Exhaust backpressure checks are essential

Editor's note: This article was originally published March 1, 2007. Some of the information may no longer be relevant, so please use it at your discretion.

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.

Symptoms

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.

Inspection first

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.

First, pierce a hole at the selected spot. Never pierce a hole in the converter or muffler housing. (It's best to start ahead of the converter as it's the most likely culprit.) This also gives you an initial backpressure reading that you can use for comparison as you work rearward. After you install the threaded adapter in the hole, you're ready to connect your gauge. Although no exhaust flows into the gauge and a standard hose would work, heat will destroy the end of the hose in no time so a high-temperature silicone hose is your best bet here.

Take a reading

Start the engine and let it warm up to normal operating temperature. Of course, if the engine won't run, take a backpressure reading with the engine cranking. Take one reading with the engine idling in neutral and another with the engine running at 2,500 rpm. Faster engine speeds should show a higher backpressure reading.

Don't use a powered exhaust venting system when backpressure testing. The system's draw could alter the readings. If necessary, do your testing outside. If you're fortunate enough to have a chassis dyno, run the car under road load conditions to heat the exhaust system up more. Sometimes, backpressure slowly builds as the exhaust system heats up. If you don't have a chassis dyno, carefully route the hose so it doesn't drag or get pinched. Take the car for a ride and check backpressure under load.

What the readings mean

If you're curious about how much backpressure is too much, the answer is that it depends. Ideally, a proven, cast-in-stone rule would be nice, but it's somewhat unrealistic. Since the engine is nothing more than an air pump, it stands to reason that different engine sizes would have different flow rates, and so do their exhaust systems. The readings also depend slightly on the location in the exhaust where they're taken. Given these variables, a backpressure reading that may be OK for one make and model may not be OK for another.

Even GM, probably the most outspoken carmaker regarding backpressure testing, suggests different readings throughout its carline. For most of its engines, GM recommends no more than 8.62 kPa (1.25 psi) at idle, and no more than 20.68 kPa (3 psi) at 2,000 rpm. (We stress backpressure measurements in kilopascals rather than in psi because readings in kPa offer better resolution — that is, the graduations are smaller making it easier to see differences in pressure; and gauges graduated in kPa are becoming more common.)

Naturally, the amount of restriction plays a key role in the amount of backpressure you will see. Severe cases, those with a plugged converter where the car would hardly run, can produce readings in the 137.89 kPa (20 psi) to 206.84 kPa (30 psi) range.

Isolating the cause

If your readings tell you there's too much backpressure, the next step is to find the cause. If you tapped into the system by making a hole, comparing before and after checks of exhaust components will help you isolate the culprit. If you used one of the other methods, all you can do is drop the exhaust to see if the loss of backpressure makes a difference. Since a restricted converter is the most likely cause, start at the cat and work towards the rear.

Repairing the cause

Should the source of the excessive backpressure be a plugged converter, do more than just replace it; find out why it failed. A rich mixture caused by a fault in the engine control system, a misfiring cylinder, using leaded fuel, and oil or antifreeze in the exhaust can all take their toll on a cat's life. It's also wise to check the old cat for signs of disintegration. Pieces from monolithic converters or chunks of fused beads can move downstream into the muffler and impede the flow of exhaust there, too. Being thorough can prevent a comeback on what at first appears to be a simple job.

Take a follow-up backpressure check after you've made the necessary repair. It will show the difference after the car's fixed (which you can tell your customer about) and add to your personal database of backpressure readings.

In the next issue, we’ll discuss actual testing of the catalytic converter. With the focus on I/M programs, you need to know if the cat is working right or not. See you then.