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?

[Carl Spangler]

My WAG is valve guide seals need replacing.

[Steeveedee]

Are any of the plugs you removed a lot lighter than the rest? Could be a vacuum leak to one cylinder, which might show up as a lean plug. Otherwise, that variation points to worn valve guides. A good umbrella seal could keep the oil at bay, even with worn guides. Blipping the throttle and the gage going to zero is normal, BTW. I'd personally just drive it.

[geezer#99]

You got a crate 350, but which one?
Cam, compression and timing can effect your vac readings or compression numbers.
Tell us about your motor.

[Caddylackn]

Ignition problems could be the advance weights/springs in the distributor. Make sure these are operating correctly and there is not a broken spring or egged out holes in the weights. You can hook up a timing light which reads total advance then temporarily plug the vacuum line to the vacuum advance unit. If you see the same fluctuation pattern in the total advance readings as you see in the vacuum gauge, there is your answer.

You should not get any mechanical advance at idle, you want it to start at 1,200 to 1,500 rpm. Having weak springs that bring in mechanical advance at around idle can cause the symptoms you are experiencing.

[LNP]

An uneven air/fuel mix, erratic ignition timing, a misfire, misadjusted valves or a manifold leak near one or two cylinders also are possible causes. I would be sure all these are good before thinking a bad engine. For example a misfire at idle can be covered up when the rpm is raised. This could be a , plug, plug wire, distributor cap, ignition rotor, and/or a vacuum leak. After many many many many years as a wrench I learned always start with the basics first.

[Sheepdip]

If it ain't broke...don't fix it!
That is my opinion, and that's just simply my opinion.

[cj847]

Thanks for all the input. To answer some questions and comments:

-the engine is a GM crate 350 of the 265 hp variety (I think).
-the timing is set at 32 degrees total. I don't recall any fluctuation but will double check.
-I am beginning to suspect valve adjustment. The GM spec calls for 0 lash + 1/8 turn. I know that most people set SBC at 1/2 to 3/4 turn.
-I am fine driving it like it is. My only reservation is possibly burning a valve.

At this point, I kind of want to fix it just to prove the vacuum test really tells you something; make it run a little more perfect; and help out others who may have a bad running engine with a similar reading.

[jimijam00]

It ain't broke. Plugs look fine to maybe a tad to the lean side.

[HO455]

I would make sure I had no vaccum leaks, and then running the valves would be my next step.

[cj847]

Well, I adjusted the valves and no difference.
Checked for vacuum leaks using starting fluid.
Didn't notice any fluctuation in the timing light check.

At this point, I am out of ideas. I am going to drive it and I will report back if something happens.

[j_cst_10]

I'm confused as to why you think there is something wrong with it? The vacuum reading isn't bad and the plugs look fine. If anything you can make the signal more consistent by tuning the idle circuit properly.

When the signal is bouncing it is typically from the idle circuit being too lean. But otherwise there is nothing wrong.

[Wrenchbender Ret]

Check all the vacuum units hooked to the vacuum outlet on the intake manifold in back of the carb. for leaks. Like the power brake unit or the trans modulator or A/C, heater controls. A leak in that area can affect 2 cyls. & cause that. Try pulling a plug wire one at a time with the engine idleing to compare the amount of effect it has on each cyl. That can pinpoint you to the cyl. causing the problem.
George