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The engine doctor will see you now

Troubleshooting drivability issues should start with making sure the engine is healthy.
Monday, October 12, 2009 - 23:00
The manifold vacuum gauge has been around for as long as I can remember. That’s because it is still a valuable, if not high tech, tool.

The miss was just barely apparent at idle. As soon as the throttle plate was opened, even slightly, the miss went away. Out on the road, the engine performed perfectly at any speed, under any load. The culprit turned out to be a burnt exhaust valve on the rear bank of this Chrysler minivan causing just a slight loss of compression, but I didn't find it until I had nearly exhausted all other possibilities.

Up until that moment, I had considered myself a competent technician, but this van taught me a few valuable lessons in basic troubleshooting and actually was instrumental in my becoming a student of diagnostic techniques and methods.

Now, verifying the engine's overall health is one of the first steps I take when tackling a misfire diagnosis or troubleshooting an engine performance problem that has no related codes recorded.

This is the cranking starter draw on a Ford Focus. The even peaks indicate that all four cylinders are creating the same amount of resistance to movement in the starter motor. This indicates that the compression of each is similar.

The Easy Way First
In the initial stages of my troubleshooting, I'm not looking to prove that the engine is in 100 percent prime condition. I am looking for diagnostic direction, though, and for any early indications that there is a problem in the engine's ability to compress the air/fuel mixture or in its ability to flow that air in and out of the combustion chamber.

Traditional compression tests and cylinder leakdown tests certainly have their place, but the layout of many engine designs make performing these tests a time consuming proposition. Being on flat rate, this is time that I'm not getting paid for unless the customer authorizes it. And I can't ask the customer to spend that additional money unless I can show justification for it.

Manifold Vacuum
One of the first indicators of engine health you can use is an oldie but a goodie — manifold vacuum. To check manifold vacuum, connect your vacuum gauge to an intake vacuum port as close to the intake manifold as you can. Long distance connections can dampen the vacuum pulses at the gauge and limit its diagnostic use. The vacuum brake booster feed line works well and is usually easy to get to. A running engine at idle should produce between 18 inches/Hg to 21 inches/Hg and the needle should remain steady. Significantly lower readings or a gauge needle that bounces around are signs that further, more detailed tests should be done.

Here’s a six-cylinder with a plug removed to show you the difference it causes in starter current draw.

While you've got the vacuum gauge connected, snap the throttle to wide open and release a few times while watching the gauge. The needle should drop to roughly 5 inches/Hg or less, rebound quickly and surge past the idle reading, then fall as quickly back to the idle reading. Any delay in needle response can indicate a problem with airflow through the engine, usually in the exhaust. Or, hold the throttle at a constant 2,500 rpm. The gauge should read near the idle reading and stay there. Lowering vacuum readings while maintaining a steady rpm are another indicator of a restricted exhaust.

Use of the manifold vacuum gauge as a troubleshooting tool has dwindled over the years. Current intake designs impact the vacuum pulses we used to rely on to diagnose all sorts of engine problems. But that doesn't make it any less useful. Weaknesses indicated by this simple test are, at the least, justification you can take to your customer when seeking authorization to dig deeper.

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