VCI logo Archive Index | Current Issue
The Leak Down Test
John Parker

I get frequent calls from Volvo owners who want to improve the performance of their cars but are not sure whether their current engine, or the one in a car they are considering purchasing, may need a rebuild. The leak down test is a simple procedure that will usually provide the answers. It has become the standard method of determining the condition of an engine.

Most of us are familiar with compression testing which helps, but does not give you all of the information you need. In this process, the test gauge is connected to the spark plug hole and the engine is turned over with the starter. The gauge reads in pounds per square inch (PSI) and the reading is compared with the specifications for that engine, previous test results, and the readings from the other cylinders in order to determine the health of the particular cylinder or engine. The gauge is measures the engine's ability to pump air at the starter RPM and changes in RPM can mean changes in recorded pressure. It is not uncommon to have lower pressures for each successive cylinder just because the battery is running down. If a there is a low reading, it is difficult to tell whether the problem is in the rings, valves, head gasket, etc. (although the trick of squirting oil into the cylinder to see if the problem is in the rings sometimes works). Some performance modifications result in lower compression readings, at least partly because the modifications are designed to improve the engine's performance at high RPM, not at cranking speed. A worn cam may give a lower reading, but so will changing to a high performance camshaft with longer duration and significant overlap. Thus a racing engine in good condition may give a lower compression test reading than a stock engine in average condition.

Doing a leak down test, particularly in conjunction with a compression test, will give you a great deal of additional information and allows problems to be pinpointed. The condition of the engine is determined by measuring the degree to which a cylinder with valves closed leaks air. In simplified terms, if air is pumped into the cylinder at 100 PSI and the gauge reads 97 PSI, then the leak down percentage is 3%. Doing a leak down test is a fairly simple matter. The tester is connected to an air source and is attached to the engine through the spark plug hole. There are two types of instruments, those with single and double gauges. The cost of the tester is usually between $50 and $100 depending on whether it is the single or double gauge and where it is purchased. The double gauge constantly measures the input pressure and the cylinder pressure. The single gauge instrument relies on checking the input pressure and then switching to measure the cylinder pressure. Each requires some adjustment to be accurate. The engine should be rotated so that the valves in the cylinder to be tested are closed and the piston is at top dead center. Tests should be conducted when the engine is warm.

Problems are pinpointed simply by determining where the air is leaking out of the cylinder. Air leaking out of the exhaust system (you can hear it in the exhaust pipe) indicates a problem with the exhaust valve. Air coming out of the carb indicates a bad intake valve or seat. Air going into the crankcase is leaking past the rings and does not indicate a problem if the percentage is low enough. A leak where the air is going into an adjacent cylinder or into the coolant indicates a blown head gasket or cracked head.

A brand new street engine might measure from 5% to 8% depending on the engine, manufacturer, and degree of break in. A B20 engine with unknown history I tested recently measured 10% to 11% per cylinder. Although indicating some wear, the consistency between cylinders and the fact that all of the air was leaking past the rings into the crankcase indicated a reasonable street engine for daily driving that did not need any immediate work. A new race engine should measure under 2%. My current race engine measured 0% after one race and a dyno session. Three years later, it measured between 2% and 3%. After six years, two cylinders are still around 2% while two others are at 5% and 6%. For race engines a leak down test is usually done after each race. The most common problem is valve seats that have been beat up by high RPMs and valve float. Find out which ones are leaking, touch them up and they are good for several more races. For street engines the most common problem is non-hardened exhaust seats that have eroded due to the lack of lead in the fuel.

Using the leak down test, especially in conjunction with a compression test, should allow you to quickly determine the basic condition of any engine. If you are not sure what the levels should be, differences in readings between cylinders is a key indication of a problem. These are tests that can and should be done by any competent garage. After testing, you will know whether the top or bottom end really needs a rebuild.

If you have any questions about the process, please feel free to contact me.

Back to the Top