1990 535i Vacuum reading

[scott 4991]

I read this on the group.. not sure which car...

At idle 650 rpm = 8.5 inHG steady
At 1000 rpm = 10 inHG and steady
I held the RPM's at several levels and the vacuum increased with rpm's and held steady in each case with no fluctuations at each rpm.

I wanted to check my vacuum, and as close as I can tell I read 3 inHG at idle.

****** Is this to low? *****
Should I be looking into a vacuum leak somewhere?
*** What issues will I see if the vacuum is to low? ***

My idle is rock solid.

Also, as I increase RPM the vacuum goes down a bit, then returns to 3.

I have looked in the Bentley manual, dont see any reference to this.

Thanks for any comments.

sc

[Martin in Bellevue]

My 535 with a 284 regrind cam, idles at 19"Hg. The purge valve is blocked, as is the vacuum line to the breather hose between the valve cover & the intake tube in front of the throttle.
I am using a map sensor as replacement for the stock afm. My 535 idles right over 800rpm, when warm. If your car runs fine, why are you interested in the vacuum?

[scott 4991]

Thanks for the info Martin - I am interested as I have never really monitored the Vacuum before - so I don't know much about it. It's a learning process for me.

What do you mean by this? "The purge valve is blocked, as is the vacuum line to the breather hose between the valve cover & the intake tube in front of the throttle."

Should this read, "The purge valve is blocked, (IT) is the vacuum line to the breather hose between the valve cover & the intake tube in front of the throttle."

I will look up the Purge valve - but maybe you can explain a bit more for me?

BTW - your motor looks GREAT - what do you use on it? Mine will look like this soon!

sc

[winfred]

the purge valve is the valve that directs the stored gas tank vapors into the intake when the engine is started, the other line plugs into the bottom of the breather line that goes to the air boot from the valve cover

[scott 4991]

OK,the Chilton's book I have tells me how to check this valve and circuit! I did not see it on the Bentley book.. hummm..

Anyway, it indicated the valve is located under the air intake hose or intake manifold - I know where this is. The valve has 1 electricial connector and 2 vacuum lines, 1 leading to the manifold vacuum and the other leading to the carbon canister. I will check this tomorrow. Thanks for the hint - hope I find something.

sc

[Martin in Bellevue]

Should this read, "The purge valve is blocked, (IT) is the vacuum line to the breather hose between the valve cover & the intake tube in front of the throttle."

I will look up the Purge valve - but maybe you can explain a bit more for me?
sc

I meant that, on my motor, I blocked off the intake manifold port to the evaporative cannister as well as the manifold port to the intake tube. This was to condition the manifold vacuum signal to the split second box using vacuum & rpm to provide a simulated afm signal.

[scott 4991]

Oh, I get it now.. I will still check mine tomorrow, but wonder what others read with the car w/o mods.

sc

[scott 4991]

How is vacuum created in the car? Where does it originate? I'd like to track this and see it, at some point, my vacuum increases to 10 inHG or something like this.

Thanks

sc

[scott 4991]

Tested at the manifold, 15"HG and steady - this was a good sign for me. I then cleaned the oil stick "O" ring and tightened the valve cover 1/8 turn on each - the vacuum went up to 7.5 and steady at the carb - I think this is a good reading lacking anyone elses input. I'd way up from 3 at the same measuring point yesterday.

sc