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Vacuum Gauge Usage For Gas Engines

by Jerry Fitzgerald

When I had gasoline powered motorhomes, I used a vacuum gauge for years and have collected several articles on them. The following is intended to help those that already have a vacuum gauge and convince others to use a vacuum gauge with their gasoline engines.

They can be used on gasoline engines (carburetor or fuel injected) and cost about $50. The most difficult part of the installation is finding a location for it on your dashboard area. Next you need to connect the negative wire to a ground and the positive wire to one of your current dash lights. This is so the light in the vacuum gauge will dim when you dim the dash lights. Finally you feed the vacuum tubing from the gauge through the firewall, cut one of the engines vacuum lines, and connect into that vacuum line using a plastic tee.

It's easy, the higher the vacuum reading the better will be your gas mileage. When cruising at 55-60 MPH on flat ground the vacuum gauge will be whatever it is for your engine, probably about 15-16 inches.

When climbing a slight hill try to keep the vacuum gauge at 9-10 inches or higher. This may mean backing off the gas pedal a little. If the hill is a big long run one try not let the gauge go below 5 inches of vacuum for best gas mileage. You will slow up your motorhome more than you would have by keeping the pedal to the metal but you will get better gas mileage and your engine will not get nearly as hot. NOTE: at about 4 inches of vacuum both carburetor and fuel injected engines change the fuel/air mixture to burn much more gas. For those periods when you are below 5 inches of vacuum you will be using about 20 percent more fuel.

I have tested this on long 15-20 mile 6 percent uphill runs. For example last summer the outside temperature was 106 degrees F and after driving about ten miles with the vacuum gauge between 4-5 inches I took note of the temperature gauge and reduced the throttle so the vacuum gauge was at 7-8 inches of vacuum. Within one mile I could see that the temperature gauge went down (cooler) a small amount. My speed reduced about 2-3 MPH but the engine temperature went down while I was still climbing the mountain.

My bottom line is, I try never to let the vacuum gauge go below 5 inches. Also, I do NOT let the cruise control down shift the transmission. If you have the cruise control on and let your motorhome lose speed on an uphill until the cruise control forces a automatic down shift by the transmission you are burning lots of excess fuel and some unburned gas goes into the manifold overheating it.

On ther other hand, if you need to climb the hill at the maximum speed your coach can obtain,just don't look at the vacuum during that assent. One further tip, if there is a slight downhill or a long flat stretch before the climb "crank 'er up" and enter the climb at a higher speed. In other words use momentum.

With the engine FULLY warmed up and idling, quickly floor the gas pedal and immediately let your foot off it. The needle should drop to about 5-7 inches and quickly rise to its normal reading it it's a healthy engine.

A healthy engine at idle reads about 18-22 inches (your engine/chassis manual may tell you more).

At idle a steady but low reading in the 10-14 inch range can indicate warn piston rings, incorrect ignition timing, or a vacuum leak.

A reading that is steady but higher than 22 inches could indicate a blocked air intake. Remove the air cleaner and retest.

If the needle swings back and forth between 10 and 20 inches there is probably a problem with the engine's valves.

A smooth fluctuation of the needle in cadence with the engines idling points toward a leaking head gasket and/or lost compression.

These diagnostics are indicators only and further checking should be done.