mx5cartalk.com

i installed a mechanical auto meter vac/boost gauge in the SP a few weeks ago, and the vacuum readings seemed pretty normal.
vacuum readings were:
- cold start idle: 19 inHg
- warm idle: 21 inHg
- while engine braking: 25 inHg

2 days ago however, i noticed that my vacuum readings had changed.
vacuum readings are now:
- cold start idle: 14 inHg
- warm idle: 16-17 inHg
- while engine braking: 19 inHg

all readings above are with AC off.

the car still drives like normal - no noticeable power loss, no noticeable change in fuel consumption, idle speed is normal.

naturally i suspected a leak in the pressure line to the boost gauge, so i checked it over but can't find any problems. there are no hissing noises in the cabin or in the engine bay. at WOT, i'm still getting 7.5-8 psi boost, which seems to indicate that the problem is not a leak in the pressure line to the gauge.

since the car is behaving normally i suspect a fault in the gauge itself, but what are the symptoms of a low vacuum problem?

does anybody have a hand held vacuum gauge i could borrow?

Considerint the differences in vacuum between cold idle, warm idle and engine braking have stayed similar, it sounds like its a calibration issue with your sensor to me.