Low Vacuum at Idle

XJ-S
1

1988 5.3 V12 with about 50,000 miles

Once warmed up, I’m only getting 16 inHG vacuum at idle (in Park). When going into Drive, it drops down to 14 inHG. This probably explains my not-perfect idle, and bad gas mileage. Car otherwise runs very well, plenty of power.

I know these cars are supposed to have 18inHG at idle minimum. Roger Bywater says if it’s lower, then something is wrong with the engine.

I have checked compression and it is high across the board, 215 - 225. I have set timing to 20 BTDC at 3000rpm. It’s about 4 BTDC at idle. As a test, I applied full vacuum to vacuum advance to advance the timing at idle, and the vacuum only went up from 16 to 17. Another test, I tripped the full throttle enrichment at idle, and the vacuum went up from 16 to 17.

I know I need to give this car an Italian tune up (I will get new tires in the Spring), but my questions:

  1. If an ITU will burn off carbon deposits and help bring up the vacuum at idle, then how are the carbon deposits lowering vacuum? (just trying to learn)

  2. Or could I have a vacuum leak somewhere? I have not found any, but am thinking about a thorough smoke test to make sure. Or does a vacuum leak not make a difference to engine vacuum? It’s still sucking the same, just getting air from alternative sources…

  3. Could PCV system contribute at all to a low vacuum?

2

Advancing your timing a little more should increase your manifold vacuum.

3

Even in P, an A/T will cause lower vacuum readings than a M/T because the engine is still driving the fluid pump in the tranny.

4

Those timing numbers are a bit odd. A generic comment as I am not familiar with this engine.,

  1. Static, at rest is similar to idle. 10 to 14 BTDC ?

  2. Vacum advance brings it up, more than 16 or so?

  3. Centrigugal; adds more.

  4. All in more like 30 or so???

5

The US imported V12’s with lower compression are supposed to be 18 BTDC @3000 rpm with no vacuum advance. Idle supposed to be about 0-4 BTDC with no vacuum advance.

W/ vacuum advance, I think mine is about 14 BTDC at idle? I also have the vacuum switch/dump valve which supplies about 8 inHG to vacuum advance at idle.