Understanding Dual-Cast Exhaust Systems Will Assist Diagnosis
Though most cars and light trucks have exhaust systems with only a single catalytic converter, there are some with dual converters or piggyback converters.
One example of a dual-cat system is found on 1984-96 Corvettes. The 5.7L V8 engine has a separate converter for each bank of cylinders.
Several dual-cat configurations can be found on a variety of Ford applications from the mid-1980s up to 1994. On 1986-93 Ford Mustangs with a 5.0L V8 engine, a pair of "mini-converters" (one for each side) is located in the Y-pipe.
A similar setup is used on the 1985-87 Lincoln Continental and 1988-93 Mark Series cars with a 5.0L V8 engine. On 1983-85 Ford Thunderbirds and Mercury Cougars with 5.0L V8 engines, there is a single exhaust but each leg in the Y-pipe has its own mini-converter.
On 1986-89 Ford Aerostar minivans, the Y-pipe has a mini-converter just aft of the point where the two legs join together, but there’s also a second larger converter just behind the Y-pipe.
A "piggyback" configuration can be found under 1988-89 Ford Bronco II and Ranger with a 2.9L engine, and 1991-92 Ford Explorer and Ranger with the 4.0L engine.
Service and repair procedures are pretty much the same on the dual-cat systems as those with only a single converter, but there are some tricks that can help you figure out which converter is the one that needs to be replaced.
The Converter’s Job
Original equipment catalytic converters are designed to last the life of the vehicle. These days, that typically means 150,000 miles or more. But many converters never go the distance for a variety of reasons. Some succumb to rust and corrosion. Others are damaged by running over something. The most common cause of failure, though, is plugging caused by overheating.
Converters are essentially afterburners that reburn carbon monoxide and hydrocarbons with additional oxygen from an air pump or aspirator valve. The catalytic reactions produce heat, which the converter can handle as long as there are normal amounts of these pollutants in the exhaust.
But when too much unburned fuel enters the exhaust because of a rich fuel mixture, misfiring spark plug or leaky exhaust valve, the converter’s operating temperature can soar. This can cause the converter’s innards to melt, resulting in a partial or complete blockage.
Converters are also vulnerable to contamination. As a converter ages, its operating efficiency can decline as contaminants accumulate inside on the reactive surfaces. Particularly bad are phosphorus deposits from oil burning (worn valve seals, guides and/or rings), or silicone deposits from internal coolant leaks (cracked head or block, or a leaky head gasket).
As the contaminants build up, HC, CO and NOX emissions begin to rise. Eventually, the converter’s ability to destroy pollutants falls to the point where it does little or nothing to clean up the exhaust. There’s no effect on engine performance, but the drop in emissions reduction may cause the vehicle to fail an emissions test.
There’s no way to rejuvenate a dead or contaminated converter, so replacement is the only option. The converter is covered by federal emission regulations, so it can only be replaced by a car dealer unless it’s out of warranty and a legitimate need for replacement has been documented. Up to model year 1995, converters were covered by a 5-year/50,000-mile federal emissions warranty (7 years or 70,000 miles in California). In 1995, the warranty jumped to 8 years and 80,000 miles.
You must also get the customer’s repair authorization for repairs in writing, keep the paperwork for six months and the old converter for 15 days. The replacement converter must be the same type as the original and be installed in the same location.
Diagnosis Options
In many instances, the only clue that the converter may need to be replaced is an emissions failure or a performance complaint Ñ lack of power, poor fuel economy, stalling after starting.
The first thing you should do is visually inspect the converter and air pump plumbing for obvious problems like rust, damage or leaky connections. Any loose heat shields can create annoying rattles and vibrations.
Severe discoloration on the converter shell may be an indication of overheating and possible internal damage, but you can’t always count on it.
In the case of a failed emissions test, higher than normal HC and CO tailpipe readings from a properly tuned engine, combined with higher than normal O2 readings and lower than normal CO2 readings, would tell you the converter probably needs to be replaced.
On 1996 and newer vehicles with OBDII, the on-board diagnostics will usually catch a bad converter. The OBDII system monitors converter efficiency with a second oxygen sensor mounted behind the converter. The system compares oxygen levels ahead of and behind the converter to see if there’s a difference.
Downstream oxygen levels should be lower if the converter is working properly. Little or no change in the downstream O2 readings means nothing is happening inside the converter.
This will usually cause the OBDII system to illuminate the malfunction indicator lamp (MIL) or the Check-Engine light and set a fault code. The generic OBDII code for low converter efficiency is P0420.
On pre-OBDII vehicles, converter efficiency can be checked with a four-gas exhaust analyzer Ñ fine if you have one Ñ but rule out this method if you don’t. One way to check converter performance is to read the tailpipe emission levels, then disable the air pump or aspirator and read the emission levels again.
If the converter is working, HC and CO should both be lower, and CO2 should be higher. O2 should also be higher when the air pump is working. No change in HC, CO and CO2 readings would tell you the converter has reached the end of the road.
Another diagnostic method is to look for an increase in temperature as the exhaust passes through it. This can be done by checking the exhaust temperature ahead and aft of the converter. We’ve heard of some people drilling small test ports into the pipes and inserting a temperature probe to measure exhaust temperatures. This technique works, but requires welding or plugging the holes afterwards. A faster and easier way is to use a non-contact infrared thermometer. These are relatively inexpensive and easy to use. Just point the gun at the pipe and read the temperature.
With the engine at normal temperature and idling, the outlet temperature should be higher than the inlet temperature if the converter is functioning. On 1980 and older vehicles with two-way converters, the difference should be at least 100ûF.
But on 1981 and newer vehicles with three-way converters, the difference may only be 20 to 30 degrees. No difference in temperature indicates a defective converter or no air from the air pump. In this case, the air pump diverter valve and plumbing will need to be checked.
An increase of 500 degrees or more indicates converter overheating because of a rich fuel condition (check the fuel system), misfiring spark plugs or compression leaks.
Replacement Diagnosis
Replacing the converter will restore proper emissions performance. But the new converter will suffer the same fate as the old one unless you figure out what caused the original problem. Look for fouled spark plugs or wires, low or no cylinder compression, or a computerized feedback system that stays in open loop all the time.
On a dual-cat system, the side with the bad converter tells you which cylinder bank to check. If the converter on the right is bad, for example, check the O2 sensor, spark plugs and compression on the right cylinder bank.
Always replace the oxygen sensor. Converters needsan air/fuel mixture that is constantly flip-flopping from rich to lean. If the sensor is sluggish or dead, the fuel feedback loop will flip-flop too slowly or remain rich all the time.
Bad coolant sensors will also keep the fuel system in open loop. A thermostat that’s stuck open or is too cold for the application can also cause similar problems.
It’s also important to check the air pump and related plumbing as these components provide fresh air for the converter to reburn the pollutants in the exhaust. If the air pump isn’t working right, it can reduce the converter’s operating efficiency significantly.