In "Engine mechanical testing: Good, better and best," June 2018, I shared two fundamental engine mechanical tests performed with a scope: cranking relative compression and cranking intake vacuum. So today, let's take it one step further.
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Please keep in mind that the goal of all of these engine mechanical tests is to determine not only whether or not there is a mechanical problem present but to also determine what the problem is without engine teardown for visual inspection which is very time consuming and may yield a situation where the car owner may bail out on the repair but now the vehicle can no longer be operated unless the engine is reassembled. I’m sure many of us have been down this road and it is one I like to avoid.
First let me categorize the types of problems that may be present before testing begins and the problems that will be uncovered with these tests. Modern engines can suffer from any of the following problems; cylinder sealing issues causing compression loss, incorrect valve timing from failed cam drive systems, incorrect ignition timing, shifted or out of synch engine rotation sensor signals, intake path restrictions, exhaust path restrictions and bank to bank breathing issues - all of which can be traced down using these tests. While I have documentation on all of these issues and I could fill a small textbook with all this information, it is beyond the scope of this article to discuss each problem in detail. It is my intention to whet your appetite so you’ll invest in the tools needed to perform these tests and begin using them in your diagnostic routine. There are many excellent training classes on this subject and further research will be required to become proficient with this testing, but we must start somewhere so let’s get started now.
Start at the beginning
To begin, you must have a scope and pressure transducer along with an assortment of compression test hoses for the different spark plugs used in the many engines found in the marketplace. The first item to be aware of is there cannot be a Schrader valve in the test hose when using a transducer! If you have a gauge-style compression test kit with a hose assortment, you can use these hoses but be sure to remove the Schrader valve when using a transducer. The Schrader valve is the reason a mechanical gauge builds pressure and shows compression puffs or pulses, but this is not how pressure builds in the cylinder and you will see that the pressure pulses shown on a scope are all the same on a normal engine during cranking.
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We will not pay too much attention to the first or last pulse in a transducer generated compression waveform because you don’t know where the piston was when the engine began to crank or stops turning. If cranking pressure peaks are varying during the test we have already uncovered a problem and one that a conventional gauge will never be able to show us! The sequence of in-cylinder pressure testing should be a cranking test first, then a running test with a snap throttle event to allow analysis of the various problems mentioned above. You must prevent the engine from starting during your cranking tests, preferably by removing fuel. Scoping the ignition event will also be needed during certain tests.
Two cranking pressure waveforms will be shown to illustrate a normal waveform and a problem vehicle. The first waveform shows normal compression pressure from the 4-cyl Honda engine shown in Figure 1. Notice the first compression event is low due to the piston being somewhere above bottom dead center when the engine began to crank but all of the subsequent compression events are at the same pressure of 175psi (Figure 2). This engine had a leaking exhaust valve in cylinder #2 and when tested, the cranking compression was only 149psi - below the 15 percent variance that would be considered a max variance on an OBDII vehicle with misfire detection.