How To Test Compression Drop in Hot or Cold Engine - Engine Builder Magazine

How To Test Compression Drop in Hot or Cold Engine

This procedure could be a course of action taken in
response to a customer complaint of engine miss, either hot or cold. To
find out what cylinder is affected, complete the steps below.

First, check the compression using standard procedures (remove all spark
plugs, open throttle valve, etc.). Write down the compression readings.
Now, install all but one of the spark plugs. In the remaining cylinder,
install a compression gauge with a pressure release.

Start the engine and let it idle. Release the pressure at the gauge
and then let the compression build up, again with the engine still at
idle. Repeat this procedure for all of the cylinders and write the new
compression numbers below the standard procedure readings.

The amount of compression at idle is not an important factor in
itself, what is important is the difference between the two readings.
The difference between the compression at cranking speed and the
compression at idle should be equal for all of the cylinders. If the
engine miss complaint is with a cold engine, do this test cold.

For a warm engine, do the test warm. Hopefully this simple test will
save you some diagnostic time when searching for the dreaded engine

Some or all of this information was provided by the Automotive
Parts Remanufacturers Association (APRA). For more information on
technical bulletins available through APRA call 703-968-2772 or visit

You May Also Like

Shop Solutions January 2023

Next time you have set of large journal small block Chevy connecting rods to resize, consider honing the big ends of them for a +.002” outside diameter bearing that the LS engines with fracture cap rods use.

Engine and Machine Shop Tips and Tricks


For proper block cleaning, the oil bypass valves in Gen 5 and 6 big block Chevys need to be removed. We made a couple different sized “hook” tools for a slide hammer. This tool will easily pull the valves out of the block and sometimes without damaging them.

Shop Solutions December 2022

Everyone misses occasionally, and this helps avoid dents and damage.

Jesel Certified Performance Rebuilds

Engine components are serious investments for any racer and maintaining that investment could be the difference between winning a championship and losing it.

Going the Extra Mile with Cylinder Head Porting

It’s not just the port work alone that creates spectacular cylinder head performance. The most critical areas of a cylinder head are those which pass the most air at the highest speed and for the longest duration. Your bowl area, the valve job, the throat diameter, and combustion chamber are all crucial parts. 

Tight Tolerances and Building Power

As you ascend Mt. Everest, you reach an area called the death zone. Once you climb high enough, the margin of error becomes perilously thin. That death zone also applies to engines. As the horsepower per cubic inch and rpm increase, the margin of error decreases. 

Other Posts

CNC Update: Features and Automation

Precision is key when it comes to automotive parts; the complex designs of connecting rods, pistons and rings, blocks, cylinder heads, and other parts require super tight tolerances that are getting more and more difficult to be met by hand or with other machining processes outside of CNC.

All Things Media Blasting

Engine building is a segment of the automotive industry that has always been ahead of the curve in media blasting, and no matter the engine shop, cleaning equipment is a common bond.

Engine & Hub Dynos: Necessary Tools and Additional Revenue

Being able to see the horsepower and the direct correlation to what is lost in the driveline is invaluable – dynos offer a myriad of benefits for the modern engine shop.

November 2022 Shop Solutions

November tricks and tips for the shop!