fusion-turbo

I was curious how much engine vacuum is normal at idle, and also at highway coasting? My brake power booster doesnt seem to "boost" enough power, and i was wondering if this could be part of the problem.

Im seeing about -10 psi at idle and -21 or so while coasting.

THANKS!

88t2romad

no, mines at +10 psi, lol.

Mine reads about like that, i got an autometer boost gauge

NZConvertible

The numbers aren't psi, they're inches of mercury (inHg). 10inHg is very low, and indicates either a vac leak or low compression.

inflatablepets

17-22 is where vacuum should be at idle. INHg that is.

WAYNE88N/A

Sounds like a compression test is in order

fusion-turbo

The compression since i last tested is fine. I have a FAZE boost guage and it measures in PSI, not inHg.

Correct me if im wrong, but cant you measure vacuum in PSI as well? The car see's -13 or so PSI at idle. How much would that be in inHg?

NZConvertible

You can measure vacuum in psi but almost nobody does. All non-metric automotive boost gauges seem to use psi for the boost side of the scale and inHg for the vacuum side. Have a closer look at yours and it will most likely say that. 1psi is approximately 2inHg.

I know your gauge has vacuum marked in inHg because in your first post you said you see "21psi" under decel. Under those conditions 21inHg of vacuum is normal but 21psi of vacuum is physically impossible, since absolute zero pressure is -14.7psi. You can't go any lower.

Now you say you have 13 "psi" at idle, before it was 10. Which is it?

fusion-turbo

Ok, then it must be inHg, although it only says "VAC"; not what scale its measured by.

The vacuum at idle really depends. If the car is on a cold start, it will measure about 14 or so inHg, but the vacuum will vary as the idle jumps around (car has a rough cold start... still trying to figure that one out. Any ideas?)

But when the car is at Normal Opp Temp. it will generally sit at around 11 inHg at idle.

NZConvertible

That's pretty low. Assuming idle speed is above 700rpm, low compression and/or a vac leak are about the only things that'll cause such a low idle vac.