In today’s modern engines, what should the manifold vacuum read? Idle, steady cruise, etc. Full throttle I know. Is the value now lower compared to the old days? My 2005 Infiniti fx35 is about 10-15 in. Hg steady cruise. 20 at idle. What’s the consensus? Thanks.
Proper vacuum specs haven’t changed. 20in is pretty normal. The standard range is 16-22in at idle depending partly on altitude. The reading should be very steady at idle - no twitches or floating around.
While driving the vacuum will go up and down a lot - mostly according to what the throttle plate is doing. Anytime you open up the throttle the vacuum drops. Close the throttle and it jumps. So 10-15in cruising is plausible depending on that - though engine rpm also plays a role, though not as much a role as throttle opening.
It varies based on altitude, barometric pressure, engine type and wear and so on but I’d say what you have sounds normal.
Around here (elevation about 1100 feet above seal level) most cars run about 17-19 at idle.
20in vacuum at idle is at the upper end of the normal scale and would seem to indicate that the engine is in excellent condition and well tuned. I must concur with the previous comments. But I am curious. What prompted you to test the vacuum? Testing manifold vacuum became a lost art it seems.