In-cylinder pressure test waveform interpretation

Joined
Jan 16, 2009
Messages
88
Location
Midwest
Backstory - 2005 Dodge Grand Caravan, 190k miles, loss of power, transmission won't shift into drive unless you let completely off the accelerator, excessive "hissing/whistling" that sounds like an exhaust leak. I'm thinking the catalytic converter or exhaust system is plugged but want to verify before throwing parts at it.

I borrowed a pressure transducer from work (10vdc output at 300 psi) and cobbled up a mess of fittings to adapt it to my compression tester (after removing the Schrader valve). Now I've never captured or analyzed an in-cylinder pressure waveform before, but if I've correctly identified the exhaust valve opening then I'm seeing 492mV or about 14.8 psi at idle. At 2500 rpm, pressure increases to about 512mV or about 15.4 psi. So unless there is something I'm missing, I'm thinking this confirms a plugged exhaust.

Thoughts?
 

Attachments

  • pressure.jpg
    pressure.jpg
    198.3 KB · Views: 71
  • pt.jpg
    pt.jpg
    99.7 KB · Views: 66
I just bought a Picoscope and a pressure transducer. Haven't had a chance to use it yet.

@Timmastertech has a lot of experience with this type of diag work.
 
I think south main auto on youtube has some videos where he covers this particular topic, though I can't find them right now
 
The Pine Hollow video is interesting. His backpressure increased with engine speed but mine didn't. Any possible explanations for that?
 
You don't need all these electron microscopes for engine work.

Put your hand over the tail pipe opening. Do you FEEL any air coming out of the pipe? Does it SOUND restricted with elevated revs?
 
So back to your readings.

14.7psi is normal air pressure or 1 atmosphere/bar. You were measuring nothing elevated at idle.

At 2500 revs, you got 15.4psi. So the simple math here is 0.6psi. So far, I'm not seeing any clogged coverts.

Do the same at 4000-5000rpm. If you get 30psi then..........
 
So back to your readings.

14.7psi is normal air pressure or 1 atmosphere/bar. You were measuring nothing elevated at idle.

At 2500 revs, you got 15.4psi. So the simple math here is 0.6psi. So far, I'm not seeing any clogged coverts.

Do the same at 4000-5000rpm. If you get 30psi then..........
Well it depends if the gauge is calibrated to 0 at 1 atmosphere or not. If so, then he's seeing 2 atmosphere which seems like a clogged cat to me.

You don't need all these electron microscopes for engine work.

Put your hand over the tail pipe opening. Do you FEEL any air coming out of the pipe? Does it SOUND restricted with elevated revs?
Using cylinder pressure graphs and things like vibration charts is pretty common now, it allows you to diagnose issues more precisely.
 
The quickest and easiest way to check for an exhaust restriction is with a vacuum gauge.
 
So back to your readings.

14.7psi is normal air pressure or 1 atmosphere/bar. You were measuring nothing elevated at idle.

At 2500 revs, you got 15.4psi. So the simple math here is 0.6psi. So far, I'm not seeing any clogged coverts.

Do the same at 4000-5000rpm. If you get 30psi then..........

Pressure transducer is calibrated for 0-300 psig; not psia.

It reads nearly zero (20 millivolts) with vehicle not running.
 
My current plan is to drill a hole in the exhaust downstream of the converter and see if I can hold a piece of vacuum tubing over it and get a pressure reading on the transducer or a 15 psi analog gauge.

Maybe somebody put a potato in my tailpipe...
 
Doesn’t it have a downstream O2 sensor you could remove for testing?
Good point. But I don't know if I have fittings to adapt to it. Maybe it is the same thread that my compression tester has? If I drill a hole, I can always weld it shut or put a self-tapping screw in it.
 
Good point. But I don't know if I have fittings to adapt to it. Maybe it is the same thread that my compression tester has?
Yes, it will have threads equivalent to one of the spark plug sizes, so if your tester has some adapters, it should work.
 
There are some things in your waveform that are not logical that generally relate to the transducer, such as your intake vacuum pocket being much deeper than exhaust. If youa re using a regular compression test hose that will also change the waveform as the hose flexes. Hoses for in cylinder testing are very rigid so you do not lose definition in the waveform. Your waveform IMO is useless. Most transducers that are not designed for automotive use are too slow to be useful in this type of testing. Your best bet would be to pull the upstream o2 and put a backpressure gauge in the hole and measure backpressure.
 
There are some things in your waveform that are not logical that generally relate to the transducer, such as your intake vacuum pocket being much deeper than exhaust. If youa re using a regular compression test hose that will also change the waveform as the hose flexes. Hoses for in cylinder testing are very rigid so you do not lose definition in the waveform. Your waveform IMO is useless. Most transducers that are not designed for automotive use are too slow to be useful in this type of testing. Your best bet would be to pull the upstream o2 and put a backpressure gauge in the hole and measure backpressure.

I know nothing about intake and exhaust pockets and how they should look. :)

I agree the hose may be flexing. But I guarantee the transducer is fast enough as it has a 1 millisecond response time and we routinely use them for high speed applications at my day job.
 
Back
Top