Upgrade Vacuum Gauge on MityVac?

[pgtr]

I've got an older MityVac vacuum pump w/ a small vacuum gauge threaded into the top. It's a super handy tool for testing/measuring vacuum advance distributors and (older) cars w/ lots of vacuum actuated devices and such.

On those occasions I have the MityVac hooked up to a running car - the needle shakes and vibrates like crazy - almost unreadable. These cars are in good tune and other vac gauges don't bounce so I think the one on the MityVac is likely the problem.

So I was thinking of buying a new gauge and threading it onto the MightyVac. Online search yields (too) many including many that are fluid filled (glycerin, etc.). I just need something for occasional automotive use and will be reasonably accurate and not overly sensitive to shaking/vibration from a running engine vacuum source.

Any suggestions on what I should look for? Or avoid? Thanks,

 

[TurnipTruck]

Look for the largest case gauge with the finest graduations on the dial, and liquid filled. Finer graduations are a clue to the higher precision of a gauge. Having a zeroing screw or adjusting dial are a clue to the accuracy.
Don’t bother with a “snubber” gauge, go liquid filled but be aware you have to open the vent when using and close the vent when storing or the glycerin or mineral oil will leak out.
IIRC the mityvac uses 1/8” npt, and the gauge style is Leg mount, unless you want Back mount. 1-1/16 face is common, but 3-1/2 can be found.
If they are still made, look for a mechanism that is advertised as multiple coil Bourdon tube instead of a single “C” shaped Bourdon, like Helicoil or similar. These won’t be liquid filled.
Digital gauges are also available for a couple hundred bucks.

 

[308guru]

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[Schurkey]

...go liquid filled but be aware you have to open the vent when using and close the vent when storing or the glycerin or mineral oil will leak out.

Quoted for truth. Most folks don't realize that liquid-filled gauges are sensitive to temperature change. The have to be "burped" at minimum before each use--equalizing the pressure inside the gauge mechanism with ambient pressure ("barometric pressure".)

Permanently-mounted liquid-filled gauges can have the rubber seal pierced so there's a tiny vent-hole, so long as that vent is at the top so the liquid doesn't leak-out, and provided that there's no vibration to cause the liquid to slosh and leak-out. I have gauges on my home water supply, the rubber plugs have a molded ****** that can be cut to vent the gauge. (I didn't.)

Automotive forums have multiple instances of folks chasing fuel-pressure "problems". "The fuel pressure goes down as the car warms up..." And yet, they DON'T complain that the engine runs worse. What's actually happening is that the gauge warms-up, the rubber plug keeps the gauge mechanism sealed, and the expanding liquid inside the gauge restricts gauge movement, so it appears that the pressure is reducing when it's simply a matter of internal gauge pressure counteracting the pressure of the fluid being measured--fuel, in this case.

 

[pgtr]

Thanks,

Appreciate the help!

My MityVac is an older model 'similar' to this one. It has a 2 in diam gauge, cheap plastic lens, 0.5 in gradations. I think it's a 1/4" mount on the bottom (but haven't removed yet).

 

[larry_g]

On those occasions I have the MityVac hooked up to a running car - the needle shakes and vibrates like crazy - almost unreadable. These cars are in good tune and other vac gauges don't bounce so I think the one on the MityVac is likely the problem.


Any suggestions on what I should look for? Or avoid? Thanks,

I would suggest that you remove the gauge from the tool and take a direct reading of the vacuum source with that gauge. That will confirm that the gauge is the problem or if the tool is causing your problem.

lg
no neat sig line

 

[rust in the eye]

I would suggest that you remove the gauge from the tool and take a direct reading of the vacuum source with that gauge. That will confirm that the gauge is the problem or if the tool is causing your problem.

lg
no neat sig line

^^THIS
sounds like a relief valve on the tool fluttering
Automotive vacuum guages are cheap as chips, no need to abuse this tool.

 

[pgtr]

Good point.
Perhaps not exactly the same definitive test but I've also sometimes 'Teed' in my ancient guage that has a standard hose ****** connector to the same hose I run my MityVac on. My ancient gauge (reads about 3 psi lower at idle than MityVac and) doesn't flutter like crazy when hooked to same engine. Though perhaps it wouldn't pick up on flutter since it's got about a of vacuum hose between it and the MityVac - e.g. that ancient gauge isn't mounted directly on the MityVac. I'll post a follow-up once I get engine running again with modified vacuum advance...

FOLLOW UP QUESTION: On a related note - are glycerin or liquid filled gauges appropriate for automotive use? Sometimes fluttering can be a symptom or indicator or an engine problem and if the liquid sufficiently dampens movement - could it mask a potential underlying issue?

Thanks,

 

[A0x0max0a]

I ran into an issue trying to fix the plastic and Zamac mityvacs.

The guage is not threaded on at least some of them, making it more or less proprietary. It is fastened by the cotter pin through the body.

 

[OccupantRJ]

A small orifice before the gauge will help dampen the needle fluctuations. The 4 gang gauges for motorcycles had small holes drilled into the ends of the metal extensions for the hoses that were screwed into the carbs for this reason. A small canister to act as a surge tank will also do the job. If you look at the threaded end of some gauges there is an orifice already incorporated.

 

[bonneyman]

I ran into an issue trying to fix the plastic and Zamac mityvacs.

The guage is not threaded on at least some of them, making it more or less proprietary. It is fastened by the cotter pin through the body.

I don't own any of the pistol-grip vacuum pump tools, so didn't know the gauges were press fit in. Could one remove the gauge and adapt a threaded adaptor so one could use a threaded gauge? If so, I'd recommend Marsh gauges. They've done great for me over the years.

 

[whateg01]

I would suggest that you remove the gauge from the tool and take a direct reading of the vacuum source with that gauge. That will confirm that the gauge is the problem or if the tool is causing your problem.

lg
no neat sig line

I was going to suggest a separate gauge for use on a running engine. Can even add a magnet so it can be stuck on the bottom of the hood for easy, hands-free viewing. All you need is the gauge, a barb, and a hose then. For times when I'm using the mityvac, I don't think I would want to add a lot of weight to the pump.

 

[pgtr]

Based on comment(s) above - I disassembled/cleaned/lubricated the MityVac. It was a bit dirty inside including the little round thin rubber 'valve'. The needle still has a bit of 'vibration' when hooked to running engine, but it's not as bad and readable. Thanks for the comments!

I also took my older standalone gauge apart and slightly tweaked the internal linkage to get it to read the same as the MityVac gauge. Both now read about 18 inches (albeit the old standalone gauge is steadier) on my old Chevy at idle which is about right.