The Volvo Parts, Accessories &
Performance Specialists Since 1963

What Makes Your Volvo Tick? The Basics of Spark, Fuel & Compression

2011-03-04 - Scott Hart

Spark, Fuel and Compression

Most poor running problems can be quickly identified once you understand how the 3 key ingredients of combustion (spark, fuel and compression) affect the engine when out of sync. The brief explanations in this article will hopefully help you to better understand the mechanical marvel under the hood of your car and how to troubleshoot it when there is a problem.

In simple terms, a gasoline engine needs 3 things to run. Spark, fuel and compression all properly timed. Too much air creates a “lean” condition and too much fuel creates a “rich” condition. For a more thorough explanation of air fuel mixtures I recommend the book “Bosch Fuel Injection & Engine Management” published by Robert Bentley. The book provides in-depth explanations of air fuel mixture, oxygen sensor (Lambda) systems, ignition & valve timing, injection systems and how changes in these systems effect combustion & performance. Again, if you find the information in this article interesting, I can’t recommend the Bentley book enough. It really cleared up a lot of “mysteries” for me.


The ignition system in most cars is quite complicated, so we will only cover confirmation of spark. The simplest and safest method is to keep a new spark plug in your toolbox and simply connect it into the end of the spark plug wire, make sure it is properly grounded and check for spark while cranking the engine. Do this for each cylinder and note the color, consistency, and intensity of each test. Refer to your workshop manual for clarification. If you find a no spark or weak spark condition, swap the wires to see if the problem is in the wire. If the problem persists with the substituted wire, then the problem will most likely require professional diagnosis. If you have a good workshop manual, you can perform further ignition system tests, but more sophisticated tools and techniques are required, usually beyond the scope of the average do-it-yourselfer.

Fuel System

Next on the list is the fuel system. You’ll need a good service manual for your year and model as well as specialized tools to perform fuel pressure and volume tests. If you feel uncertain about your ability to perform these tests after reading your manual, I would recommend having them done by your local Volvo specialist for safety reasons. Check the spray patterns of the injectors according to your manual. Problems with electric injectors (LH-jet) are infrequent compared to the early continuous (K-jet) style. A common problem is dirty injectors which cause poor spray patterns, rough idle, reduced fuel economy and reduced power. Once you’ve determined that the fuel system is functioning normally and that there are no significant air leaks, you can move on to checking the condition of the engine with a compression and leak down test.

Vacuum & Air Leaks

The most common driveability problems are usually caused by air leaks in the intake system which affect vacuum. Symptoms are high or inconsistent idle, popping back through the intake when accelerating, stalling when shifting the car from park to drive and knocking or pinging when accelerating. The most common culprits are cracked or worn hoses connected to the intake, dried up or missing injector seals and loose connections to the intake manifold. Visually inspect the hoses and use a spray bottle with water to check for leaks around the injectors. The water will temporarily seal the leak and the idle should drop. Another good tool for troubleshooting air leaks is a 3-foot length of rubber hose that you can use to listen for leaks. Hold one end of the hose to your ear while you scout around the engine compartment with the other end. When you hear rushing air or whistling, you’ve located the leak.

Compression & Leak Down Tests

A compression test will give you a good idea of overall engine condition. If a compression test indicates a problem (low compression), then you’ll need to perform the more involved leak down test using a leak down tester to pinpoint the cause. By filling the sealed engine cylinder (with the piston at top dead center) with a known pressure of air, the amount of air that leaks out due to a worn valve or worn rings can be expressed as a percentage of leakage. For example if you pressurized the #1 cylinder to 100 psi and it held 90 psi, you would say that the cylinder had 10% leakage. The leak down tester makes this an easy task. The tool screws into the spark plug hole and when connected to an air compressor it will quickly allow you to pinpoint the problem. If you hear air escaping out through the exhaust pipe, you’ve probably got a burnt exhaust valve, if you hear air escaping out through the intake manifold or air box, then the intake valve seat is suspect. If you hear air escaping out through the oil filler cap or block breather then the problem is with the rings or piston. When checking leak down, 10% is acceptable. Beyond 10%, you’ll want to check for consistency from cylinder to cylinder, as a difference of 5% between cylinders could cause a significant imbalance and rough running. If the compression is within 30% of the factory specs, you can usually continue to operate the engine although the performance will be reduced. Beyond 30% wear, the engine will usually begin to smoke and have a difficult time passing smog tests, not to mention the lack of power. For most do-it-yourselfers, a compression gauge will be all that is needed, as leak down testing is best left for the experienced mechanic.