Editor's Note: This article was orginally published Oct. 14, 2015. Some of the information may no longer be relevant, so please use it at your discretion.
In our previous article, we discussed various methods of using the scan tool to obtain information useful in determining the cause of misfires. In this article, we will combine the information gained from the scan tool ,along with other diagnostic tools to determine the root cause of the misfire and whether it was caused by an ignition or mechanical issue.
Step 1: Review diagnostic worksheet and scan data
Looking back at the diagnostic worksheet on this vehicle, there are some items that stand out:
- Misfire codes on the No. 2 and No. 4 cylinders
- Fuel trims appear to be normal
- Ignition components are worn
- Engine vacuum is low (16”) and is pulsing
Let’s take each item in order. Having misfire codes on multiple cylinders is not uncommon, but the cylinder locations can tell you quite a bit about possible causes. If the cylinders are next to each other, mechanical issues (such as a compression leak) are a possibility. If they are opposing cylinders on valve overlap, it can indicate an intake valve sealing issue.
If they are next to each other in the firing order, it can indicate an ignition system issue caused by an ignition system or crank/cam sensor fault. It is also helpful to remember that the same issue causing a misfire on one cylinder may not be the same cause of a miss on another cylinder. An example could be an ignition miss on one cylinder and a mechanical issue on another.
The next item is fuel trims. The trims are within a normal range, but you need to look at the loop status. In this case, it is in open loop. That means the oxygen sensor data is not being reported to the PCM, therefore fuel trims may not be accurate. Fuel trims can provide very good information regarding what type of misfire is occurring, and possibly its cause. In next month’s issue, we will be looking at fueling misfires and get deeper into fuel trim diagnosis.
Part of the initial diagnosis included inspecting the ignition components. The plugs were worn, indicating a possible issue.
Lastly, observe the engine vacuum. It is low, meaning there is an issue ranging from a vacuum leak, a camshaft timing issue or another type of mechanical issue. Since the vacuum reading is not steady, we will go in the direction of a mechanical issue. Pulsations in the engine vacuum indicate a cylinder sealing issue, (typically a valve not sealing) allowing compression to leak back into the intake manifold.
Step 2: Obtain vehicle specifications
Due to suspecting a mechanical issue, it is a good idea to obtain specific information regarding its specifications. Using ALLDATA, we found that normal engine vacuum should be between 18-22 in-Hg and cranking compression should be a minimum of 120 psi.
Step 3: Perform cranking compression test
Using our Snap-on compression tester, we performed a compression test with the following results:
Remember to have sufficient and consistent battery voltage during the test and to open the throttle allowing sufficient air to enter the cylinders.
- Cylinder No. 1 – 80 lbs
- Cylinder No. 2 – 80 lbs
- Cylinder No. 3 – 85 lbs
- Cylinder No. 4 – 115 lbs
At first look, the compression readings are confusing due to having higher compression on cylinder No. 4, which is one of the cylinders with a misfire.
Step 4: Perform pressure/vacuum test with lab scope
A very good tool that can be used to diagnose causes of a mechanical misfire is a lab scope with a vacuum/pressure transducer. As with any test using a lab scope, you can gain much more information looking at data in near real time. Using a vacuum gauge and a compression tester may show readings that differ than the vehicle manufacturer’s specifications, but using a lab scope will allow you to observe vacuum and pressure at the same time on each cylinder.
The transducer is installed into the combustion chamber in place of a spark plug and then connected to the lab scope electronically providing a digital positive voltage for pressure and a negative voltage for vacuum. The voltage amount corresponds to the pressure or vacuum reading, for example 10 psi would provide a 10V positive reading. This test can be performed either in an engine cranking mode or an engine running mode.
Keep in mind that, like many diagnostic test procedures, the pressure tests you can perform with a lab scope are comparative, meaning that you may not see the same pressures or vacuum readings on every vehicle, but you can use them to compare cylinders on the same vehicle.
First, allow the engine to reach operating temperature. Disable the ignition system and remove the spark plug from the No. 1 cylinder. Install the pressure transducer in place of the spark plug using adapters as necessary (in many instances your compression tester hoses can be easily adapted). In order to use the compression tester hose for adapting to either a vacuum/pressure transducer or a leak-down tester, the valve core must be removed from the hose. Just as in compression testing, assure the battery voltage is sufficient and consistent during the test process. Connect the transducer appropriately to your lab scope. Crank the engine 8-10 times, making sure the throttle is fully open. If at this time you choose to perform a running compression test, re-enable the ignition system and start the engine. Allow the engine to idle for about 30 seconds while recording with the lab scope. Turn off the engine and reinstall the spark plug into the No. 1 cylinder. Repeat the process with all cylinders, taking care to duplicate the tests as close as possible (including engine temperature).
In the images of the cranking compression test, there is some good information. Note the waveform pressure readings on all the cylinders. The pressure (positive voltage on cylinders No. 1, No. 2 and No. 3) are all very similar at approximately 50 psi. The voltage on cylinder No. 4 is significantly higher (approximately 120 psi). Notice the vacuum readings on all cylinder are approximately the same (15 in-Hg).
Observing the images of the running compression tests provided significantly more information. As with the cranking compression results, the pressure readings of cylinders No. 1 No. 2 and No. 3 were very similar and cylinder No. 4 was substantially higher. Here is the value of using a lab scope with a running compression test; being able to observe what is happening during the complete engine cycle showed a substantial drop in the vacuum voltage reading between cylinder No. 4 and the others. When the engine is running, the ability of each cylinder to intake enough air can be different than when the engine is cranking due to turbulence in the intake manifold and other factors.
Step 5: Perform cylinder leak-down test
Since we determined that the compression was lower than the manufacturer’s specifications, we would like to determine where compression is leaking in each cylinder. It can leak past the piston rings, past the valves, between cylinders or into the cooling system. The way to determine where compression is leaking is to use a cylinder leak-down tester. This tool allows the technician to put air into each cylinder and measure the percentage that is leaking out and by listening for air (where it is leaking from). It is best to perform this test on an engine that is at operating temperature because some issues can cause less of a leak when the engine is cold.
Start with the No. 1 cylinder and remove the spark plug. Rotate the engine until the cylinder is at top dead center with both valves completely closed. Install the tester into the spark plug hole and use shop air to fill the cylinder. The attached gauge on the tester will show you the percentage of the air going into the cylinder is leaking.
Listen to the throttle body, the exhaust pipe and the crankcase to determine where you hear air. Since air leaking past the exhaust valve maybe difficult to hear in the exhaust pipe, placing a glove over the exhaust pipe and looking for it to expand can show even the smallest leak. Once you determine where the air is coming from, you can decide the extent of the repair necessary. The results we found on this vehicle were cylinders No. 1, No. 2 and No. 3, all had around 40 percent leakage (and we could ear air in the intake and exhaust). Cylinder No. 4 had about 15 percent leakage and we were unable to determine the source of the leak.
Step 6: Analyze the results
In order to determine possible causes of the misfire, let’s look at the results of our tests:
- Multiple misfire codes (Cylinder No. 2 and 4)
- Worn ignition components
- No conclusive freeze frame data
- Low and pulsing engine vacuum
- Lower than expected compression readings
- Cylinder leak-down test showed substantial air loss through the intake and exhaust on cylinders No. 1, No. 2 and No. 3
- Different vacuum and pressure voltage readings on cylinder No. 4
Knowing the overall compression and engine vacuum readings were low, and the vacuum was pulsing we can be pretty sure there is a mechanical issue causing the misfire. It would make sense that since cylinder No. 4 had a misfire code and the results of the tests we performed showed differences from the other cylinders that the issue would be found in cylinder No. 4. In most cases, that would be an accurate conclusion, but having higher compression will rarely cause a cylinder to misfire.
Remembering that the PCM uses a strategy that determines if a misfire is occurring by measuring gains in rpm from one cylinder to the next. In this case, there are three cylinders all with relatively the same compression and vacuum, all lower than the manufacturer’s specification and one cylinder with higher compression matching the manufacturer specification. Because the PCM is looking for increases in crank speed to determine if a misfire occurred, it cannot determine the increase in crank speed on cylinder No. 4 is actually because that cylinder is working more efficiently than the others and the issue is actually the other three cylinders not contributing enough, verified by the cylinder leak-down test. Since we had substantial air loss from both the intake and exhaust valves, we determined the best course for the repair would be to remove the cylinder head.
