While I am certainly a high-tech equipment junky, I am also a big fan of simple low-tech but effective testing techniques. One of my favorite low-tech tools is a simple bottle of propane, the type you would use for a propane torch. It's an easily applied source of air/fuel (A/F) enrichment to any type of engine management system. The great part about using propane is that the bottle-mounted regulator knob can be used to vary the amount of enrichment applied, unlike aerosol cans of carburetor cleaner or starting fluid. Being that propane is a gas, it also distributes more evenly to all cylinders than does a liquid. Regulation and even distribution of enrichment allow us to use propane as a cylinder-specific misfire diagnostic tool.
When faced with what seems to be a cylinder-specific misfire at idle, one of the first things we need to know is if the cylinder is misfiring because it's too lean or too rich, if there is insufficient spark or if there is a mechanical defect resulting in a loss of cylinder filling, emptying or sealing. When faced with these possibilities, we always begin with the least time-consuming tests. Propane enrichment to test for a lean misfire condition is certainly one of those tests.
Understand that a cylinder can fire successfully with A/F mixtures in a range from 16:1 (lean) to 13:1 (rich). What we are after with the application of propane is an overall engine mixture that allows the lean misfiring cylinders to become rich enough to fire without allowing the overall engine mixture to become so rich that it causes a misfire in the good cylinders.
To do this, introduce the propane gas into the intake manifold either before or after the throttle body, first in smaller amounts and then in gradually larger amounts. Each time you increase the flow, wait a few seconds for the engine management system to react to the increased enrichment. When you are first learning to perform this technique, watch your Short Term Fuel Trim (STFT) as you are adding the propane. Each time you increase the flow, expect the engine management system to drive the STFT greater than 10 percent negative to bring the total mixture back toward the target of 14.7:1. Allow the engine management system time to stabilize so the good cylinders do not end up with a rich misfire just as the lean cylinders come back to life due to the enrichment.
You will know the engine management system has successfully adjusted the overall A/F mixture when the STFT stabilizes around 0 percent ± 10 percent. This tells you the Long Term Fuel Trim (LTFT) has compensated for the added propane. After a few minutes of playing with the propane bottle flow knob to achieve this balance, you will be flowing just enough propane to bring the lean cylinders back to life but not enough to cause misfire in the other cylinders.If all cylinders are now firing, you have determined for sure that the cylinders that were previously misfiring were too lean. Now cut off the propane supply abruptly. Expect the cylinders that were previously misfiring to start misfiring again, only now much worse since the engine management system has the LTFT adjusted lean on the entire engine. After practicing this technique on a few vehicles, you can easily perform this test without the aid of a scan tool.
In the scan data shown in Figure 1, a 1997 Chevrolet pickup has DTC P0306. This vehicle is misfiring at idle due to an internal intake manifold leak around the No. 6 intake runner. Figure 2 is the same vehicle at idle while carefully regulating propane into the engine at the airbox inlet.If the lean condition is due to a vacuum leak, either internal or external, then the misfire should subside as you raise the rpm or drive the vehicle under load. This is due to the vacuum leak being a fixed mechanical size that will have less effect at faster engine speeds and the resulting higher intake airflow.
If the lean condition is due to a cylinder-specific fuel delivery problem, like a severely clogged injector, then that cylinder will probably remain lean throughout the entire engine speed and load range.
Of course, be careful using propane to find an external vacuum leak. Propane is easily ignitable by the smallest of sparks, which makes this an extremely dangerous technique if the ignition wires are leaking. But with practice and attention to detail, you can learn to use propane enrichment to find a lean misfire quickly and with great precision.
Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.
