Exhaust Pulses | Using exhaust pulses to identify a misfire.

A long time ago I asked for help in a forum about using tailpipe pressure for diagnostic purposes. I was told tailpipe pressure for misfire diagnosis is worthless these days being we have misfire monitors. He was wrong! I've personally seen other technicians/mechanics criticize others just because they themselves do not have information or knowledge on the subject. That's pretty sad...
I ended up doing my own research, watching videos and having some fun experimenting just to learn more on the subject. Using tailpipe pressure to identify a misfire will only take about 5 minutes!
This vehicle turned out to have a faulty ignition coil. The pressure sensor used was the WPS 500.

[@8power0]

I HAVE USED THIS TEST BEFORE ... YOU JUST HAVE TO MAKE SURE YOUR IDENTIFYING THE CORRECT CYLINDER ,,,,,,,, THANKS FOR YOUR TIME !!!!!

[@detailedautodiagnostics]

This is good....many debate and argue things that they don't understand. There are MANY Ford vehicles that will misfire and NOT identify the exact cylinder. This is a great way to verify which cylinder and to also ensure that the misfire counter is telling the truth and not identifying the incorrect cylinder 👍

[@autotechtogo6290]

Thanks. I was told by someone that this test was a waste of time. I find it to be a very good test at certain times.

[@tillwv11]

The pressure drop is due to a vacuum pocket in the cylinder when the cylinder misfire. A in cylinder running compression test shows the vacuum pocket. If a injector is causing the misfire, injecting propane into the intake manifold will help improve the pattern and remove the misfire.

[@alexmessina3383]

Yes, but he says every time the exhaust valve opens there is a "drop in pressure". Why so on a good cylinder that would have combustion pressure and then release that pressure into the exhaust stream when Ex valve opens??Would not a firing cylinder show an spike in pressure? Or is he referring to an in cylinder compression test with transducer where there is not combusion so the the cylinder would be in vacuunm when Ex valve opens, hence the pressure drop ... can someone clarify what Ex pressure pulse pattern is expected for a good firing cylinder?

[@autotechtogo6290]

The exhaust pressure waveform in this video is referring to an ignition or fuel misfire. If an exhaust valve opens without prior combustion, then there will be a high amount of negative pressure from the expansion stroke. This low pressure is the drop we can see from the ignition or fuel misfire. Combustion on the other can release a positive pressure(10-20psi) when the exhaust valve opens. After a cylinder has exhausted its positive pressure, a slight drop in pressure towards atmosphere will occur. My Pico wpx500 shown in this video shows a positive overall exhaust system pressure on a healthy running engine. This can only be seen with an absolute pressure sensor Watch the video "Brandon Steckler Applying Advanced Drivability Diagnosis". Brian shows cylinder pressure with a one spark plug removed on a 5.7L hemi. Its near the end of the video.

[@rafaelgarcia-xk7eu]

I've been practicing lately and trying to learn this way of testing. I watched lots of videos but most are done with 4 cyl engines. As you may know, V8s are harder to read the pulses. What sensor did you use? And was this test done cranking or idling?

[@autotechtogo6290]

This was done on a V type engine at idle. The sensor was either the Pico WPS 500 or the Ditex Pressure Pulse sensor. Either can be used in this low pressure application.

[@alexmessina3383]

You say every time the exhaust valve opens there is a "drop in pressure". Why so on a good cylinder that would have positive combustion pressure and then release that pressure into the exhaust stream when Ex valve opens??Would not a firing cylinder show an spike in pressure? Or are you referring to an in cylinder compression test with transducer where there is no combusion so the the cylinder would be in vacuum when Ex valve opens, hence the pressure drop ... can someone clarify what Ex pressure pulse pattern is expected for a good firing cylinder?

[@autotechtogo6290]

The exhaust pressure waveform in this video is referring to an ignition or fuel misfire. If an exhaust valve opens without prior combustion, then there will be a high amount of negative pressure from the expansion stroke. This low pressure is the drop we can see from the ignition or fuel misfire. Combustion on the other can release a positive pressure(10-20psi) when the exhaust valve opens. After a cylinder has exhausted its positive pressure, a slight drop in pressure towards atmosphere will occur. My Pico wpx500 shown in this video shows a positive overall exhaust system pressure on a healthy running engine. This can only be seen with an absolute pressure sensor Watch the video "Brandon Steckler Applying Advanced Drivability Diagnosis". Brian shows cylinder pressure with a one spark plug removed on a 5.7L hemi. Its near the end of the video.

[@DJ-tn7vj]

Will try it and let you know if it works

[@thesmartguy3523]

And did it ?

[@duanedonaldson2262]

Not a good overlay, you aligned the left side, adjusted the right side, and had to come back to the left side again, new pieces of software don't have this, meaning there are better overlays than this one so I recommend against it. If you want to try this one, start your alignment from the right side??? #1 sync is on top, not all overlays sync on the top so be aware of this when you start to use your new software. You are correct, sometimes what the scan tool states are the missing cylinder, is incorrect. If codes and Mode 06 data were recently erased, you are out of luck.

[@autotechtogo6290]

I don't care where I start the overlay. Left or right it doesn't matter. This is the only only overlay I've used with the Pico so far.

[@cjolney]

I get so tired of watching internet experts tell people they should just buy a $5000 snap-on modis and plug it into the port that exists on every car ever in their imagination. These unorthodox methods of testing can help us in situations where a code reader isn't available. I'd rather be by myself stranded in the desert and be limited to indirect testing of broken components than be with 100 people who only know how to read the manufactures book or some shitty website and think know how to hold a wrench.

[@autotechtogo6290]

Agreed. Oscilloscope users give techs a huge advantage over the competition. A little but of research and training goes a long way.