While trying to increase MPG I installed an autometer phantom vacuum guage. What i notice is that upon starting the truck the vacuum reads 20 in. which is good, as the vehical warms up the reading drops to 15 in. This isn't normal is it? Or do I have a problem? The truck runs fine no noticable miss or stamer on accelertion, just a drop in vacuum.

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I was wondering the same thing. I installed a Vacuum Gauge today and mine idles at 14. Is this normal?

automatic choke, dude.. think about it.

Where's the carb?

same concept tho, no? higher rpm's until the engine is warmed up some, right?

yea, the engine is at higher rpms at first start up...so that would explain this.....you should notice the same thing with your oil psi....

when checking vacuum on my other car, it maintains 20 in' no matter how warm it is. It only drops when you accelerate.

i may be wrong, but i also think the vac should be the same at warm idle 600 rpm as cold idle 1000 rpm. if you want to investigate some, temporarily disconnect and plug all the vac port at the manifold. if vac improves - it points to a vac leak on that line.

I installed a vac gauge a few months ago and at cold or warm idle, it reads 20.

When I installed the gauge, I accidentlly bent the nylon tubing when trying to wire it throught the firewall and stuff. I was mad but ohh well. That's what it reads at idle....I get about 300 miles per tank full. After the install i noticed that when i drive with O/D off my vacuum is higher than driving with O/D on, even on the highway. So Im going to do a test run and see if i get better mpg driving with O/D off. I run about 2500 rpm on highway with O/D off going about 65mph - 70mph.