Using a Vacuum Gauge for Engine Mechanical Diagnostics
 

A simple vacuum gauge is a powerful tool for analyzing engine mechanical defects. Below is a summary of the instructions that came with my Lisle gauge (about $20), they're similar to instructions and diagnostics that come with other vacuum gauges. Note that I've scaled the values for a 4-cyl engine. Decrease all readings by 1 in. Hg for every 1000 feet of alititude.

1. The procedures and responses below apply to typical stock street motors which have cams that do not have significant overlap. Cams with significant overlap produce much lower manifold vacuum, and results must be scaled accordingly.

2. The engine should be at operating temperature and idling 100-200 rpm higher than normal (set with the air bleed screw, about 1200 rpm).

3. Connect the gauge directly to the intake manifold (plenum box).

Reading: Needle holds steady at 14 to 16 in. Hg. As you hit and release the throttle suddenly, the needle drops below 5 and bounces up to over 20.
Meaning: Normal response.

Reading: Needle holds steady at 10 to 13 in. Hg. Drops to below 5 and bounces back to near 20 when you hith the throttle.
Meaning: Low intake vacuum, likely due to leaking piston rings - check with compression tester.

Reading: Needle drops intermittently 3 to 4 in. Hg.
Meaning: Sticking valves.

Reading: Needle drops to zero as engine speed is increased.
Meaning: Restricted exhaust system.

Reading: Needle drops back to a low reading, returns to normal, drops back, etc., at a regular interval.
Meaning: Burned or leaking valve.

Reading: Needle vibrates rapidly at idle - steadies as engine speed is increased.
Meaning: Worn valve guides.

Reading: Needle holds steady at an extremely low reading.
Meaning: Air leak in the intake manifold or large valve overlap.

Reading: Needle fluctuates regularly between a high and low reading.
Meaning: Blown head gasket between two adjacent cylinders - check with a compression tester

Reading: Needle fluctuates over a range of about 2 in. Hg.
Meaning: Spark plug gap is too narrow.

Good post Brad.

I have used a vacuum gauge many times for tuning and diagnosis. I have even had them installed on the dash in a couple of cars years ago. If you have a non-restrictive intake (Webers for example) the needle really responds rapidly to throttle input and is kind of cool to watch as you drive.

James

Thanks, James. Another use of a vacuum gauge is a quick way of determining if there are major engine mechanical problems with a car you are thinking about buying. Only takes about 1 minute to verify.

Reading: Needle drops to zero as engine speed is increased. Meaning: Restricted exhaust system.

In what way could the exhaust be restricted?
I had a "Performance" muffler installed and was wondering if a new muffler can cause this to happen?
My vacuum needle drops to zero as engine speed increases and right back up to 15-16 in. Hg when I let it come back to idle.
When I hit the throttle the vacuum drops to zero then right up to 20+ then slows to 15-16 in. Hg.
No vacuum leaks that I can find.
bruce

Learning how to interpret vac gauge readings takes a bit of experience. When you rap the throttle abruptly to a wider throttle angle (but still far less than wide open), the vacuum will initially dip to near zero, then return to some level between the idle level and zero. When you have exhaust restriction (e.g. disintegrated cat converter), as you smoothly increase engine speed, instead of a modest dip in manifold vacuum, you see it dip all the way to zero, as the restriction permits complete pumping of the intake system. I seriously doubt that a new muffler would result in this type of problem.

OK thanks Brad. BTW... really enjoy reading your site. Lots of info that's helped me quite often.
bruce

"Reading: Needle fluctuates over a range of about 2 in. Hg.
Meaning: Spark plug gap is too narrow."

WHAT?!

I was wondering the same thing. Would that be misfiring?

That would be my guess. It could be caused by a ton of things other than spark plug gaps!

It should probably be changed to "misfiring", with a list of probable causes, which would include "narrow spark plug gap".

The list I posted is a compliation of cause/effect pairs from several sources.