Exhaust Backpressure Checks are Essential

Troubleshooting exhaust backpressure issues.


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.

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