4 |
3 |
/ |
6 |
5 |
7 |
2 |
Vacuum order |
6 |
5 |
7 |
2 |
/ |
1 |
8 |
4 |
3 |
Cylinders #2 and #3 show weaker vacuum signals than the others.
