Help me learn my vacuum gauge

I probably go in the declining mechanical aptitude category.

I am new to the vacuum gauge and having instructions and having experience are two different things.

The current car in question is a '97 Ford Escort (2.0L, SOHC, 4cyl, 8 valve) - 220K, but at the moment I’m mostly trying to figure out how to interpret the gauge.

My “shop” is my driveway which is at a little over 400ft above sea level. The car is mostly running fine but has a mild roughness to the idle that can come and go, and recently had a couple of misfire incidents.

Anyway…with the car warm and idling I get about 16.5lb of pressure - given that I’ve seen 17 as the bottom of the normal range I could justify subtracting a 1/2 lb or so for the 400 ft of sea level and take it as normal enough.

But, the 16.5 is not rock solid, nor does it change a lot. The needle flickers somewhat irregularly, but no more than about a pound - down to about 15.5 or up to about 17, but without any obvious rhythm. This does tend to go with the bit of roughness in the idle (idle falls a little, vac falls a little). It will increase and stabilize if I rev the engine a little, so I know that its not just that the gauge is a little whippy.

Of all of the instructions I’ve seen and found, none of them tell me what to do with that. I do get the general idea of engine vacuum and how the gauge works, but for an effect this small I’m not sure how to read it.

Does it most likely mean that things are probably fine, but that I have a small rough idle condition that shows up as these needle flickers, and could be caused by any of those things that cause rough idles (air, fuel, spark).

Or does it tell me something more specific?

Vacuum is measured in inches of mercury. 400 feet of elevation is nothing as far as atmospheric pressure is concerned. You probably have burned exhaust valves. If you do have that condition; you need a newer car. You can try having a remanufactured head installed or just replace the exhaust valves and hope they seal. A compression test will give you a better idea than a vacuum test at this point. Your vacuum readings seem a little low to me but I don’t know how much your engine should have.

yes sorry - change all of those lb to inches.

And thanks.

The symptom you have usually points to an ignition miss; plugs, wires, etc.
However, an ignition miss can be caused by low compression. Considering the engine has 220k miles on it the plugs should be removed and the compression tested as Step No. 1.

4450,I know you have a VW background.

VW was my first car to work on professionaly, and I learned not to make any diagnosis or conclusion without first checking compression from working on air colled VW’s.

It was amazing how many you would find with low compression,number 3 exaust valve was the bad one.

The checking compression first rule has served me well.

15-16 inches is pretty low for idling in neutral. What does the instructions say about idle vacuum? Does the vacuum come up when the engine is reved up and holding steady rpm? If the base ignition timing is correct, I would suspect you have an intake manifold leak – either a leaking gasket, hose off, cracked hose, or leaking vacuum device. It is also possible that this engine is just worn out and air is getting by the valve guides and seals and by the piston rings. How is the blowby generally?

I have not yet found specific vacuum pressure specs for my car, but I keep seeing the generic vacuum gauge instructions giving 17-20 inches, minus 1 in. per 1,000 ft elev as a rule of thumb. That includes the minimal instructions that came with my gauge (Actron off the shelf from an AP chain).

So 16 in. & quivering might be a bit low. The engine being generally worn out is a distinct possibility - many many miles and I didn’t have it for the first 90K of those. I’ve never had blowby tested, but despite the age the engine burns no oil and the condition of the oil (which I changes regularly 3-5K) never suggests any serious problem there. I do wonder about the exhaust valves (as pleasedodgevan mentioned), but I imagine that they can’t be terrible if all I get is an irregular needle twitch/vibration - my understanding of the burned valve is fairly large and and regular needle movement. I suppose they could be just gunky enough to create a relatively small sealing problem.

I have looked for a vacuum leak, and haven’t found one - but it must be pretty small. I’m going to look some more.

I should probably look into the timing, but it is PCM controlled, so I presume that it would point to cam or crank shaft sensors.

But anyway…I’m just driving this car until its dead and it really isn’t running all that poorly. One of the reasons I bought a vacuum gauge is that I want to have one and know how to use it. One of the reasons is that I have a Ford Escort with 220K and when it dies, I want to carry a vacuum gauge in my pocket whenever I go to look at used cars.

I’m guessing that the general answer to the question of my current readings is that “it could mean a lot of different things” - ?

Thanks to all for their time

I recommend a text book, Automotive Technology,a Systems Approach 4th Ed. Jack Erjavec.

You’re exactly right about compression. The way I was taught when I first started wrenching many years ago that a compression test was always performed with any tune-up or maintenance type service no matter the mileage. The plugs are out anyway so kill that issue right off the bat.

One would think that very low miles automatically means the engine mechanicals are good but I’ve run across a number of very low miles cars that had an engine compression problem and in which every part in the book had been thrown at them in an attempt to make it run right.
(The most glaring example was that 6 month old, 7k miles Subaru in which someone had misadjusted the valves at a 1000 miles. This led to a total trashing of both cylinder heads; neither one was even repairable. Another was a VW Quantum (5 cylinder) with less than 500 miles on it and the cylinder head was totally wiped out due to overheating. I was absolutely stunned when I tore that one apart.)

OP, I would not shoot for 20" of vacuum. I live in OK (about 1000 ft. elevation) and I’ve never seen an engine on any type of vehicle, and no matter the barometric pressure, exceed 17-18" of vacuum. It’s also possible to have lowered compression and little oil burning but that’s another subject.

Hi admitted amateur;
Years ago my 88 Taurus Vacuum was 18 inches @ idle, engine hot ,in neutral ,no accessories on. My 2000 Honda accord was 21 inches, both had a very steady needle.
Don’t know if they will allow me to type all the following.
Normal engine should read 17 to 22 inches, engine hot ect. as above.

  1. A low steady reading usually means a leaking gasket betweel intake manifold & cyl. head or throttle body, a leaky vacuum hose, late ignition timing ,or incorrect camshaft timing.
  2. If reading is 3 to 8 inches below normal & fluctuates at that low reading ,suspect a leak at an intake port, or a faulty fuel injector.
  3. If the needle has regular drops of about 2 to 4 inches at a steady rate ,the valves are likely leaking.
  4. An irregular drop or down flick of the needle can be caused by a sticking valve or an ignition misfire.
  5. A rapid vibration of about 4 inches combined with exhaust smoke indicates worn valve guides. If rapid vibration occurs when you increase engine speed,check for leaking intake manifold gasket, or head gasket, weak valve springs, burned valves or ignition misfire.
  6. A slight fluctuation say 1 inch up & down may mean ignition problems.
  7. If there is a large fluctuation, perform a compression or leak down test & look for a weak or dead cyl. or a blown head gasket.
  8. If the needle moves slowly through a wide range, check for clogged PCV system, incorrect idle fuel mixture, throttle body or intake manifold gasket leaks.
  9. Check for a slow return after revving engine quickly snapping the throttle open to about 2500 RPM & let it shut quickly, normally the reading should drop to near zero then rise above normal by about 5 inches & the return to the normal idle reading. If the vacuum returns slowly & doesn’t peak when the throttle is snapped shut, the rings may be worn. If there is no long delay, look for a restricted exhaust system or catalytic converter. Hope you can understand all this & can figure out your problem.


I’m not sure if I’ll worry about getting the 16" up, but I’ve begun to wonder if the quivering of the needle might be something like either slightly worn valve guides, or perhaps just somewhat gunky valves that then don’t seat quite right. As I understand it, if my quivers were exaggerated a by several more inches they would look something like one of those two things. (I have run Sea Foam through the intake, but I don’t think that does much if the exhaust valves are gunky and it certainly won’t fix worn guides).

At the moment I was trying to better understand how to read/interpret the gauge, though its obviously got me wondering about smoothing out that idle too. But if there is something going on with the valves, I’ll just try to finish shopping for another car before this one goes b/c I’m not tearing down the engine on an 11 year old 220K Ford Escort. Sometimes you just pat a car on its head and let it rest after a job well done.

Considering 220k miles on the car I would consider a slightly twitchy 16" of vacuum as fairly decent.
My guess is the compression is dropping; through a combination of ring wear and valve/valve seat wear. Valve wear can show up as slight twitches or major ones; it all depends on how many are at fault and how bad they are.
The problem with leaky exhaust valves is that once they quit sealing the downhill spiral can be comparatively quick as hot exhaust gasses will start eating the face of the valve and the valve seat up.

You’re doing a pretty good thing by using that vacuum gauge as it can tell so much about what is going on inside the engine as it runs; and can even detect partially clogged catalytic converters that may not even give the car owner any noticeable symptoms.

Here is a good article, containing pictures and drawings:

Thank you Kit . . . interesting article. Rocketman

I also found that to be both interesting and useful - thanks.

In case anyone is interested, this site doesn’t explain the “story” as well, but in my searches have found it to be the best one for learning the typical kinds of readings/problems - b/c of the animated gauge at the bottom:

Excellent link. Thank you.