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:
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)
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…
Could PCV system contribute at all to a low vacuum?