S: Gauge shows low vacuum while idling
S: Your gauge shows low vacuum idling. Car feels very shaky – may stall if revved & throttle is released.
C: There could be several causes why vacuum is low.
Starting with the easiest to the more painful to fix, here they are:
1. vacuum leak(s) on the intake piping (usually via vacuum hoses or cracks in the piping)
2. vacuum hoses on the intake manifold (past throttle body)
Usually the greater the difference between good vacuum level and bad vacuum level indicates how big the air leak is.
3. EGR – the diapraghm is known to leak.
4. Leak around throttle body.
5. Intake manifold gasket leak.
6. BLOWN PISTON RINGS or BENT VALVES / Head Gasket (?) *** (check compression / leak-down)
Solution: To diagnose vacuum leaks – visual inspection helps but it can be hard to trace in a maze of hoses & cracks almost impossible to see. Instead opt for air compressor – just ensure it is set to moderate pressure levels. Do not worry 100 psi will not blow away your manifold right away or create leaks if you only have a 1/4″ hose – but pay attention to your boost gauge or whatever gauge you have on the large piping to ensure you do not run over pressure it is rated for. I am checking my stock ka-t setup to 20 psi and can easily spot if something is not right – I found that my EGR leaked past 4 psi so that got fixed
Once you find out you do not hear audible leaks and haven’t found the reason for the leak, do a compression test. And if it looks good – and you are sure there’s no leaks, it’s probably your timing that’s off – a symptom of that would be really bad uneven idle.