Coming In or Going Out?
If you’re VE test results suggest a restriction to airflow, you’ll need to narrow it down to a problem on the intake side of the engine or the exhaust side. One way you can narrow which half to focus on is an “old school” test called the running compression test, using your mechanical compression gauge.
First, do a normal cranking compression test. Be sure the battery is healthy, the fuel system is disabled (to avoid washing the cylinders down), and the throttle is wide open. Record the readings for all cylinders. Next, enable the fuel system and reinstall any spark plugs you’ve removed except for one. Put your compression tester in this hole, leaving the line’s Schrader valve in place.
With the compression tester installed, start the engine and “burp” the gauge a few times using the pressure relief on the tool itself. Then snap the throttle to WOT and record the peak reading on the gauge. Repeat, one cylinder at a time, until you’ve gotten results for all of them.
Normal snap throttle running pressures are roughly 80 percent of cranking compression pressures. If the restriction is preventing air from reaching the combustion chamber, this number will be lower. And if the restriction is preventing the air from getting out through the exhaust, the number will be higher.
If you’ve added a pressure transducer to your lab scope arsenal, you can use it to perform an in-cylinder running pressure test. This method not only show you that there is a restriction, it can also provide the clues you need to identify the cause of the restriction. Problems with valve openings and cam timing are easily seen using the technique. Build up in the exhaust plateau (pressure) can indicate a plugged catalytic converter, even if only one side of a dual cat system has a problem.
Here’s a tip: If you do have a clogged exhaust on only one side of a dual cat system, take a look at fuel trims. One bank will typically show a lean correction, while the other side will show a rich correction. Why? The MAF is measuring total airflow and the ECM thinks each bank is getting an equal share, adjusting fuel accordingly. In reality, one side is getting more than the other. The oxygen sensor on the side getting more is reporting a lean condition and the ECM adds fuel. The sensor on the side getting less is reporting a rich mix and the ECM takes fuel away. The side with the negative trim is the side with the restricted exhaust.