Scope & Scan: Hidden Pressure Hunt

Jan. 1, 2020
When only one catalyst on one bank has become restricted, the typical complaint is a lack of overall engine power. If the restriction is severe enough, there will be a misfire on only one bank, and not necessarily on the bank with the defect. This ba
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This month, we will use our in-cylinder pressure transducer to take a look at a defect many shops have trouble nailing down even by other more time-consuming methods. This defect is a clogged catalyst on only one bank of a two-bank engine equipped with a dual catalyst system.

When only one catalyst on one bank has become restricted, the typical complaint is a lack of overall engine power. If the restriction is severe enough, there will be a misfire on only one bank, and not necessarily on the bank with the defect. This bank-isolated misfire is due to the effects on airflow through the engine with one side of the exhaust system restricted.

When one side is restricted, less than 50 percent of the total air entering the engine ends up being passed through that side's cylinder bank. Fifty percent of the fuel mass is delivered via the injectors to that restricted bank. As a result, one bank ends up very lean, and the other bank very rich. The first sign of an unequal airflow due to restriction on a V-type engine is the fuel trims.

Figure 1 is a P0300 Failure Record captured on code set from a 2003 Chevrolet pickup with a 4.8L pushrod V8. Bank one is the side with the clogged catalyst, and as such has a negative total fuel trim (STFT+LTFT) correction of -21 percent, while bank two is showing a positive total fuel trim correction of +25 percent.

How would you diagnose this condition to be a clogged exhaust before purchasing an expensive catalyst? I prefer removing one spark plug from each bank in turn and installing a pressure transducer. Look at the saved pressure waveforms, and you get an easily acquired and highly accurate comparison of in-cylinder conditions bank-to-bank.
Start at idle speed so you have a relatively slow crankshaft speed and plenty of engine vacuum. Then snap the throttle open. Initially, the crankshaft has not yet begun to spin faster, so the extra air has lots of time to fill the cylinders before it attempts to exit the exhaust ports.

In Figure 2, I have captured bank two, cylinder six, the good side of this engine. Compare these pressure waves seen from the good side to the clogged bank one, cylinder seven shown in Figure 3, the restricted side. In the zoomed out wave capture shown in the lower right corner of the images, you can see that the cylinder from the restricted bank built up more pressure on average than the cylinder from the unrestricted bank.

If we zoom in on the highest in-cylinder pressure peak section of the wave, we can see the cycle-by-cycle difference between the cylinders on the different banks. On the bad cylinder, the end of the exhaust stroke pressure wave plateau peaks at a high of 0.78V. On the good cylinder, the exhaust stroke pressure wave plateau peaks at 0.53V and permanently drops off as the intake valve opens during the overlap period.

The scaling on this sensor is 1.0V = 75 psi, so the variation between the restricted and unrestricted cylinders is 0.25V or 18.75 psi. It is impossible to get this kind of accuracy and detail using older methods for testing for backpressure. Even a slightly clogged catalyst will show up by using in-cylinder pressure measurements.

Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.

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