TST held its 10th Annual Big Event this past March in Fishkill, N.Y. Presenting at the event were John Thornton and David DeCourcey. DeCourcey is the owner of D&D Professional Automotive Services, a mobile diagnostic service and training provider based in Worcester, Mass., and is appearing here as a guest columnist. To see more from Dave, visit his website at DDProfessional.com.
Mastering Misfire Diagnostics
As one of the two presenters, my topic was “Misfire Strategies for Today’s Vehicles.” My goal was to teach the attendees during the event to concentrate on three topics during their troubleshooting process.
How do you go from swapping coils to checking cam timing? By not following a logical process, that’s how.
First, I want the main focus of any misfire diagnosis to be on the combustion process. Anything that negatively affects the combustion process will create a misfire. I refer to these negative effects as simply the three Ls:
• Lack of a combustion process.
• Low combustion process.
• Late (or early) combustion process.
Second (and most important) is locating the actual root cause of the misfire. In most cases, the effects of the misfire are easily detected. Locating the root cause for the rough idle, sputtering on acceleration, or any other outward indication of the miss can be more elusive, especially with intermittent faults such as leaking fuel injectors or a weak, leaking or overheating ignition coil.
Third is the importance of using a solid diagnostic process for locating the root cause of the misfire. The idea is multifold as to keep focused, to avoid random testing, to prevent replacement of good parts and to reduce the total time spent on the diagnosis.
The cranking amperage test quickly revealed a mechanical problem with two cylinders.
To give you an idea of the process I use to isolate a misfire’s root cause, follow me as we take a look at a 2004 Kia Sedona minivan with a 3.0-liter V6. The little van has approximately 78,000 miles on the odometer and a local dealer had recently installed a used engine. The shop that called for my help reported the customer was complaining of a rough-running engine and a lack of power when accelerating. The Malfunction Indicator Lamp (MIL) was on and on occasion, flashing. All sure signs of the effects a misfiring cylinder would cause.
The shop’s tech easily had verified the customer’s complaint and performed a visual inspection. He had retrieved a series of Diagnostic Trouble Codes (DTCs) indicating misfires in cylinders 1 and 2 (P0301, P0302) as well as a random misfire code (P0300).
During his attempts at repairing the problem, he replaced two of the three ignition coils on this Distributorless Ignition System (DIS) and torn down the front of the engine to inspect the cam-to-crank timing. That, of course, proved to be correct.
From what you know so far, do you see any mistakes this tech had made?
For one, he’s testing randomly, isn’t he? How do you jump from swapping coils to pulling the engine apart? This is because he’s not following a solid diagnostic process. He’s not focusing on the three Ls. And he’s replacing parts unnecessarily.
Pete and G. have said it often. Test, don’t guess!
Let’s Start Over
Whenever I’m called in by another shop to troubleshoot a problem, I always come in “cold.” I really don’t care what you’ve done so far and I’ll often ask a shop to deliberately not tell me what they’ve done. I start from the beginning as if the car had just been brought to me in the first place.
A reference trace is any waveform that helps you identify where you are at in another. Here I’m using cylinder 6 ignition and injector events to identify the weak cylinders by firing order.
And where does that start? I’ve developed my own Misfire Diagnostic Work Sheets. They provide a simple step-by-step diagnostic process for determining the root cause of any misfire condition. There are a total of six worksheets, but since I’ve been using this diagnostic process I’ve found that in most cases I’ll rarely complete the third worksheet before the root cause is revealed. (Editor’s note: Contact David via his website to request more information on these worksheets and how they are used.)
Let’s start in on this soccer mom’s car using the first page of the worksheet.
1. Verify the customer’s complaint. That can be as simple as checking for codes or taking a test drive. All I had to do on this one was start it and let it idle to know that it had a dead hole.
2. Perform a visual inspection. Note all recent work, bodywork, and aftermarket accessories, but do not disturb anything. If I see a possible issue, I will verify it as being the actual problem during the diagnostic process. Nothing stood out on the Kia so I moved to the next step.
With the references added, I know now that cylinder 1is a dead hole and cylinder 3 is weak.
3. Check the charging system. Check for large voltage drops and excessive EMI (electric noise). I prefer 150mv AC or less, and my personal maximum allowable spec is 300mv AC. It’s amazing how many cars I’ve fixed by simply correcting a bad cable ground but that wasn’t the case on the Sedona.
4. Check for binding/seizing accessory drive components. A jerking motion of the belt tensioner will be a giveaway of a binding accessory drive and can be enough to set a misfire code.
5. Perform a Relative Compression test. Also known as a cranking amperage test, this is a quick way to see if all the cylinders are contributing their share.
And that’s where I found my cause.
I only connect a high amp clamp when I perform my cranking amperage test. I just want to very quickly get a gauge on the health of the engine. And, in the case of the Kia, the current draw pattern clearly showed that there was one cylinder that was totally dead and another that was low. Now the extra effort is justified to hook up another channel on the scope and get a reference I can use to identify exactly what cylinder is causing me problems.
This piece of zip tie found its way into the intake during assembly and wedged itself into the intake valve, holding it open.
A side note here. You might think it’s going to be cylinders 1 and 2 based on the codes the first tech pulled from the ECM. But I’ve found more than one case where the codes lied about the cylinder that was truly weak and rarely rely on that alone. Let’s see if that plays out on the van.
The ignition firing event occurs closest to Top Dead Center (TDC) on the compression stroke and makes a great reference. Synch off any accessible ignition coil, then use the firing order to determine the non-contributing cylinder. I synched off the cylinder 6 ignition coil and the firing order is 1-2-3-4-5-6. Keep in mind that this is a waste spark ignition system; each ignition coil fires twice every 720 degrees of crankshaft rotation. How can I be sure I am actually syncing off the ignition event for cylinder 6? Or am I looking at the waste spark?
How would you further identify the cylinders?
What about the injectors? They inject fuel near TDC, but on the exhaust stroke 360 degrees before the combustion event. Adding the injector reference now allows me to exactly identify the non-contributing cylinders by firing order. I know for certain now that cylinder 1 is contributing very little and cylinder 3 is weak.
There is no number of spark plugs, coils, injectors, etc. that can be installed in an engine to repair a low or non-contributing cylinder. With the weak cylinder identified, it was easy to dig a bit deeper and find the actual cause of the compression loss. A piece of wiring harness zip tie had been accidentally dropped into the intake manifold during assembly and had become lodged in the cylinder 1 intake valve. Had the first tech focused on the combustion process, and followed a solid diagnostic process, this shop could have saved the cost of my diagnostic fees, unnecessary parts replacement and wasted time. So remember those three Ls when you get your next misfire complaint.
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