Exhaust backpressure can cause a variety of problems. A plugged catalytic converter can strangle engine breathing and cause a big drop in engine performance and fuel economy. And if the converter plugs up completely, it can make the engine stall. The same thing can happen if a muffler, resonator or double walled exhaust pipe collapses internally. Anything that restricts exhaust flow will create excessive backpressure in the exhaust system.
The classic symptoms of too much backpressure include things like a lack of high speed power, poor fuel economy and even overheating. Anything that backs up exhaust pressure into the engine will also back up heat. About a third of the heat produced by combustion goes out the tailpipe as waste heat, so if the heat can't escape it can overload the cooling system and make the engine run hotter than normal, especially at highway speeds.
If there is a complete blockage in the exhaust, the engine may start and idle fine for a minute or two, then die as backpressure builds up and strangles the engine. In some instances, backpressure may buildup to such a degree that it blow out a pipe connector or the converter shell. That makes diagnosis a lot easier, but in most cases you may not be sure if there is an exhaust restriction or not. So in these instances, you need to measure backpressure.
To measure exhaust backpressure, you need a pressure gauge with a scale that reads zero to 15 psi, or zero to 100 kPa or higher (note: 1 psi equals 6.89 kPa, and 1 kPa equals 0.145 psi). If you don't have a low pressure gauge, you can buy a basic exhaust backpressure test kit for around $60. If you want to be really accurate you can use a digital manometer or pressure gauge that displays pressure readings in a variety of different units of measurement (psi, kPa, inches Hg, inches H2O, bar, etc.). A tool that reads from 0 to 15 psi will typically cost around $170.
Measuring exhaust backpressure is not as easy as it sounds because there is no quick and easy way to tap into the exhaust system. If the engine has an air pump, you can tap into the exhaust system at the air pump check valve. Disconnect the check valve and install a pressure gauge. For accurate test results, however, the check valve must connect to the exhaust system ahead of the converter. Note: if the air pump plumbing hooks up at the converter, this technique won't give you reliable results.
You can also check backpressure by removing an oxygen sensor from the exhaust manifold, and connecting a hose fitting to your pressure gauge. If the O2 sensor is fairly easy to reach and the vehicle is fairly new, it should come out without too much effort. But on an older vehicle, O2 sensors can be difficult to remove. And there is always the risk of damaging the sensor.
A third option is to drill a small hole into the exhaust pipe just ahead of the converter and attach a fitting for your pressure gauge or manometer. This may be easier than trying to remove a 10 year old O2 sensor, but it also means you'll have to plug the hole afterwards with a self-tapping screw or a small spot weld.
On some diesel engines (Ford diesel trucks, for example), there is a backpressure sensor in the exhaust system that measures exhaust backpressure directly. You can see the actual value by using a scan tool and looking at the exhaust backpressure sensor PID.
Backpressure readings at idle on most engines should generally be less than 1.5 psi (10 kPa). This will vary somewhat from one vehicle to another depending on the design of the exhaust system, the size of the pipes, how restrictive the converter, muffler and/or resonator is, and whether it is single or dual exhausts. We've seen some idle readings as high as 2.75 psi on a few vehicles, but for most 1.5 psi or less at idle is normal.
A partially restricted converter, muffler or pipe may flow enough exhaust at idle not to cause a problem, but chokes breathing at higher engine speeds. So to test this possibility, you need to rev and hold the engine at 2000 rpm. A "good" reading on most engines at 2000 rpm should be 3 psi (20 to 21 kPa) or less. Again, there may be some vehicles that will read a little higher that don't have a problem, but the reading should not be significantly higher.
Pay close attention to what the backpressure reading does while you are holding it at 2000 rpm. If it remains steady, chances are there is no restriction. But if the reading gradually increases, it means backpressure is building up and there may be a blockage.
If you want to rev the engine higher, say to 4000 rpm and hold it, the backpressure numbers will shoot up. Most stock exhaust systems will show backpressure readings from 4 to 8 psi (27 to 55 kPa), or even higher. As before, if the backpressure reading is unusually high or it continues to climb at a steady rpm, it usually means there is an abnormal restriction causing an unhealthy increase in backpressure.
Higher than normal backpressure readings mean something is restricting the flow of exhaust out the tailpipe. Though the converter is usually the trouble spot, restrictions can also occur inside mufflers and resonators if a baffle collapses or the fiberglass sound-absorbing roving clogs up an internal passageway. Double-wall exhaust pipes can also collapse internally causing a blockage.
If the system has a blockage, inspect the exhaust system end to end for any obvious signs of damage like a crushed pipe, severe corrosion, etc. You can thunk the converter to see if it rattles inside (indicating the catalyst substrate is broken).
The next step would be to disconnect the exhaust pipe just behind the converter to see if that makes a difference in the backpressure readings. The readings will go down a bit when the exhaust system aft of the converter is disconnected, but if you don't see any drop it means the converter is probably plugged. The other possibility is that the head pipe between the exhaust manifold and converter has collapsed internally.
If you see a big drop in the backpressure readings when the exhaust system aft of the converter is disconnected, it means the converter is flowing okay and the blockage is somewhere in the rest of the system (bad muffler, resonator or tailpipe).
Another way to check for a backpressure problem is to check intake vacuum at the engine. It's much easier to hook up a vacuum gauge to a vacuum hose or port than it is to remove an O2 sensor. Vacuum gauges typically display readings in inches of Hg (Mercury). One inch Hg equals a pressure reading of 0.49 psi or 3.38 kPa.
Normal atmospheric pressure is around 14.7 lbs. per square inch at seal level. This will vary a bit with temperature and humidity, and it goes down at higher elevations. Vacuum is created inside the engine's intake manifold by the intake stoke of the pistons trying to overcome the restriction created by the throttle butterfly. On most engines will develop 16 to 22 inches Hg of vacuum at idle (except for most diesels which have no intake vacuum because they have no throttle plate). The vacuum reading at idle will depend on engine wear, throttle opening, camshaft overlap, exhaust backpressure, air temperature and density.
To check vacuum at the intake manifold, start with the engine off, and disable the EGR valve by removing or disconnecting its hose or one of its solenoids. Connect a vacuum gauge to a ported vacuum source on the intake manifold or throttle body. Start the engine and note the vacuum reading at idle with the transmission in neutral.
If the idle vacuum reading is lower than normal, or it continues to drop while the engine idles, an exhaust restriction is causing exhaust pressure to build up and backup into the engine.
If you increase engine speed, the vacuum reading will drop slightly, then stabilize and rise back up to within 2 to 3 inches of the vacuum reading you noted at idle. Any sudden drop of over 10 inches Hg of vacuum may indicate a blockage problem. Erratic swings of the vacuum indicator may indicate periodic blockages caused by loose components temporarily blocking the exhaust system.
Vacuum readings can also be affected by other factors such as weak or broken valve springs, overadvanced or retarded valve timing and/or ignition timing. So if the needle on the gauge is bouncing around, it could indicate a mechanical problem in the engine.
Reducing exhaust backpressure can improve fuel economy and performance. Reducing restrictions in the exhaust system allows the exhaust to flow more easily so the engine can breathe more efficiently. The most common modification is to replace the stock muffler with a low restriction aftermarket performance muffler, or to replace the entire stock exhaust system from the catalytic converter back with a free-flowing aftermarket performance exhaust system.