Afternoon,

As I've been continuing my diagnosis for my rough idle/jerkiness of my N12, I found my differential pressure sensor (MAP/vacuum sensor) to be reading 970hpa at idle and most rpms with little fluctuation. After some converting to units I understand (inches of vacuum), I found 970hpa to be 2in of vacuum (nearly no vacuum) At sea level which is where I am, normal vacuum at idle should be between 17-22in. From here, I removed the MAP sensor and inserted a vacuum gauge to verify if the engine actually only had 2in, or if my sensor was faulty. To my surprise, my manifold vacuum was actually 2in--Not good!

I then ran static and dynamic compression tests (cranking and running) both of which past with flying colors (cranking PSI was between 210-220psi and running was 170psi) I have compression which eliminates a few variables.

At this point I decided to disconnect my VANOS solenoids. What do you know? I HAVE VACUUM WITH EITHER VANOS DISCONNECTED A full 18inhg at idle with VANOS solenoids disconnected. I AM STUMPED as to what would cause this except improper engine timing (chain skipped or is slack) but it still within enough of a threshold that VANOS can keep the timing proper but throw vacuum out the window

Has anyone seen this before or have any ideas?

Thanks in advance!

 

Vacuum is from the piston going down with the intake valve open, pulling against a closed throttle plate. So if you get vacuum only with the VANOS disconnected then there is probably overlap of both intake and exhaust valves open. (N18 has the Valvetronic variable intake valve lift system so it only has vacuum at cold idle) I am not familiar with the N12 but it sounds like cleaning the VANOS solenoids would be a good first step.. Or just replace them, not too expensive.

Any codes present?

 

thanks for the response!

I’ve already removed the solenoids and cleaned with no change. I’m going to run liqui molley engine flush through the engine tomorrow with an oil change after and see if the VANOS actuators themselves are sticky when cold with gunk.

no codes present—quite frustrating

 

I went ahead and ran liqui molly engine cleaner in the engine before my oil change—once the oil change was done the engine nearly stalled (hasn’t been that bad before) but once a few seconds past it was fine. The attached video is of the cold start VANOS data. Anyone read this and make sense of it? still no vacuum

 

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