Vacuum Gauge - an engine diagnostic tool

Mon Sep 17, 2007 12:02 pm ... gage+guage

Bringing this topic up to the top of the forum. This AM used a vacuum gauge to help diagnose a hard starting with cold engine problem.

Background: Chevy 350 with 4 barrell carb. Engine started hard when cold. When started engine ran rough and would back fire through carb on acceleration. Once engine started and warmed a bit - ran great.

Cold engine. Connected vacuum gauge to an intake manifold port. Reading should be 5 in. +/-. Gauge reading while cranking engine was about 3 inches of mercury, indicating a vacuum leak.

I have shortened this story up quite a bit. Major vacuum leak found. Vacuum port with 3 vacuum hose branches was loose - not even finger tight.


Vacuum guage readings relate to engine problems

Mon Sep 17, 2007 2:12 pm

Engine pistons serve as suction pumps. The quantity and quality of suction/vacuum created is affected by the condition of the rings, valves, ignition, and carburetion. Problems in each of the indicated areas effect vacuum guage readings in a specific manner.

Engine problems that can be detected with a vacuum gauge are:

1. Defective points
2. Ignition timing
3. Spark plugs
4. Weak coil
5. Leakage at the intake manifold
6. Cylinder head gasket leakage
7. Piston rings
8. Sticky valves
9. Valve leakage
10. Weak or broken valve springs
11. Carburetion adjustments/problems
12. Exhaust back pressure

The vacuum gauge post is sort of under construction. Hopefully after we get all the information, photos and comments we can assemble into an easily readable format.

Any one having comments or additional information. - Please add.

Vacuum gauges were commonly available at the auto parts store.

Stewart Warner makes/made the gauges under the brand name of Motor Minder. These gauges should be fairly inexpensive and work well. You will have to purchase a length of vacuum hose and a hose barb to adapt the gauge to the Cub. I have two from the 1960s. I’m thinking I paid about $5- each at the time.

There are two currently on Ebay for $15.50 each, but these gauges do not have the readings in inches of mercury.

SnapOn sells a vacuum/pressure gauge set for $199.95. Heavy duty mechanics set. Do not purchase this set for occasional use on your Cub. I also own this set. For the extra $170-s you get a metal case, a number of fittings and a booklet on how the interpret readings.
Last edited by Eugene on Mon Sep 17, 2007 6:17 pm, edited 1 time in total.

Mon Sep 17, 2007 2:37 pm

Anyone interested can learn from this site.

An unsolicited testimonial

Mon Sep 17, 2007 7:56 pm

I located some manifold gaskets today. Took the warpage out of the manifold, cleaned up the head, and torqued the manifold down. It fired right up. I sure wish I would have thought about either the vacuum gauge or some spray/propane around the manifold, might have saved me a few hours.
My guess. It would have taken about 5 minutes with the vacuum gauge to locate the problem.

Thinking about buying the Cub down the road. Vacuum gauge check the engine condition. 10 minutes tops and $20- or so for the gauge. Results provide a good argument for a price reduction.