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All choked up

Just like you, an engine can't run very far if it can't breathe.
Tuesday, May 1, 2012 - 07:37
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Just like you, an engine can't run very far if it can't breathe.

OK, I know, what if I can’t get to the holes on the backside of the engine?

Additional Testing Tips
Some engine designs won’t allow you to perform a running compression test on every cylinder. It’s hard to run the engine with the plenum on the bench. If that is the case, then it’s time for a different approach.  You’ll have to break the engine in half (not literally!), and do a little old-fashioned troubleshooting. Let’s start with the easiest first.

The manifold vacuum gauge is still a capable diagnostic tool on many engines.Take a good look at the intake tract. Look at the condition of the air filter. Is it an aftermarket part? Perhaps it’s been over-oiled? Any critters living in the air box or its inlet? You can always try rerunning the VE test with the air filter removed to see if VE returns to normal or power is restored. I’ve had a few instances where the replacement filter was just too restrictive, even though it was brand new. And don’t forget to take a look at the inlet screen on the MAF sensor. It’s amazing what can get sucked up into the intake tract.

Next easiest step is to check for excessive exhaust backpressure using a manifold vacuum gauge. I’m not talking about one of those small face sissy gauges, I’m talking about a real, old-fashioned, gauge like the ones we used to use for testing mechanical fuel pumps with. Connect the gauge to an intake manifold vacuum test reasonably close to the source (long test hoses can dampen the vacuum pulse and make testing less conclusive). To check for a possible exhaust restriction, snap the throttle to wide open and release while watching the vacuum gauge. It should drop to near zero and then return quickly to normal idle readings. If you see the gauge returning slowly, it’s time to take a closer look at the exhaust system.

Another way to test for excessive backpressure is this little tool.Another method that makes use of the manifold vacuum gauge is to hold a steady cruise rpm of about 2,500 rpm. Look for your gauge to maintain a steady reading equal to or slightly higher than idle. A reading that progressively drops while holding a constant throttle opening is another indication of a restricted exhaust.

Do you own a Digital Storage Oscilloscope (DSO)? Improper cam timing can cause restrictions to flow by changing when the valves open and close. A quick check for cam timing issues can be performed using starter current draw and an ignition reference. Disable the fuel system to prevent the engine from starting and place your high amp clamp around either battery cable. Select a second channel to capture an ignition event for you reference. Set the voltage divisions appropriately and the time divisions to a half a second per division. Then crank the engine over and grab that waveform.

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