Turbo Dodge Engine Vacuum Testing and Diagnostics Guide

To check for an engine vacuum leak use an inexpensive vacuum gauge. The vacuum leak test is explained with a simple symptoms and diagnosis chart.

To perform an engine vacuum diagnostics test you are going to need an understanding of the common test results and what they may indicate regarding the engines general health as well as an appropriate engine vacuum diagnostics gauge. These gauges are widely available and they range in features and construction quite widely. I prefer to utilize a gauge that is specific for automotive engine diagnostics because they are manufactured with a face that are printed and marked with typical results making quick diagnostics quite easy.


Here is a picture of such a gauge


The gauge pictured above is sold under a variety of brand names and is available from a number of retailers including popular automotive parts retailers, Grainger, Harbor Freight and others. The gauge pictured above is available from Harbor Freight.




Manufacturers install ports on their manifolds for lots of different reasons: Brake Booster, PCV tube, EGR Switch, A/C vents. Find one small enough for the vacuum gauge line to slide onto firmly. A quick snap of the throttle plate should drop the vacuum to around 5 "hg vacuum, and then it should recover to a steady 21 "hg vacuum; when air mixes with atomized fuel from the fuel injectors, the engine's speed increases. For every 1000 ft. of increase in altitude, 1 "hg is subtracted from the vacuum reading.



Normal Engine: On a good engine, accelerate to around 2000 RPM and then quickly release the throttle. The engine should snap right back to a steady 17- 21 "hg vacuum.

Steady low between 5-10 "hg vacuum: This indicates that the engine's intake manifold or the intake gasket is leaking. This leak should be easy to find because it will be making a loud, hissing noise.

Steady low between 10-15 "hg vacuum: This reading indicates late valve timing. There is a good chance the vehicle has jumped timing. Check the timing belt or chain, depending on the application.

Steady low between 15-18 "hg vacuum: This just low reading indicates retarded ignition timing. Advance the timing on the distributor to correct this problem.

Fluctuating Needle: A fluctuating needle indicates there is a problem with a valve or a there is an engine misfire.

Needle drops during acceleration: If the needle drops steadily during acceleration, there is a restriction in the exhaust or intake. This is typically due to a clogged CAT or muffler.