15 in.hg. at Idle

[WhiteCloud]

I know this isn't a huge deal, but my vac is lower than it supposedly should be. I have no boost leaks (fixed the last one 2 days ago). I suppose it could be a kink in my boost gauge line.

At 850rpm idle, I have only 14-15 in.hg. at startup, and 16 when warm. When I rev the car and hold it at 1500 or so rpm, my vac drops to 20 and stays there.

Where do you think the vac leak is, or why would be vac be so low at idle, and drop upon revving the engine higher?

 

[oldman]

Make sure the gauge zeros out when you turn the key to the "on" position.

 

[Defiant]

Vacuum only exists between the back of the throttle plate and the tops of the pistons, so it greatly narrows-down where to look for leaks. Suspect the throttle shaft seals and manifold gasket, spray the areas down with the flammable aerosol of your choice (brake cleaner, WD-40, propane, etc.) and see if you get a bump in idle speed. Otherwise, valve seats or valve timing:
http://www.iwemalpg.com/Vacuum_gauge.htm

 

[WhiteCloud]

The gauge does zero out when I turn the key to the on position--I thought that might have been the issue at first too.

Defiant, I just did my tb shaft seals about a month ago, and also replaced the larger tb gaskets. My valve stem seals were replaced in January of this year, but I suppose there could be something wrong with the valves themselves.

I don't know what else to do from here, except to ignore it and wait until the situation gets worse, or the engine just doesn't start one day. What is really strange to me is that if I rev the car to 1200-1500 rpm, vac drops back to 20 in.hg.

I may do a compression test to see how compression looks.

 

[Defiant]

Throw another gauge on there and see what it reads. Could always be gauge error.

 

[WhiteCloud]

Actually, now that you mention that, I just switched boost gauges with a buddy (put in a 30-30 instead of a 30-20), and both autometers read the same vac.

One thing I did notice was that on both of our cars the plastic boost gauge line was slightly kinked near the fitting it screws into. His vac is dead on at 20, while mine is still lagging around 15 unless I rev the car to a higher idle. Weird.