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Troubleshooting exhaust restrictions

Tuesday, October 1, 2019 - 06:00
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A few years back I had a customer that parked their car in a barn for almost a year. This particular customer stored his vehicle because he was deployed to Afghanistan with the U.S. Army. Being an Army veteran myself, I felt the need to help this individual out. The Ford Focus that I had to deal with exhibited a crank no start condition. It is not uncommon if a vehicle stored in a barn, for those of us that are used to rodent damage, to exhibit this complaint chewed wires. Some initial checks were made including checking DTC’s and a quick visual inspection. No obvious faults, including nesting material or damaged wires, were found. After pumping the last few gallons of old gasoline out of the fuel tank and adding some fresh gasoline, the engine started to sound like it was trying to fire. After some additional cranking and exercising the throttle, a weird “pop” was heard and the engine roared to life. However, it still did not have the most desirable acceleration even in the shop. I found out what the “pop” was when I walked around the back of the vehicle and saw a shotgun blast of four or five mouse carcasses scattered about three feet behind the tail pipe. Who knows how many more mice, nesting material, feces and food stash still remained in the muffler? I really didn’t want to run the vehicle for too long and smell what was left cooking in there. My guess is that would have been an unpleasant odor that might not leave the shop for a week. The decision was made to replace the muffler, which was quite heavy by the way, and the car ran fine.

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The Focus did not have a typical exhaust restriction. The most common cause of a restricted exhaust is a failed catalytic converter. However, the testing techniques covered in this article will identify the issue regardless of what the restriction is. In most exhaust restriction cases the vehicle will still run but exhibit low power complaints and, if the restriction becomes bad enough, the vehicle may also exhibit misfires.

In order to diagnose a restricted exhaust, I follow a logical process that consists of basically two steps. The first step is to perform a test drive while recording some data for analysis. If a restriction is suspected, the second step is to confirm the restriction using one of a few possible physical testing methods. Let us attack these two steps individually.

The test drive

Before performing any intrusive testing, a scan tool (preferably one with good graphing capabilities) is connected and the vehicle is taken for a test drive that includes some normal driving and a wide-open-throttle portion. A handful of data PID’s are chosen and recorded. These PIDs include: RPM, MAF, O2 sensors, short term fuel trim and long term fuel trim. If you are familiar with how the particular vehicle displays its Load PID then it can be used as well. Upon return to the shop the data that was recorded can now be analyzed.

The first thing to check is the volumetric efficiency, or VE, of the engine. This is basically a measure of how well an engine can breathe. This topic has been covered in previous Motor Age articles but can be summarized as follows: MAF and RPM are noted near the peak of the wide-open-throttle portion of the test drive. These two numbers are entered into a VE calculator, along with engine displacement, and a VE number is calculated. For naturally aspirated applications we would expect somewhere around 80% or higher if the engine can breathe efficiently. A VE number in this range indicates that the exhaust is not restricted because the engine can effectively “exhale.” On the other hand, if our VE is low, then more of the recorded data PIDs need to be observed. Note: This is also where a LOAD PID can be used if you know what is known good for the vehicle being tested. If you don’t know what a good LOAD number is for the specific vehicle, the VE will still work the same for almost all naturally aspirated applications equipped with a MAF sensor.

Next, provided we have a low VE number, the oxygen sensors are observed during the wide-open-throttle portion of the drive. With a restricted exhaust the oxygen sensors go rich when the vehicle is floored. The amount of air flowing through the engine is less than it should be but is still being measured accurately. The PCM is still injecting the appropriate amount of fuel for the given air mass measurement and the oxygen sensors report accordingly… rich.  If the oxygen sensors report a very lean condition then the fault is most likely not a restricted exhaust. In that case we would suspect another culprit such as a MAF sensor or other air metering fault.

Figure 1 - Scan data recording while test driving a vehicle with a restricted converter

Figure 1 is a scan data recording of a 3.5 liter General Motors vehicle that exhibits low power due to a restricted exhaust. Engine RPM (red) is shown so we can see where the wide-open-throttle acceleration occurred. The oxygen sensor (green) does in fact go rich under load. Figure 2 shows a VE calculation, from the same test drive, of 60 percent which indicates the engine cannot breathe.

Figure 2 - Exhaust restriction causing poor Volumetric Efficiency

Finally, as a bonus, fuel trim numbers are observed when the vehicle is operating in closed loop. Exhaust restrictions have little effect on fuel trim numbers unless there are two banks with two catalytic converters. In that case, if one converter was restricted, the fuel trim numbers from bank to bank will move in opposite directions from one another. If the low power complaint were to be caused by a failed MAF sensor, or weak fuel delivery, then our trim numbers would climb higher and higher into the positive range. In the previous example fuel trim numbers were slightly negative but still within an acceptable range.

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