Question about RC Test

Chris Technician Louisiana Posted   Latest  
Resolved
Driveability
Drivetrain
1996 Chrysler Town & Country 3.3L (R) 4-spd (41AE) — 1C4GP54R6TB283125
Misfire

Hi guys. I am pretty new at the Pico and I was wondering if someone can explain what I’m seeing in this RC test. This van sat for 10 years. Customer got it running and drove it in with a dead miss. Scan tools would not communicate so I did RC test. You can hear the obvious sound of a dead cylinder. At first look, it looks to me like cylinder 6 is the problem. Manual gauge showed 125psi. Pulled the intake and found cylinder 5 port way cleaner than the rest. Camera shows 5 intake valve not opening. Question is can someone explain why it looks like 6 to me on the RC test? My Trace is cylinder 2 injector. I understand that a bad cylinder can affect the next in line. Just looking for a better explanation. Sorry for the long post. Thanks!

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Dmitriy Analyst
Ontario
Dmitriy Default
   

An injector event does not happen at the end of the compression stroke for that cylinder (talking about this case, not all engines!), so the labeling is off. To match things up, you need to add a 4-stroke overlay like here: diag​.​net/file/f663krew9…

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Matt Technician
Missouri
Matt Default
 

As the others have said injection timing isn’t based on TDC, you need ignition to determine cylinder.

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Jürgen Owner
Mexico
Jürgen Default
 

Not direct injected engines typically inject fuel at the end of the exhaust/beginning of the intake stroke. In your picture #5 cylinder appears to be injecting (companion of #2), hence #3 seems to be the offending one without compression. You could take samples of spark and injector of the same cylinder on a working vehicle to get an idea of their relationship. In general it's a good thing tu…

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Stephen Technician
Tennessee
Stephen Default
 

I agree completely with your analysis. Since it is only to ID which cylinder is in trouble there isn’t a need for precise timing measurements. An intake valve not opening will not cause a total loss of compression, there is still air in the cylinder , at atmospheric, to compress. The pressure should be near normal. Disabling the injectors is good theory but I don’t think that cylinder washdown…

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Dmitriy Analyst
Ontario
Dmitriy Default
   

Stephen, regarding whether an intake valve not opening causes compression loss... - Let‘s rewind back to the 360 deg. mark. The exhaust valve has just closed or is closing, the intake valve is always closed, pressure is atmospheric, the piston is at TDC and is about to go down. What would happen by the end of the induction stroke (540 deg.)? - Once you had enough fun with question 1, here is…

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Stephen Technician
Tennessee
Stephen Default
 

Well,,Here’s another fine mess that I’ve gotten myself into. I haven’t found it in the case study yet but I now see where my thinking went wrong. Thanks for letting me know, again.

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Dmitriy Analyst
Ontario
Dmitriy Default
 

You are welcome, glad that my leading question was helpful.

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Stephen Technician
Tennessee
Stephen Default
 

I studied the link that you provided. It verified and enhanced my thought experiment answer to your question. I am sorry to have given information to others that is incorrect.

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Chris Technician
Louisiana
Chris Default
 

Geez I didnt even think about and thats why I asked the question. Honestly I’m not sure why I grabbed the injector instead of the plug wire. Rookie I guess. I have done mostly cop systems as we don’t get to many plug wires anymore around here. That’s why you guys are here right?! Lol.

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Douglas Mobile Technician
Texas
Douglas Default
 

Put your sync on #1 ignition.

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Sean Technician
Ohio
Sean Default
 

The injector will fire before tdc.

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Jj Technician
Massachusetts
Jj Default
 

Reference off of ignition and you will see your problem I believe. Think about when the injector fires in the cycle.

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Daniel Owner/Technician
Texas
Daniel Default
 

...Best guess...it stuck a valve and bent a push rod or broke a rocker...get on with looking under the valve cover and fix what ya find...try to free up that valve​.​​.​​.​bad/varnished gas is what did this...Dan H. ...South Texas...

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Kyle Diagnostician
Pennsylvania
Kyle Default
 

Shift your identifiers one hump right, the injector fires for the cylinder hump that follows it. -Kyle W._ South Philly

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Chris Technician
Louisiana
Chris Update
 

Thanks guys. May have been a stupid question cause it was a light bulb when y’all answered. Wasn’t thinking straight but that’s why I asked. Gotta learn somehow right? I’m gonna re run the test just for my sake and records. As I said I know for sure the vale is stuck. Just trying to get some learning in while I can.

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Kyle Diagnostician
Pennsylvania
Kyle Default
 

Using the injector as a reference is fine just remember the sequence of operation. —Kyle W.- South Philly

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Jj Technician
Massachusetts
Jj Default
 

It's easy to go cross-eyed when you're learning this stuff, I know because I'm still learning it too. Don't be worried about asking "stupid" questions, be worried when you stop asking.

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Rudy Technician
California
Rudy Default
 

It wasn't a stupid question at all. Ive done it in the past and was confused for the same reason,as Im sure others have. Now you know better.

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Rick Technical Support Specialist
Missouri
Rick Default
 

Chris, You are doing great! Keep taking samples and asking questions. You will be giving great advice to others in no time!!! This day and age the fuel timing strategy can be all over the place and will not time with the combustion or compression event. Here is the rule, sync with power (Ignition). It is a good foundation for the cam timing and the combustion event. It will help to pinpoint…

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Chris Technician
Louisiana
Chris Default
 

Thank you!

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Kyle Diagnostician
Pennsylvania
Kyle Default
 

Totally agree with Rick. -Kyle W.- South Philly

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Chris Technician
Louisiana
Chris Resolution
   

Just for a follow up, I did rerun my test while hooked to cylinder 1 plug wire just for fun. I can clearly see the correct cylinder now. Unfortunately For the customer cylinder 5 intake pushrod is bent. Valve contacted the piston, is bent and will not seal. Thanks again for the learning lesson on using injector for a trace! I’m enjoyed learning this stuff.

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Daniel Owner/Technician
Texas
Daniel Default
 

...Just as I suspected...although the valve to piston contact was unfortunate...the bad fuel caused it to stick ,most likely. ...Dan H.

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Edwin Technician
Utah
Edwin Default
 

The beauty of this is others get to read along & learn with you. Thank you for the post.

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