Re Video;How to test for a plugged exhaust (picoscope and psi transducer)

I have a question that needs a clarification or probably a correction related to the above Video; when Paul Danner is saying that a low Baro frequency at wide open throttle might lead to either a dirty MAF or a blocked exhaust. What is the principle behind the result of a; blocked exhaust causing low frequency from a pressure sensor at WIDE OPEN THROTTLE? A low frequency means low pressure, while the perception is that a blocked exhaust will cause higher pressure in the inlet manifold.

Hey Bashar! Throwing up a link for the video you're referring to:



About the low Baro frequency, it absolutely does come down to engine breathing. In the case of a restricted exhaust, it's the reduced amount of airflow across the MAF that causes the PCM to miscalculate the barometric pressure reading.

A restricted exhaust WILL cause poor vacuum numbers, but in the case of this Ford, the PCM doesn't know that. There's no MAP or Baro sensor to let the PCM know what's happening in the intake. A restricted intake will cause the same thing, but result in vacuum building in the intake at WOT.

As Tyler said it is an airflow thing, if the exhaust is plugged after the first few revolutions of the crank the back pressure builds up in the exhaust and the gas can't escape from the cylinder through the exhaust valve so at the end of exhaust stroke when the inlet valve opens the cylinder is already full of gas under pressure so can't draw fresh inlet air from the intake.

The fancy name for clearing exhaust gas from the cylinder and getting fresh air in is called scavenging really just clearing out the rubbish.
Good scavenging is what makes modern multi-valve engines more efficent than the old push rod and flat head engines.

Dear Tyler and Andy thank you for your replay, but the answers are still not clear to me, possibly due to to my question was not clear, so I'm trying again. The aim is to correct my wrong understanding on this issue, or get a clarification. When Paul Danner in the above Video was saying that one possibility for a low Baro freq. (and the principle is the same if it was an actual or an ECM calculated) at WIDE OPEN THROTTLE; is a plugged exhaust, then my understanding is:
Low Baro freq.= More Vacuum = Less Pressure.
How to expect a low inlet Vac. at WOT from in-cylinders high pressure due to a plugged exhaust? Clear understanding of this is a very useful knowledge

A bit of correction for the previous message; I meant how to expect low pressure (more Vacuum), at WISE OPEN THROTTLE when the engine cylinders are at high pressure due to a plugged exhaust?

Sorry sir. :blush: Lemme try again?

To be clear, none of the Baro frequencies you'll ever see on a calculated system like this indicate vacuum. It's all related to atmospheric pressure at sea level. A higher Baro frequency relates to a lower altitude, and lower relates to a higher altitude. Most Baro PIDs I see come out to around 156 or 157 Hz in my area, for example. This chart by Wells may make things clearer?

www.wellsve.com/ds_maf.html