Using the vacuum gauge

[cj847]

The other day I happened to check my engine vacuum while working on a transmission shift problem. I know that you can tell a lot about the internals of your engine by using a vac gage and I have used one several times in the past.

However, I have a really good running crate 350 and was expecting a "no problems" reading. Unfortunately, I didn't get that. At idle, the needle bounced 2" between 17 and 19. It would smooth out at about 1500 rpm at 20". With a quick throttle blip, the gage would go to zero.

Now let me say, I am perfectly willing to let sleeping dogs lie as the engine runs perfectly, doesn't use oil, doesn't smoke at start up. It is the best running SBC I've ever had and I've owned at least 15 over the years.

So, I checked the various websites for diagnosis:



The truck was about due for a tune up so I thought I would try it. The result was the vac gage read exactly the same.

Next I looked at On All Cylinders:


I first did a compression test to hopefully rule out worn rings (based on the zero at throttle. All cylinders were 140 to 145 psi. To me this indicates an engine with some miles on it but certainly not worn out. It doesn't burn any oil. I did squirt some oil in 2 or 3 cylinders and the compression did rise to 160, which was the reading I was hoping for. This befuddles me - the engine doesn't burn oil, doesn't have excess crank pressure causing oil to blow out the valve cover, etc. No, indications of even the amount of wear I found here. PS> I did drive the truck on a 75 mile cruise after the tune up and the spark plugs look great.




The 2" problem should be worn valve guides according to on all cylinders. I haven't had time to check them yet but will report back. I can say for certain that the guides are not worn enough to have effected the seals as I do not have an issue with smoke at start up.

Long story short, I have a perfectly running engine, that apparently has something wrong with it.

What should I do: continue to investigate or stop looking at the vac gauge and be fat, dumb, and happy?