Despite all the technology on today’s cars and the diagnostic equipment that goes along with it, one often overlooked area of diagnostics is exhaust backpressure. Rarely a problem before catalytic converters, excessive backpressure has as much of an effect on drivability as dirty injectors or carbon in the throttle body. Once you see how easy and important it is to check backpressure, you'll be convinced that it belongs on your drivability checklist.
Excessive backpressure often mimics late valve timing, and has fooled more than one technician into replacing a timing chain or belt. Common symptoms include poor power and fuel economy, a slipping automatic transmission or altered transmission shift points, backfiring through the intake manifold, hesitation, stalling and, if bad enough, an engine that won't run at all. Excessive backpressure also tricks backpressure-sensitive EGR valves into recycling too much exhaust gas. This destroys drivability because the extra volume of exhaust gas displaces the air/fuel mixture.
Before jumping right into a backpressure reading, take a couple of minutes to look for obvious causes of restrictions like a pinched tail pipe or exhaust pipe, or even mud packed into the end of the tail pipe. If the car has been in below-freezing temperatures, check the muffler and tail pipe for ice. The muffler should sound hollow when you tap on it. If you get a dull thud, suspect a chunk of ice inside.
When equipped, check the heat riser or EFE valve to make sure they operate freely. If either is stuck in the closed position, backpressure will increase. Lube the valve and work it back and forth until it moves freely.
The oldest trick in the book
For years, the only industry-wide method of checking backpressure was simply a vacuum gauge connected to the intake manifold. First, disable the EGR valve by removing and plugging its hose, or disconnecting one of its control solenoids. Start the engine and take a vacuum reading at idle with the transmission in neutral, then take another reading at 2,500 rpm. The vacuum reading should increase slightly. If the reading decreases, there's a good chance that the exhaust system is restricted. (Remember, vacuum is also affected by other factors including valve and ignition timing.)
Although still a valid test, there are better ways of checking backpressure.
A direct approach
Although the vacuum gauge test may work for some backpressure problems, there are occasions when backpressure affects drivability, but not the vacuum reading. You may end up overlooking exhaust restrictions that really are a problem. The best way to check backpressure is directly from the exhaust system. You can do this in several ways.
Tap into the injection manifolds. On older engines with air injection or pulse air, the air manifolds and connections for these systems provide a convenient access point for checking backpressure. With the engine cool, remove the air injection check valve. Connect a rubber adapter cone to the check-valve connection and then connect your gauge hose to the adapter.
Tap into the oxygen sensor hole. The oxygen sensor mounting hole offers another handy port to check backpressure. Since all sensors use an 18 mm threaded hole, one adapter fits all cars equipped with an oxygen sensor. With the engine off, carefully remove the sensor (a shot of penetrating oil and rocking the sensor helps). Install the adapter in the hole and tighten it according to the manufacturer's torque specification.
Tap into the exhaust system with special tools. A universal backpressure kit is probably the slickest way to get a backpressure reading. Unlike tapping into the air injection system or the oxygen sensor hole, this kit lets you tap into the exhaust system anywhere, like ahead and behind the converter or muffler, without having to remove any parts. Also, by checking backpressure ahead and behind suspected restrictions, you can reduce the time spent pulling off sections of the exhaust system to isolate the source.
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