* Throttle position: More throttle means higher manifold pressure.
* Engine speed (RPM): Higher RPM generally leads to higher manifold pressure (though this is also dependent on the camshaft profile and other factors).
* Engine load: Heavier loads (like towing or climbing a hill) will result in higher manifold pressure.
* Vacuum leaks: Leaks in the intake system will reduce manifold pressure.
What you *should* be concerned with is the *vacuum* reading at idle. A healthy 1986 305ci engine should have a manifold vacuum reading of between 17 and 22 inches of mercury (inHg) at idle. This indicates proper engine operation and a relatively tight intake system. Readings significantly lower than this might point to problems such as vacuum leaks, worn engine components (like rings or valves), or a faulty PCV system. Readings higher than this are less common but can be caused by issues like a misadjusted throttle linkage.
You should measure manifold vacuum using a vacuum gauge connected to a port on the intake manifold. You cannot determine the appropriate manifold pressure from any fixed setting. It's dynamic and reflects the engine's overall operating condition.
