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Testing Converter Backpressure

Catalytic converters are one of the greatest emission add-ons ever to be installed on vehicles. By cleaning up the pollutants left over from combustion, they reduce tailpipe emissions of hydrocarbons (HC) and carbon monoxide (CO) to extremely low levels when everything is operating normally, that is. But sometimes things don't operate normally, and when that happens engine performance may suffer or the vehicle may fail an emissions test.

Driveability symptoms such as a drop in fuel economy, lack of high speed power, rough idle or stalling are classic symptoms of excessive backpressure due to a plugged converter. Checking exhaust backpressure and/or intake vacuum will tell you if there's a blockage (more on this subject in a minute).

Elevated HC and CO tailpipe emissions, on the other hand, are often symptoms of a fouled converter or a faulty air supply (bad or leaky air pump, diverter valve or pulse air system). A fouled converter may not cause any increase in backpressure, so other methods of checking the converter are required for this type of problem (which we'll also get to shortly).

The important point to remember here is that converters don't just plug up or die for no good reason. There is usually an underlying cause which must also be diagnosed and corrected before the problem can be eliminated. Diagnosing a plugged or fouled converter is only half the fix. Replacing a bad converter will only temporarily restore things to normal because unless the underlying problem that caused the original converter to fail is identified and fixed, the replacement converter will likely suffer the same fate.

Under normal operating conditions, the converter shouldn't have to work very hard to accomplish its job. If an engine has good compression, isn't sucking oil down the valve guides, and the fuel, ignition and engine management system are all working properly, there should be relatively little HC and CO in the exhaust for the converter to burn (a few tenths of a percent CO and less than 150 ppm of HC when the engine is warm). In many late-model engines with multipoint fuel injection, combustion is so clean that the converter has little to do and the difference between the inlet and outlet temperature may only be 30 degrees F at 2,500 rpm which is a lot less than the old rule of thumb that says a good converter should show at least a 100 degree F difference fore and aft at cruise. At idle, the converter in many late-model vehicles may cool off so much that there's almost no measurable difference in fore and aft temperatures. So checking exhaust temperatures fore and aft of the converter at idle and 2,500 rpm may not be the most accurate way to determine if the converter is working or not.

One thing temperature measurements will tell you, however, is if the converter is working too hard. An infrared noncontact pyrometer or a temperature probe will tell you if the converter is running unusually or dangerously hot. If the converter outlet temperature is 200 or more degrees higher then the inlet temperature, it means the engine is running rich and there's a lot of CO in the exhaust that needs to be burned. A rich fuel mixture will often produce a "rotten egg" odor in the exhaust (the smell is hydrogen sulfide). Underlying problems may include an engine management system that is not going into closed loop (check the coolant and oxygen sensors, or for a thermostat stick in the open position), plugged PCV valve, or excessive fuel pressure (bad fuel regulator). High CO levels in the exhaust can also be caused by an inoperative air pump system.

If the outlet temperature is a lot hotter (more than 500 degrees F) than the inlet temperature, it indicates unburned fuel in the exhaust. The most likely cause would be ignition misfire (fouled spark plug, shorted or open plug wire, cracked distributor cap, arcing rotor or weak coil), or a compression leak (burned exhaust valve). But other causes may include lean misfire (check for vacuum leaks, leaky EGR valve, low fuel pressure or dirty injectors). A single misfiring spark plug can cause an increase in HC emissions of 2,500 or more parts per million, which can push the converter's operating temperature well above its normal range.

A common external clue of overheating to look for is a badly discolored or warped converter shell.

Prolonged overheating or short term severe overheating are the leading causes of converter plugging. The average light off temperature at which the converter begins to function ranges from 400 to 600 degrees F. The normal operating temperature can range up to 1,200 to 1,600 degrees F. But as the amount of pollutants in the exhaust go up, so does the converter's operating temperature. If the temperature gets up around 2,000 degrees F or higher, several things happen. The aluminum oxide honeycomb begins to degrade and weaken. The platinum and palladium coating on the honeycomb also starts to melt and sink into the ceramic substrate reducing its effect on the exhaust. This accelerates the aging process and causes the converter to lose efficiency.

If the overheating condition persists for a long period of time, or if the temperature soars high enough, the honeycomb itself may breakdown and melt forming a partial or complete obstruction and causing a sharp rise in backpressure. A complete blockage will cause the engine to stall shortly after starting, and will not allow exhaust to exit the engine.

Some degree of restriction inside the converter honeycomb can also be caused by accumulated deposits: phosphorus from oil burning and/or carbon from oil burning, a rich fuel mixture or frequent short trip driving where the converter rarely reaches light-off temperature). Physical damage to the honeycomb as a result of road hazards or severe jolts may cause the relatively brittle ceramic honeycomb to break or crumble inside the converter shell. A rattling noise when you shake or thump the converter would tell you there's loose debris inside. A undamaged monolith converter should make no noise.

To diagnose a plugged converter, you can check intake vacuum or exhaust backpressure. To check intake vacuum, connect a vacuum gauge to a vacuum port on the intake manifold. Start the engine and note the vacuum reading at idle. Then increase engine speed to about 2,500 rpm and hold steady. Normal vacuum at idle for most engines should be 18 to 22 inches Hg. When the engine speed is increased there should be a momentary drop in vacuum before it returns to within a couple of inches of the idle reading. If the vacuum reading is lower than normal and/or continues to drop as the engine runs, it probably indicates a buildup of backpressure in the exhaust. Remember, though, that intake vacuum can also be affected by retarded ignition timing and valve timing. What's more, some engines are much more sensitive to small changes in intake vacuum than others, so checking backpressure rather than intake vacuum may give you a better indication of what's going on.

Checking backpressure requires connecting a pressure gauge to the exhaust system. Use a gauge that reads up to 8 to 10 psi and is calibrated in 1/2 inch increments. Or, use a metric pressure gauge calibrated in kilo-Pascals (kPa). One psi equals 6.895 kPa.

A backpressure gauge can be connected to the exhaust system one of several ways: by removing the oxygen sensor and connecting the gauge to the hole in the exhaust manifold; by removing the air check valve in the air pump or pulse air system and connecting the gauge here; or by drilling a small hole into the head pipe just ahead of the converter to attach the gauge (never drill a hole into the converter itself!). One drawback of drilling a hole is that the hole will have to be plugged by a self-tapping screw, plug or welded shut after you've taken your measurements. Also, drilling is not recommended if the head pipe has a double-wall construction.

Once you've made your connection, start the engine and note the backpressure reading. Depending on the application, the amount of backpressure that's considered "normal" will vary. On some vehicles, backpressure should read near zero at idle, and should not exceed 1.25 psi at 2,500 rpm. Others can handle 0.5 to 1.25 psi at idle, but should have more than 4 psi during a snap acceleration test.

If you find a relatively high backpressure reading (say 8 to 10 or more psi), there's obviously an exhaust restriction that will require further diagnosis. Don't jump to conclusions and assume the converter is plugged because it might be a collapsed pipe or muffler.

One way to rule out the pipes and muffler is to visually inspect the exhaust system for damaged components. Another way is to drill a small hole in the pipe aft of the converter and check backpressure here. If the reading is lower (or is less than about 1 psi), the rest of the system is OK and the converter is what's causing the restriction. Or, disconnect the exhaust pipe aft of the converter. No change in backpressure would indicate a blockage at or ahead of the converter. If backpressure drops back to normal, the problem is not the converter but a collapsed pipe or muffler.

If you suspect the converter is plugged, you can disconnect and remove it. Then hold a shop light by one end of the converter and look in the other end. If you can't see the light shining through the honeycomb, the converter is plugged and needs to be replaced.

You can also recheck backpressure readings with the converter removed. If readings are at or near zero, you've found the problem. But if backpressure is still high, there's an obstruction in the head pipe or manifold. Sometimes a collapsed inner tube inside a double-wall head pipe will create an obstruction that acts just like a plugged converter. Another cause can be a heat riser valve on an older V6 or V8 exhaust manifold stuck in the closed position.

To clean the exhaust, the catalyst inside the converter must be exposed to the hot exhaust gases. Lead, phosphorous and silicone can contaminate the catalyst and prevent it from working its magic. Lead used to be the most common contaminant, but is no more since it was eliminated from gasoline. Phosphorus is still a threat, and comes from motor oil. So if an engine is burning oil because of worn valve guides or rings, phosphorus will shorten the life of the converter. Blue smoke in the exhaust and an emissions failure are pretty good clues that the converter has been fouled with phosphorus.

The new "SJ" rated motor oils contain less phosphorus than earlier SH rated oils. The difference isn't much (about 20% less compared to SH oils), but over time the lower level of phosphorus reduces contamination to extend the life of the converter.

Silicone can find its way into the exhaust if the engine develops an internal coolant leak through a crack in a combustion chamber or a head gasket. Silicone will ruin the oxygen sensor as well as the catalytic converter, so chances are if the converter has been fouled the O2 sensor will also need to be replaced. White smoke in the exhaust is a clue that there's an internal coolant leak.

If a converter is not plugged and passes exhaust normally, and there are no other engine performance problems (fuel, ignition and compression all OK, and the computer going into closed loop), but HC and CO levels in the exhaust are higher than they should be, the converter may be fouled. Most original equipment converters are designed for a service life of well beyond 100,000 miles, so if the converter has failed at low mileage contamination may be the culprit.

Checking converter operating efficiency can be done several ways. One "low tech" method is to make the fuel mixture momentarily rich by disconnecting the MAP sensor, or by creating excess HC in the exhaust by disconnecting and grounding a plug wire. Either condition should make the converter's operating temperature rise sharply, with the outlet temperature rising several hundred degrees over the inlet temperature. No change in temperature would tell you the catalyst is fouled and nothing is happening. Do not run this test for more than about two minutes because there's a risk of overheating and damaging what might be a good converter. Also, disconnecting the MAP sensor will likely set a trouble code, and on OBDII-equipped vehicles pulling a plug wire may set a misfire code.

The better approach is to read the composition of the exhaust gases with a 4- or 5-gas exhaust analyzer. A number of companies have recently introduced small portable exhaust analyzers that are relatively affordable ($2,500 to $5,500) and can be used for a variety of diagnostic purposes.

Checking emission readings at the tailpipe will tell you whether or not they are within normal ranges and help you diagnose the cause if emissions are high. Doing a "cold start" emissions check when the engine is first started will tell you if there are any engine problems that need attention. A cold start, in this situation, is when the converter has cooled down for at least 20 minutes. It will take a couple of minutes for the converter to warm up to light off temperature, so during this time you have a relatively clear window of what's coming out of the engine. When the converter reaches operating temperature, there should be a measurable drop in HC and CO readings (the amount will depend on how dirty the baseline readings were). No change in readings would indicate a dead converter.

Another test is to create a momentary rich condition or a misfire (as described earlier) to see if the converter can clean it up. As the converter starts to react to the excess pollutants, it's operating temperature should go up as the tailpipe emission readings come down.

If the vehicle in question is equipped with OBDII, the built-in catalyst monitor compares O2 sensor readings upstream and downstream of the converter. If the downstream readings start to match those of the upstream O2 sensor, it indicates a drop off in operating efficiency and sets a code P0402.


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