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General Q on Vacuum Gauges

I don’t know if this question will make sense.



If I put a vacuum gauge on an engine and it gives me all of the signs of a “healthy” engine, what does that really tell me about the engine’s condition in terms of longevity?



E.g. if I get a nice steady 18-20in of vacuum at idle; a drop to 3-4in & jump to 24-25 on a throttle blip…stuff like that, what have I really learned?



Presumably it means that at that particular moment, the valves are functioning well, the head gasket probably isn’t leaking, there are likely no vacuum leaks, the compression is generally good, the exhaust system isn’t clogged…



But it can’t tell me that the car won’t spin a bearing or throw a rod tomorrow does it? I’m guessing nothing tells you that. But if everything looks really good “today” does it make it unlikely that something catastrophic will happen tomorrow? I don’t know…maybe I should have been a mechanic for a living.



I’m still just working on predicting the future and when I figure it out I’ll let everyone know so that you can win the lottery too.

You’re right; a vacuum gauge will not give you any indication about the crankshaft or bearing condition. It can only point out things related to head gaskets, valves, valve springs, rings, ignition timing, vacuum leaks, and so on.

If that example you give you actually do have an exhaust restriction if the needle returns to a higher figure before dropping back to normal.

E.G. 19" steady at idle, blip the throttle and the needle drops to zero (or should), and when the throttle is closed the needle should drop back quickly to the original 19". If it drops back slowly or returns to the 24" you mention before dropping back to 19" then there is a restriction in the exhaust system (usually the converter).

It’s also quite possible to have a car that appears to run fine and yet will have a partially clogged converter.
I ran into this on my SAAB a few months back while doing some maintenance on it. Just for the heck of it I threw a vacuum gauge on it and the needle would drop back about 1.5-2" above normal before settling back down after closing the throttle.
Since the converter is easy to get off of this car I yanked it off, knocked some of the substrate out of it, and sure enough; the converter was about 1/3 clogged.
(I’ve got a small pile of SAAB parts so another converter was not a problem.)

Hope some of that helps.

Extremely helpful - that’s what I need is the voice of experience.

All of the “instructions” I’ve seen though say that a throttle blip should drop the vacuum to just above zero (but not all the way), then jump back to 24-25, and settle back to normal idle. (If not it says worn rings). And that you look for exhaust restrictions by revving to about a steady 2K rpm and watch for a slow and steady drop. Do I have that wrong?

The long & short is that I am shopping used cars and all I really want to know about is the general health of the engine & transmission. My best tool for the engine is a vacuum gauge, and for the transmission is a smell of the fluid; a check for leaks, and a test drive.

When I find the right one I might take it to my shop for a once over - but that costs $$ and they probably mostly do what I do already.

While it’s possible I suppose to check for a restriction that way I’ve never done it by that method.
While it’s been many decades now the way I was taught back in some service schools I attended way back when was the method I mentioned.

To be honest, I never know whether to put much faith into the written word sometimes.
Some for instances.
I’ve seen recommendations for pulling a vacuum on an A/C system down to 29" and I’ve never, ever seen or heard of a vacuum pump being able to do this. Through several electric Robinaires and one air-operated pump 21-22" is about it.

I’ve also never seen a car that was able to pull a vacuum gauge down to 25". While this varies based on altitude, barometric pressure, humidity, etc. the absolute tops I’ve ever seen on any make of car was around 20-21". Here in my area of OK at about a 1100 feet altitude, 17-18" is the norm.
Maybe at sea level during high pressure it’s possible; I don’t know.

There’s also been some service manuals published with cylinder compression readings and some of them state that 120-130 PSI of compression is normal. This is utter garbage IMHO and any engine that has figures this low has problems.
The general rule of thumb on that is multiply the compression ratio X 18-20, etc.
(Again, varies by altitude, baro. pressure, etc.) and this will tell you what the figure should be on a good engine.

“To be honest, I never know whether to put much faith into the written word”

Amen. That’s partly why I’m here. (yes, this is sort of written word - but it more just normal conversation among folk).

btw: the other thing I’ve seen in general instructions is that a slow, steady fall at idle (rather than 2k rpm) indicates an exhaust clog. I’m guessing how the the gauge reads it will partly depend on how bad it is - so the SAAB wasn’t that bad (1/3 clogged is still 2/3 open), but if it was 3/4 or more clogged - likely a more obvious kind of reading that might show at idle I suppose.

The gauge isn’t too useful when the readings are normal. I guess you know that already. If the needle jumps wildly at idle, you might have something to worry about. It can also tell you if the ported vacuum valves are working properly, but you will first have to find out when they are supposed to provide vacuum. I am not tempted anymore to use anything that will fit in the hole. I used a wrong ported vacuum valve once and the heat riser valve was working the opposite of the way it was supposed to.