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.
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 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.
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.
To summarize the data analysis: if the VE measurement is low, the oxygen sensors display rich and fuel trim numbers are not ridiculously positive then a restricted exhaust is suspect. Once we have analyzed the data, and our conclusions strongly suggest a restricted exhaust, it’s time to get dirty and confirm our hypothesis.
Physical testing
There are quite a few methods used to test for exhaust restrictions. Some are better than others. They will be covered one by one.
Drop the exhaust and drive the car
This method to me is pretty “shade tree” to say it politely. It involves disconnecting the exhaust before the catalytic converter and test driving the vehicle again to see if power returns. Although this technique is somewhat effective, it can be labor intensive and will definitely be very loud during the drive. I think there are better options.
Vacuum testing method
This method involves connecting a vacuum gage to the intake manifold and revving the engine up while observing the gage. I believe this test is flawed because an exhaust would have to be extremely restricted to see any discernable change in manifold vacuum. Again, I feel there are better, and more accurate, methods of proving our hypothesis.
Backpressure at the O2
This technique is the most common test that has been used by technicians for many years. It requires either a dedicated backpressure tester or some creative connections with tooling you may already have. This creative tooling includes a compression gage hose and a standard vacuum pressure gage. If present, the Schrader valve should be removed from the compression hose and the gage can be connected to the end of the hose with a piece of vacuum line or similar tubing. Effectively you are building your own backpressure gage.
The backpressure tester, dedicated (Figure 3) or homemade, is designed to be screwed into the oxygen sensor mounting bung just before the converter. The vehicle is started and the throttle is snapped. A good vehicle should have little or no backpressure which indicates that the exhaust is freely exiting the engine and exhaust system. If the backpressure gage spikes 4 psi…10 psi…or sometimes even worse, then an exhaust restriction is present.
There are some problems with this test. First, access to oxygen sensors on some vehicles can be very difficult and time consuming. Second, if you live in an area that is prone to rust, removing the sensor can be even more difficult and can result in thread damage of the mounting bung, oxygen sensor or both. Again, more time consumption.
However, this test does have an advantage over some of the testing that will be covered shortly. If this test is repeated with the gage connected to the downstream oxygen sensor location and the results indicate high pressure then the restriction is further back in the system and not the catalytic converter.
Backpressure with a drill
This technique works exactly the same as the previous method but requires the technician to drill a hole in the exhaust ahead of the converter, install an adapter and connect the backpressure gage. Although this test allows easier access, it requires damaging the exhaust and then an additional repair after the test is complete.
In-cylinder
This test is by far the easiest to perform in my opinion. It does require the use of an oscilloscope and a pressure transducer. It also has an advantage that I believe is extremely valuable: ease of access to a test point. The only component we need to access on the vehicle is a spark plug. I know that some spark plugs can be located in some difficult spots, but it has been my experience that it is almost always easier to get to ONE of the spark plugs opposed to an oxygen sensor. In addition, unlike oxygen sensors, spark plugs almost always come out. Unless you are working on a 5.4 liter 3 valve Ford… but I digress.
To perform the test a pressure transducer is installed in one of the spark plug holes as shown in Figure 4. Either disable the spark for that cylinder or install a spark tester and be very careful not to expose the pressure transducer to the resulting secondary voltage. Some of the transducers on the market do not like to take a 60 KV hit and I would hate to damage a potentially expensive piece of diagnostic equipment. Next, the engine is started, a throttle snap is performed and the resulting pressures are observed on an oscilloscope. In order to explain how a restricted exhaust behaves we should know what good is first.
Figure 5 shows a known good engine running at idle. All four strokes of the cylinder are visible in the capture. The red box is calling attention to the pressure in the cylinder during the exhaust stroke. At this point in the four stroke cycle the exhaust valve is open the cylinder is directly connected to the exhaust system. Therefore, the pressure in the cylinder is the same as the pressure in the exhaust. In this case I placed the horizontal cursor at 0 psi and no exhaust backpressure can be seen.
Figure 6 is the same vehicle as figure 5 when the throttle is snapped. In the capture we can also see 0 psi during the exhaust stroke. This confirms the vehicle does not have an exhaust restriction.
Now that we know what known good looks like, let’s take a look at how a restriction behaves. The subject vehicle is a 2006 Buick Rendezvous with a 3.5 liter engine. The customer’s complaint was low power on acceleration. The test drive and VE calculation mentioned earlier in this article (Figure 1 and Figure 2) were performed. Scan data indicated poor VE, rich oxygen sensor readings and relatively normal fuel trim numbers. This was enough to warrant testing exhaust back pressure. Figure 7 is an in-cylinder capture that was obtained when the throttle was snapped in the bay. The labeling of the image is the same as the previous two images. However, I added a second horizontal cursor to measure the pressure in the cylinder. In this case the vehicle was generating in excess of 32 psi on the exhaust stroke. This would be the same measurement we would obtain if we had installed a backpressure gage in the exhaust system. This vehicle had a very restricted exhaust system. Replacing the catalytic converter resolved the issue and restored the vehicle’s acceleration. Access to a spark plug, scope connections and obtaining the capture were extremely quick and easy. I hope this helps illustrate the value of using a pressure transducer and oscilloscope over the older, but still effective, exhaust backpressure testing methods.
Catalyst efficiency side note
I know that catalyst efficiency DTCs do not exactly fit this article, but I wanted to take a moment to address a question that has been asked many times while I have been teaching around the country. The question: Why does a restricted catalyst usually not set a P0420 or P0430? The two most common reasons for this are: the catalyst efficiency monitor is suspended or the enable criteria to run the catalyst monitor have not been met. First, if there is a current misfire, or even a history misfire DTC stored, then the catalyst monitor will be suspended. There is a strong possibility that the misfire was the cause of the catalyst failure to begin with and the PCM will not even attempt to run the monitor in these cases. Second, if a catalyst becomes restricted for whatever reason, the engine load PID (or other possible data) can be out of the range of the enable criteria for the catalyst monitor to run. If this is the case, the vehicle will continue to operate while the converter continues to degrade and the PCM will not execute the monitor and set a catalyst efficiency DTC.
Remember, a catalytic converter can fail in two ways: efficiency or restriction. This article covered the restriction aspect. Efficiency issues require a different logical diagnostic approach.
Summary
When an exhaust restriction is suspected analyze some scan data to back up your theory, choose your physical testing method to prove the restriction, make the repair and perform a repair verification test drive (with a scan tool) to confirm success!
