Leak Down Test

by Dennis Haynes

A number of people have asked "What is a LEAK DOWN test?" and "Can I do it myself?". First, let me explain the concept. We already use a compression test to determine an engines condiditon. the problem with this test is that there are too many variables. It can only be used to check engine condition by comparing cylinders to each other or a past norm. Variables such as cam profile, engine cranking speed etc, will affect the readings.

A leak down tester uses air from a compressor and measures the rate at which it leaks through the engine. This is done with the engine not running and the beauty of this is that toy can find the source of the leakage by listening for the escaped air. Internal leakge is found by air bubbles in the cooling system. Tools need are a leakdown tester, and an air compressor. The tester is available from Milton at about $60.00. Compressor should be at least 2hp and deliver 90psi.

Procedure: Remove spark plugs. Set engine to TDC #1. Calibrate test gauge per the instructions. Lock engine so it can not turn. Conect hose to spark plug hole, Conect pressurized gauge to hose connection. Read leakage. If looking for coolant leaks and nothing obvious shows up, bypass gauge and connect shop air direct to cylinder. Open radiator cap before this. If coolant sprays out, you have a head problem. Do same test on rest of cylinders. Remember to set TDC of each piston for compression stroke. This test is also great for air cooled haed leaks and valve problems. Note that all engines will have some leakge past the rings. I always do full pressure test when I suspect a problem. Make sure engine is secured with full pressure test. it will spin violently. With gauge connected, you can rock crankshaft to see if leakage changes. If so, this is a sign that the ring lands are wearing, new engines will also do this until the rings are seated.

I know these instructions are kinda flaky, but I hope this gives some insight as to this type of test. If having a mechanic work on your vehicle, he should be familiar with the leakdown test. I would be concerned if he isn't. This is a basic troubleshooting tool that all fleets use. Especially on diesels.

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