How-to: Diagnose engine problems, with a vacuum guage

[nedge2k]

I get these regular emails from Wolf Motorsport with tuning and diagnostic tips. More often than not it's fairly basic stuff but occasionally there's a really interesting one. The following is an excerpt from this weeks email, from the way it's worded it should apply to most engines....

When diagnosing problems on a vehicle you must have a good understanding of what an engine requires.

An engine requires good airflow in and out of the engine, spark and compression. Any one of these not working correctly and you cannot expect the engine to run correctly.

Most of these problems can be indicated with a Vacuum gauge. If you connect a vacuum gauge to the inlet manifold and look at the readings, on a standard engine the manifold vacuum at idle will read approximately 21in/lb.

At a wide open throttle you should see no vacuum in the manifold and the gauge should read 0 in/lb

I will now tell you of some quick vacuum gauge tests you can perform yourself.

When using the gauge look at the needle if the following happens .....

Needle stays still at idle around 21in/lb - this is a normal engine.

Needle has a pulse at idle between 15-21 in/lb approx, the engine has a burnt or leaky valve. This causes a pulse in the manifold when the valve leaks.

Needle is still at idle but pulses at high revs. This indicates weak valve springs, as they are not sealing under higher revs.

Needle is bouncing at idle but is steady as you rev the engine up. This can indicate worn valve guides ( you must not get mixed up by having modified cams ).

As the throttle is moved to WOT (Wide Open Throttle), the Needle will move slowly from the initial vacuum reading to 0 vacuum (it should move as soon as the throttle is moved without delay). Do this test at 2,000 RPM or more to accentuate the symptom. This may indicate a blocked or choked exhaust.

Needle will initially go to 0 on sharp acceleration (at lower RPM). While holding throttle wide open, the needle will start to show more and more vacuum as the engine RPM increases. This can be a restriction in the intake system. A restriction can be too small a throttle plate, or an air filter restriction (maybe a dirty air filter).