The peak on each starter current hump is roughly equivalent to Top Dead Center (TDC) of each cylinder’s compression stroke. And when does ignition occur? Just before that, right? And that’s where you should see your ignition trace intersect the current pattern, just before the peak. If the timing is off due to cam timing, the ignition pattern will occur well before (or well after) its normal position. Keep in mind, though, that this is not foolproof. It’s possible to have only one cam in a multiple cam engine to be out of whack. (You can see that in the in-cylinder transducer test I told you about earlier.)
Now We Have To Take Stuff Off
Further isolation of a restriction is going to require some work from here on. You can start by removing the upstream oxygen sensors (pre-cat) and installing a backpressure test gauge in its place. Borescopes are increasingly popular tools for actually taking a look at the cat’s condition, and can also be used in the cylinder to see if the valves are, indeed, opening as they should. When you know that you’re facing a VE problem, and your diagnostic tool arsenal is limited, start by verifying and eliminating the obvious first before moving on to the harder stuff. Just remember to think “air flow” and don’t focus on “restriction.”
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