In-cylinder, cranking, low compression waveform looks like a leaking exhaust valve, but it isn't

Ray Diagnostician North York, Ontario Posted   Latest   Edited  
Case Study
Driveability
Propulsion
2006 Ford Escape XLS 3.0L (1) 4-spd
Crank / No Start

The vehicle is an 06 Ford Escape 3.0 cranking, no start, no codes.

In the first capture, the red Channel B is a "First Look Sensor" pressure transducer, on the dipstick tube, to compare each cylinder's "blow by" pressures past the piston rings into the crankcase. 

The green CH C is the relative compression and the higher crankcase pressure, in CH B for cylinder 2, lines up under the low starter amp draw for cylinder 2.

The tan CH D is a WPS500 pressure transducer in cylinder 2 and the compression is measured at 78 psi. The compression/expansion stroke tower is leaning to the right and the vacuum pocket at the bottom of the expansion stroke measures at 9.9 psi or near 20 "vac. Looks like a leaking exhaust valve.

In the second capture, CH C is the relative compression amps.

CH D is the WPS500 on the dipstick tube and the high crankcase pressures line up under the low starter amps. An old COP spark plug boot works well to connect the WPS500 to the dipstick tube.

In the third capture, CH C is the relative compression amps.

In CH D, I put some oil into cylinder 2 and now the cranking compression measures near 200 psi.

In the fourth capture, CH C is the relative compression amps.

The blue CH A is the injector 2 voltage on times. The maximum cold cranking injector on time is near 20 ms.

A used PCM was the fix and the engine now runs well.

+15
Michael Mobile Technician
Clinton, Utah
Michael Default
 

Wow Ray, You really thought that through. It is nice being the hero for your customer. Great Work!

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Mario Diagnostician
Weston, Florida
Mario Default
 

Nicely done! I warn everyone i can about assuming it's a mechanical failure. Always tell them to rule out a washed cylinder first! I hope you don't mind me sharing one of my personal experiences of a similar situation. youtu​.​be/J0nOJGvrweM

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

Nice work Ray!! I learned a long time ago, when using a scope for testing mechanical engine problems, especially testing for compression loss to test everything possible; valve leakage, ring leakage and leakage into the cooling system.

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Brandon Diagnostician
Reading , Pennsylvania
Brandon Default
 

AWESOME!...thanks so much for sharing. Goes to prove, once-again that we must ALWAYS get to the root-cause of a problem, not just the results of a problem 👍

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Ray Diagnostician
North York, Ontario
Ray Default
 

Thanks guys, much appreciated! Ray

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Frank Owner/Technician
Toronto, Ontario
Frank Default
 

Well done Ray! Lessons having been taught or brought again to light...either way a win for us!

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Orlando Technician
Toronto, Ontario
Orlando Default
 

Awesome work, Ray. I love these types of diags. Show the power of the scope and the knowledge to use the information it's giving you to make the right call. I'm curious about how the captures might have lead one to think of a leaking exhaust valve. I didn't see anything in your captures that would have lead me to think of a leaking exhaust valve as a suspect. Thanks.

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Ray Diagnostician
North York, Ontario
Ray Default
 

In the first capture, the WPS500 is in cylinder 2 while cranking and it shows a typical waveform of a leaking exhaust valve. There is the low cranking compression of 77 psi. The compression/expansion strokes are leaning to the right and the vacuum pocket at the bottom of the expansion stroke measures 20 inches of vacuum.

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

why do you say it looks like a leaking exhaust valve? How would the in cylinder waveform shape differ between a leaking exhaust valve, a leaking intake valve, or a leaking head gasket or piston ring? All I see in the waveform is a compression leak, and the direction to make another test to find out exactly where.

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Ray Diagnostician
North York, Ontario
Ray Default
 

You're right Albin, the reason that I started this post, is for techs to verify the reason for a low compression cylinder, such as a cylinder leakage test.

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