'87 Chevy G20 Van
At Idle, I have about 19-20 inches of vacuum...running down the road at
50-60 there is about 9! as soon as a mild hill is encountered, 3 inches of vacuum! It pulls the hill with little loss of speed.Why is the vacuum so low going down the road?
The vacuum drops as the throttle plates are opened....this is normal behaviour. Pick a cruise speed where you have maximum vacuum and this where you will get the best mileage.
Get a backpressure test done. Probably the CAT is plugged up big time.
I know this! That IS the best the vacuum gets! It should be MUCH higher!
No cat. Was removed by a previous owner.
My uncle used to time his truck by the vacuum gage. Don't remember exactly how he did it, but I do remember the drastic difference a few degrees would make.
Maybe the timing has slipped or the chain has jumped a notch.
That would affect idle vacuum to though. I am installing a new timing set this week along with a water pump. Both have 140K on em. I will see if that does anything.
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