What good is an in-dash vacuum gauge?

This 1989 Dodge B250 camper van with a 318 V8 has a vacuum gauge on the dash. I know vacuum leaks can cause problems like engine running rough, etc, and that’s when the real mechanics pull out the old vacuum gauge…but if the vehicle is running well, can the driver tell anything useful from the vacuum readings? I noticed on my highway drive today that with no or little load, the vacuum was around 22-25 inches Hg, and dropped down to under 10 when accelerating. I’m used to keeping an eye on oil pressure and voltage gauges, but not a vacuum gauge. How do I interpret what it’s telling me when the engine seems to be running fine…and why is it even there?

Keeping manifold vacuum steady and high results in the optimum fuel economy.

If you have a carburated engine it tells you if the secondary and primary parts of the carb are working if you already know the proper vacuum at each stage. Also it can say a hose on the complicated vacuum driven emissions and heating and cooling doors may be malfunctioning.

I second @Rod Knox. I have a vacuum gauge hooked up in one of my cars for fuel efficiency. A vacuum gauge can also tell you a lot about your engine.


Optimum fuel economy, LOL. This thing has a ginormous fiberglass camper top and a frontal area the size of TWO barn doors…I’ll consider it a miracle if I can get 15 mpg hwy, and a not surprising if under 10…tho I guess I can be grateful it has a 318 and not a 360!

Knox is right…They were once marketed as “Mileage Meters”… The higher the vacuum, the better the mileage. They can also spot trouble brewing as when the needle starts dancing at idle…Old time mechanics would set the timing to get the highest possible vacuum at idle as long as the setting was within reason…

Euryale1, I believe this thing is the first year of the switch from a carb to single-port FI for the Ram vans (1989).

So, if normal highway cruising vacuum is, say, 22 inches, and one day I notice that the “new normal” is 15, that’s a cue that the emissions system may be malfunctioning?

Which begs the question, what SHOULD the normal (ie highway steady state) vacuum reading be, and if no one here knows, where can I find that spec?

Problem is this is a motorhome, not a lot of vacuum specs available…better to get it running well, see what the vacuum is, us that as your ‘normal’.

The 22" reading is not likely when cruising in a 3 ton truck. Cruising would very likely require <13". If the throttle is released at cruising speed 20" + is possible. Coasting down a long hill at highway speed with the accelerator up might pull a great deal of vacuum, possibly past 22" An engine in good condition and well tuned will usually idle at about 18" My SWAG is from a murky memory of using a vacuum gauge to diagnose driveability problems on medium duty trucks. But I feel sure my numbers are in the ball park.

Yes, be grateful for the 318…It will run forever…The 360, not so…Your temperature gauge will tell you when you are pushing it too hard…

Rod Knox, I’m positive the gauge was reading a steady 22 inches or so at 55 mph hwy cruising. I suppose the gauge could be out of cal, but I checked it many times during the 45+ minute drive, and it was always somewhere between the 20 and 30 inch marks at 50-55 mph.

But I gather from your comment that if 18" normal vacuum is good, 22" is even better. Yes?

Caddyman, I was going easy on the throttle at 50-55 mph because of the suspected U-joint rumble/vibe problem I mentioned in another thread! So, mercifully, overtaxing the engine was definitely not a danger…but I am grateful for the 318, which I’ve read is venerable and fairly bullet-proof.

Here’s an informative chart I just found. My gauge readings seem to correspond best to the “normal” scenario shown in the upper left corner. The 22" or so reading I noted at highway speed is slightly above the dark green 18-20" zone showed on the chart, but within the light green 20-25" zone.


The normal vacuum reading will vary based on a lot of factors; altitude, barometric pressure, ignition timing, type of engine and wear, etc, etc.
Most vehicles (in my experience anyway) do not run anywhere near 22" of vacuum with most being in the 17-20 range.
It could be that the 22" on your van is due to the camshaft profile which is probably used for a heavy camper van and that low end grunt so 22 may be normal for that engine but not so with another variation of the 318 in a different model.

Could also be the gauge is just reading 10% too high…

You could use a calibrated gauge plugged into the system to compare the readings with the in-dash gauge to check the accuracy. Vacuum gauges are fairly cheap and plentiful to obtain.

Actually, I think he’d be better off with a 360. The 0-60 time in this beast must be around 20 seconds with a 318 that’s choked with late 80s pollution controls. I can’t imagine the mileage would be any worse than with the overburdened 318. A 360 would last just as long. I had well over 200K on the New Yorker I had with a 360 and it showed no signs of dying when I got rid of it due to transmission issues and rust. Pretty much any well designed V8 from any of the big 3 will last and last if given even minimal maintenance.

I remember when I was younger, my dad had a 70s Oldsmobile Toronado. It had a prominent gauge labeled “High Gear Economy” Basically it was a glorified vacuum gauge with red, yellow, and green graduations instead of numbers.

Right now it’s stuck at “zero to fifty” until I get the U-joints replaced…

And the only “fuel economy” tool would be a Sawzall to cut the monster fiberglass top off…but the headroom is great!

Anyway, thanks all for the short lesson in vacuum gauges. Most instructive.

There may be a few cob webs cluttering my memory, jesmed. But is there a tachometer on the truck? And if so what is the engine speed at 55 mph? Maybe I can find my vacuum gauge and dust off my memory. But I feel sure that if the truck can cruise at 55 mph and hold the vacuum above 18 it is running quite well.

No tach…but my uncalibrated ear says It was running around 2000-2500 at the 50-55 pace I drove…I do believe it was in overdrive…

When I take it in for new U-joints, etc, I might ask the mechanic to throw his vacuum gauge on it and check the cal on my in-dash gauge as Busted suggested.