BlueOvalTech Logo

Diagnose Your Engine

Picture of a vacuum gaugeThis tech article was originally posted at FordF150.net. It is primarily intended for 1997-2003 Ford trucks but most likely also applies to other Ford models and years.

How to use a vacuum gauge to diagnose engine related & other problems by William R. Watt. See the webmaster's note at the bottom

Diagnosing car engines with a vacuum gauge. A vacuum gauge gives a quick and cheap (under $15) indication of engine problems. To isolate a problem further diagnostics are usually needed.

This is a consolidation of diagnostics from three sources:

  1. Instructions for Equus vacuum gauge
  2. Chilton general car care manual
  3. Haynes emissions control manual

Connecting the vacuum gauge

Engine must be warm or the gauge reading will be too high and computer will be in warmup mode. Select a vacuum hose connected directly to the car's intake manifold or select an unused port on the manifold. The best alternative would be to attach a hose to an unused port on the manifold and leave it there for use with the gauge. Plug the hose when not in use. Otherwise if in doubt which hoses are connected to the intake manifold, consult the hose diagram on the emissions sticker, usually found on the underside of the hood or on the firewall, or consult a repair manual. If still in doubt the hose to the MAP (pressure) sensor is connected directly to the intake manifold. The gauge can be tapped into a hose using a T-connector. For a quick and dirty reading unplug the easy to find PCV hose on the valve cover and plug in the vacuum gauge. Using the PCV hose may not give a direct connection to the intake manifold and it will cause the engine to idle slowly, but it will give an intake vacuum reading and is easy to use when looking at a strange car, for example a prospective purchase. Start the engine and read the gauge in inches of mercury (in Hg). The dial on the gauge may be marked with the good range.

1. Equus instructions

a. testing at idle speed

b. testing at 2000 rpm

2. Chilton general car care manual.

3. Haynes emissions control manual

a. testing at various speeds

b. testing at idle speed

c. testing at higher speeds

d. testing for blocked exhaust


Webmasters Note

I have a vacuum gauge mounted right in the dash of my 98 F150 XLT. I use it to also help determine the load my engine is under while driving, and to estimate fuel economy.

Here is the gauge mounted:

My vacuum gauge installed