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Vacuum Gauge #1
barry s barry stoll alexandria, VA, USA 1972 MG MGB GT 1974 MG MGB 1980 MG MGB	Posted: March 31, 2012 07:35PM Registered: 4 years ago Posts: 508

I'd seen and heard suggestions to install a manifold vacuum gauge to get a good understanding of your engine's condition. I just installed (temporarily) a vacuum gauge. A drive around the block almost made me dizzy! The gauge moves all over the place. Step on the gas and the vacuum drops dramatically. Cruise and get high vacuum readings. What am I supposed to be looking for?

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Re: Vacuum Gauge #2
John D. Weimer Cape Girardeau, MO, USA	Posted: March 31, 2012 07:42PM Registered: 13 years ago Posts: 21,577

It's doing right, exactly as it should. When the needle starts pulsating and jerking and jumping around while the engine is running at a steady speed is when you have some kind of trouble.

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Re: Vacuum Gauge #3
Soyokaze 72MGB Ryan Foster Ð¡Ð¾Ñ„Ð¸Ñ, Bulgaria 1972 MG MGB "Bitty"	Posted: March 31, 2012 07:58PM Registered: 6 years ago Posts: 3,476

The gauge should be steady in a certain range at idle, and go up and down as you step on the gas. Do a Google search for "reading vacuum gauge" and "MGB" to get more information on how this works. You rarely look at it when driving. It is mostly used at idle and at lights.

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Re: Vacuum Gauge #4
twigworker Jack Austin Blowing Rock, NC, USA	Posted: March 31, 2012 08:27PM Registered: 7 years ago Posts: 20,766

From time to time I think of installing a permanent vac gauge but the feeling soon passes.

If you had one that was fluid dampened you might be able to stand it, but a normal gauge is far too attractive to the eye to have sitting in front of you as you drive.

If you just have to use one, tap into the intake manifold at one of the nipple openings on the top of the balance tube and run a line through one of the plastic plugged holes in the firewall so that you can prop the gauge up somewhere to monitor it as you drive and put the engine through it's paces.

I use a vacuum gauge for diagnostics far less than I should. It does help to confirm my thoughts that come to mind after other diagnosis steps have been taken though. Probably the two most common aliments that can be pin-pointed with a vac gauge is an obstructed exhaust system and an improperly timed camshaft.

All an internal combustion engine is is a sophisticated air pump and different conditions affect the pumping action in different ways.

Here is simple and easy to understand chart for reference. http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

Jack

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Edited 1 time(s). Last edit at 03/31/2012 08:27PM by twigworker.

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Re: Vacuum Gauge #5
barry s barry stoll alexandria, VA, USA 1972 MG MGB GT 1974 MG MGB 1980 MG MGB	Posted: March 31, 2012 08:45PM Registered: 4 years ago Posts: 508

Jack, Thanks for the chart. I knew I'd seen something like this before. That said, I don't think I want to have this in front of my on a continuing basis.

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Re: Vacuum Gauge #6
sweep Chris W Gosford NSW, Australia 1966 MG MGB "Basil" 1990 Nissan Patrol "(Patrol) The Beast" 1992 Subaru Legacy "(Liberty)"	Posted: April 01, 2012 03:37AM Registered: 6 years ago Posts: 4,203

Back in the days post the oil crisis of the 70's I had a Datsun bluebird which had a big vacuum gauge next to the speedo. It was marketed as an 'Economy Gauge'. (In reality I believe it was cheaper than a tacho as the higher spec model including the TRX had a Tacho there).
Anyway I got used to having it there and telling me when I was being too much of a lead foot. I had a hole in my MGB dash that the PO had an Amp meter in and I'd already put a volt meter on the other side so I got a 'Fuel Consumption' meter and popped it in hooked as as Twig has mentioned. Didn't cost much and it's another little gizmo to look at while your checking out your temp, oil and zaps. So now you can tell how much your car sucks and blows.

Chris

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Re: Vacuum Gauge #7
twigworker Jack Austin Blowing Rock, NC, USA	Posted: April 01, 2012 08:34AM Registered: 7 years ago Posts: 20,766

Sucks and blows Chris? Now be nice to the old girl.

Jack

---------------------------------------------------- Services to Interesting British Automobiles-- www.jackscars.net ----------------------------------------------------------------
-------------I like to think that I don't service automobiles so much as I provide entertainment, fulfill dreams and revive wonderful memories --------------

I am in the northwestern North Carolina mountains, right up against the Blue Ridge Parkway. Lawn chairs are in the shop and always time to talk. Drop in any time.

How to advertise on this website

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Re: Vacuum Gauge #8
bobmunch Bob Muenchausen PC, Oregon, USA 1968 MG MGB GT "Traveler"	Posted: April 01, 2012 09:22AM Registered: 13 years ago Posts: 11,537

This could also help you understand what the gauge is saying as you use it. It is not just an OK dash gauge, but an even better diagnostic tool for older cars like ours.

http://www.secondchancegarage.com/public/186.cfm

And those who were seen dancing were thought to be insane by those who could not hear the music.
Nietzsche

Edited 1 time(s). Last edit at 04/01/2012 09:23AM by bobmunch.

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Re: Vacuum Gauge #9
barry s barry stoll alexandria, VA, USA 1972 MG MGB GT 1974 MG MGB 1980 MG MGB	Posted: April 01, 2012 09:37AM Registered: 4 years ago Posts: 508

Thanks Bob

I had seen this some time ago but seem to have lost the link. Now I have it again. Barry

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Re: Vacuum Gauge #10
Denny B. Denny Brodbeck Miami, FL, USA 1971 MG MGB	Posted: April 01, 2012 09:46AM Registered: 2 years ago Posts: 931

In reply to # 2027792 by barry s Step on the gas and the vacuum drops dramatically.

Back in the 50's (when I was a kid) the windshield wipers on many cars were "vacuum operated". If you had your wipers on and gave it gas the wipers would STOP momentarily until the vacuum built back up.

A real safety feature! eye popping smiley

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Re: Vacuum Gauge #11
mowog1 Rick Ingram Saint Joseph, Illinois, USA	Posted: April 01, 2012 10:08AM Registered: 8 years ago Posts: 20,405

Moss - Using a vacuum gauge to diagnose.

1968 MGC - 1969 MGC - 1972 MGB - 1974&1/2 MGB/GT V8 conversion - 1978 MGB

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