Persistence in testing
2006 Pacifica with a customer complaint of poor running ever since another shop installed spark plugs. Also a complaint of the check engine light on but that was on before new spark plugs. Issues need to be corrected so an emissions state inspection can be performed.
One of my religious first steps to diagnosing a misfire really saved an embarrassing misdiagnosis here.
The first step I perform before staring the vehicle that has a complaint of misfiring is to hold the accelerator to the floor when cranking to enter clear flood mode and listen for an even cadence that would yield some clues to the basic sealing ability of the cylinders.
This 3.5 had that one quick revolution. Sounds like a weak cylinder.
So I know I have at the very least need to inspect the mechanical integrity of this engine.
A scan of the engine module was performed 2 codes were set diag.net/file/f1e1x49ys…
codes for improper secondary ignition ionization for cylinders 1 and 4. So those codes basically state that no work was done on the secondary side of the coils for the affected cylinders. Either no or poor spark or low compression in the cylinder.
Next the firstlook sensor was installed in to the exhaust. The scope was synced to cylinder 1 ignition coil. diag.net/file/f4j9bbd2m…
The exhaust pulses clearly indicated two cylinders misfiring but there is no ignition coil trigger for cylinder 1. The coil for cyl 1 is not being grounded . It clearly has power but no ground trigger from the engine module.
The coil circuit needs to be diagnosed. The coil was commanded on with a scan tool and still no pulse was detected at the coil. Next the left inner fender was removed to access the engine module. Connector 2 which contains all of the coil trigger wires was removed from the module. diag.net/file/f3japecyk…
The ASD relay was jumped out so the coils would have power. diag.net/file/f3man2011…
Next a meter set to 10 amps was grounded and the other meter lead was used to apply ground to the cylinder 1 coil trigger wire in module connector 2. The meter indicated there was sufficient current flow to the coil. That test verified the wiring from the engine module to the coil and the coil. The lack of trigger for cylinder 1 coil is failed in the engine module.
So cylinder 5 was then used as a sync trigger with the exhaust pulses. diag.net/file/f3peg8har…
Clearly the exhaust pulses confirm that there two cylinders not firing. It is known that cylinder 1 is not firing because of a failed coil driver in the engine module but why is cylinder 4 not firing? Time to perform some mechanical integrity testing for cylinder 4.
A relative cranking compression test was performed. A 600 amp current clamp was used around one of the battery cables. The scope was the synced to cylinder 5 coil.
The relative compression test confined my suspicion that one cylinder had low compression.
Following the firing order it is determined that cylinder 4 is the one low on compression. Also by the larger than normal requirement for starter current on cylinder 5 that would indicate possibly an exhaust valve was not sealing on cylinder 4 and the exhaust was in the intake causing high intake pressure when cylinder 5 intake valve opened. Just a suspicion. To access cylinder 4 to perform an in cylinder pressure analysis the intake would need to come off. We will do that later.
So this Pacifica has two very different problems causing misfires. One is a cylinder with low compression and the other is a failed driver in the engine module for ignition coil cylinder 1. Persistence in testing paid off. It prevented a possible embarrassing situation where possibly the engine module would be replaced only to find out the engine also had mechanical issues.
Great Find Robert!
Another example how stepping back and analyzing the situation can prevent a very sticky conversation.
Robert nice job...its always good to check the mechanical condition as you did ...it prevents egg on the face and a ticked off vehicle owner. Well done...reegards
Robert I really enjoyed reading this case study. I learned something from it. One thing I'm always having a problem with is always understanding the wave forms from the first look sensor. For some reason my mind can not filter out all the peaks and valleys, not like the relative compression wave form. Another thing I remembered from reading this is how heated I got from a conversation I had with a costumer and tech yesterday about being a diagnostician. The customer and tech responded by saying "well the car computer diagnosis it for you, it tells you whats wrong with it." In this case the first tech saw the ignition coil faults and replaced the parts because it told him "ignition coil cylinder# 4 and #1"
A very thorough investigation. Regarding the higher amp draw on cylinder 5, the way I understand it, your cylinder 5 has to work harder on the compression stroke due to less help from the preceding cylinder's weaker expansion stroke. That cylinder having less push due to lower compression.
That is true. I agree with that. I may pull off intake and do an in cylinder pattern to see why it is not sealing. Customer is in limbo as to whether or not it is going to get repaired. If I do I will surely post it here.
Thank you for sharing Robert, great case study with a thorough diagnosis. Its a pleasure to be on Diag.net with all of you great techs/diagnosticians.
I dont own any pressure transducers/first look sensors etc and definitely need more training on them, but it’s always good to see how they can be used in diagnostics. (Definitely will be adding them to the arsenal in the future)
All we need now is to get others on board to better there skills in diagnosing so customers can get there vehicles diagnosed properly the first time.
Amen to that Mr. Garcia! It is an intresting class I can tell you that. Automotive Training Authority offers that class its called Master Level Diagnostics the professor who teaches it is DR. NORMAN L. (DOC) NALL. Their they go over on how to use a first look sensor to solve driveablilty issues.