Low Vacuum Pressure
 

Hi,

My car was on the rolling road on tuesday but low vacuum pressure was detected and a boost leak the leak has been fixed but the vacuum pressure is still low. does any body have any suggestion no leaks have been found so what else could it be ??

Not sure, I would check that the throttle plate is able to close properly and air isn't leaking in through the idle control valve sticking etc. Also might be worth checking the brake master cylinder power assist vacuum chamber thing (don't know what its proper name is :lol1: ) in case that's leaking, but its probably not.

My bet is the idle control valve leaking air into the inlet manifold preventing the negative pressure reaching normal levels.

What model / year is it?

If you mean idle vacuum, it might be worthwhile doing a precautionary compression test - as much to rule that explanation out as anything else.

Assuming its not idling at high speed it unlikely that a leak would cause poor vacuum readings as it would also increase the idle speed.

A major cause of poor idle vacuum is high electrical/mechanical load (idle valve open more than it should to be in order to maintain idle speed) or incorrect cam timings. Large cams or incorrect cam timing has a significant effect on the vacuum at idle reading.

Retarded ignition timing would also have the same effect but is under ecu control so unlikely to be that.

Lastly poor compression could cause the same issue (large cams have the effect of reducing the dynamic compression ratio), but I hope its not that as that's expensive to fix.

Quote-
My bet is the idle control valve leaking air into the inlet manifold preventing the negative pressure reaching normal levels.



What should negative vacumn read then?at idle.and what does it mean if negative readings are higher than norm (eg minus 15/20)

Vac on my MY93 is circa -0.7 bar, which is also roughly...

* -10 psi
* -0.7 kg/cm^2
* -20 in Hg
* -50 cm Hg
* -500 mm Hg

...depending on your particular gauge's measuring units. :)

Yep that sounds about the same as mine. Nearly 50cm Hg with no electrical loads and hot weather.

To anyone who's checking their vacuum - it will be significantly less vacuum until the engine is warm though, and only valid for circa 800rpm idle.

If you have less vacuum it really means the engine isn't making as much power as it should for the amount of air going in - e.g. for a standard 800rpm idle more air is going in than what would normally be required. Reasons include, big cams (hurt idle significantly), retarded ignition timing, incorrect cam timing, poor compression etc etc.

Generally speaking its not a particularly useful measure as it vaires with engine load(lights on, flat battery) but if its way out then you'd want to think about getting a compression test done.

well f##k me!!all this time i thought i had something wrong.seems my gauge reads correct and achieves 14.5 / 15 psi on boost and reads -20 in hg.seems about right then :)

Yep, spot on vac. :)