There are many places that technicians might start their diagnosis when it comes to a misfire problem. I have a list of things that I suggest following to make your diagnosis fast and easy. Let’s start with a vehicle that I was called to diagnosis. The problem vehicle was a 2004 Ford Explorer 4.0L SOHC V-6. The customer’s complaint was that the engine runs rough, lacks power on acceleration and has an illuminated Malfunction Indicator Lamp (MIL).
A cranking compression test is a quick way to identify low compression cylinders early in your troubleshooting process.
Based on the diagnostic worksheet I developed, I first ask myself if the customer complaints appear to be related. Could one be the root cause and the other the effect? The technician had replaced all the ignition components without having the problem resolved. When I diagnose a vehicle, I prefer not to know about any previous repair attempts or components installed. This is one of my Golden Rules of troubleshooting: Do not be mislead by the actions of others. Do not allow other previous attempts or the vehicle’s recent repair history lead you in a diagnostic direction. It is easy to fall into a trap and move past your normal testing routines based on secondhand information you are told about the vehicle or previously performed tests. You always should start from the beginning with any diagnosis.
After that first initial analysis, I move on to my second step: look at the symptom. Third, I concentrate on the symptom. Fourth, I follow the data and test results. They will lead you to the root cause. Fifth, I trust my diagnosis. If the symptom still is present after the repair, test the replacement part before you second-guess yourself and start looking elsewhere for a possible cause. Sixth, I know my equipment before I start a diagnosis. Know their limitations and functionality. Practice with them on known good cars before you try them out on a problem child. That will help ensure accurate test results so you can trust your diagnosis. Last, when dealing misfire complaints/ symptoms I always use the Misfire Diagnostic worksheets as my guide.
All a CKP/CMP pattern will tell you is whether or not the sensors are in synch and producing correct signals. Good info, but not what I needed to find out.
Starting the diagnosis at the beginning, I perform a complete visual inspection, but do not disturb anything. If you notice a possible issue, record it. If it is actually the root cause of the customer’s complaint, you’ll find out during the diagnostic process. This could/will save valuable diagnostic time, especially when dealing with intermittent issues. The visual inspection revealed no obvious issue creating the rough running complaint. Here is some of what I observed:
- The fuel level was over a half a tank. (Remember, low fuel levels can create lean air/fuel mixtures and misfires during our road test).
- The engine fluids were at their correct levels and the fluids were in good condition.
- There were no binding tensioners, pulleys or accessory drives. (These types of issues would give us random (non-cylinder specific) misfire failed tests (DTCs).
- The charging system was functioning properly.
- There were no large voltage drops, and the ac ripple (electric noise) was 145mv.
- The Relative Compression/ Cranking Amperage test revealed no low or non-contributing cylinders.
If you do encounter a waveform that reveals a low or non-contributing cylinder, use the ignition system for sync to find the misfiring cylinder. This procedure will identify the misfiring cylinder in the firing order. I suggest performing a relative compression test before looking at stored scan tool data. Viewing scan data, I noticed that there were two failed tests, P0106 and P0340, that were results of previous testing.
I cleared the OBDII stored data and ran the vehicle in the bay for approximately 10 minutes. I had the technician perform several throttle snaps in neutral and in gear, while I gathered my equipment for the next few tests. I found it odd that no failed tests were set during this time even though the engine was noticeably running rough.
The cylinder pattern in green is obviously not a match with the pattern in blue. Other than the difference in peak pressure, is there anything else that catches your eye?
The next series of tests are intrusive starting with the Crankshaft Position (CKP) and Camshaft Position (CMP) sensor signal circuits followed by two outputs; the injector and ignition control circuits. Remember, “garbage in = garbage out.” Any CKP or CMP signal distortion could/will create misfire and/or drivability issues.
The waveform showed a good CKP and CMP correlation, and the Powertrain Control Module (PCM) was correctly controlling the injector and ignition circuits. I like to stop at this point and consider what the test results have revealed.
What type of engine design is in this engine?
How many camshafts does this engine have?
This is a V-bank engine with single overhead camshafts. I need to ask myself where the CMP sensor is located, and what is the correlation in relation to? The camshaft sensor is located on the top front of the Bank 2 valve cover on this engine. The waveform capture is the actual correlation of the crankshaft and the Bank 2 camshaft sensor positions. The synch between the CKP and CMP was good. The CMP sensor was located on the front (Bank 2) exhaust camshaft so that revealed that the front exhaust camshaft was in correlation with the crankshaft sensor position, but didn’t reveal the correlation of the other camshafts, especially the Bank 1 cams. This is what you need to keep in mind during your diagnosis. So what tests can we perform that will quickly and accurately reveal the intake and exhaust valve timing on each bank? A running compression test using an in-cylinder pressure transducer.
I performed this test by removing a single spark plug on each bank and installing a pressure transducer in its place. This test will reveal the valve timing for each bank, as well as a wealth of other information such as restricted exhaust, compression, valve opening and closing events, recessed valves, worn/broken rings and bent rods. During this misfire diagnosis, I used two pressure transducers at the same time allowing me to see results from both banks.
A quick look at the waveform shows very different compression levels of each bank. Bank 2 (Ch 4-blue) is at a normal running compression level, while Bank 1 (Ch 3-green) is significantly lower.
When you compare Bank 1 waveform to Bank 2 waveform, what differences do you see other than the compression levels? The intake and exhaust valve timing on Bank 1 is approx 30 degrees retarded compared to Bank 2, which has normal running compression levels.
This is the root cause of the rough running complaint.
My last Diagnostic Golden Rule: My test results must confirm a component has failed or is failing before I ever consider removing it from the vehicle. Using this vehicle as an example, you can easily see that there are no amounts of spark plugs, coils or sensors that will repair this rough running issue. The only repair that will correct this vehicle is the correction of the cause of the retarded cam timing. In this customer’s case, it was the replacement of the timing chains, tensioners and guides.
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