Emissions Troubleshooting on the Mazda Rotary Engine, Jon Wedeman, ImportCar, July 1996
Just the sight of a rotary engine vehicle rolling across the service drive can send chills down the spine of even a highly skilled import technician. Is it a two-cycle or a four-cycle? Well, that depends on who you ask.
The rotary engine incorporates all four events common to the two- and four-cycle engine - intake, compression, power and exhaust. However, it completes these events in one continuous circular motion - a one cycle?
The intake and exhaust events are controlled by ports in the engine housings. As the rotor moves past the ports, they are opened and closed in the process, similar to a two cycle. And since each rotor has three sides, or combustion chambers, it completes 12 cycles per revolution - a 12 cycle? However you sum it up, the rotary operates on many of the same principles as most carbureted or fuel-injected engines, while completely ignoring others.
Early Model Emissions Problems [go to top]
To try and simplify the process of diagnosing and repairing emissions-related problems, let's break them down chronologically starting with 1979. This was the first year of the RX-7, discounting the few that were titled late in '78, and the only car offering the rotary engine in the U.S. from that year on. The rotary engine was offered in several coupes, sedans, wagons and trucks prior to 1979, but the techniques used to troubleshoot and repair the emissions-related systems are similar to those of the early model (1979 & 1980) RX-7s.
Although a Mazda repair manual is a great reference to back this article, we'll try to cover the repairs from most common to least likely, rather than just opening the repair manual to the "emissions" section and wondering where to begin.
In 1979 and 1980, the RX-7 did not have catalytic converters. It incorporated in the exhaust system what is known as a thermal reactor, a large manifold looking device which mixed air into the exhaust from the air management system, consisting of ports in the engine housings, an air control valve and an air pump. Although similar in operation to a converter, the reactor did not contain any chemicals. It relied on proper fuel/air mixture and air from the air pump.
Due to the simplicity of this design, it is very easy to properly adjust an RX-7 to run well below any state specs. To set the car up properly, the initial timing should be set at 0-2º BTDC at sea level, up to 5º BTDC above 2,000 feet or a total of 29º BTDC at 4,500 RPM. Since all RX-7 models have two ignitions, leading and trailing, you will need to check the shop manual for the color and location of the proper mark on the eccentric shaft pulley. (Red is almost always trailing and yellow or white will be leading). Verify that the leading #1 wire is connected to the timing light and the timing is appropriate.
The carburetor incorporates two mixture screws. The larger, uppermost screw, controls the amount of by-pass air directly affecting idle speed. The smaller, lower screw, controls fuel and has the greatest affect on emissions. Adjust the idle speed using the upper screw as close to factory specs as possible. Then, using an infrared analyzer, turn the fuel screw clockwise (lean) until the engine runs rough. From that point, turn the fuel screw counterclockwise in small increments until the CO begins to rise to about 2 to 3 percent. At this point, CO will then begin to drop while still enriching the mixture.
This is the point where technicians become completely confused because the CO is dropping while the fuel screw is still being backed out or enriched. Keep turning counterclockwise in small increments until the CO is at 2/10 of one percent, .2 on low scale. Once you reach this point, turn the fuel screw counterclockwise an additional half turn out. You should notice a slight rumble in the engine, which is now properly set. Make any small adjustments as necessary to the air screw to adjust idle to 800 RPM.
The RX-7 will run fairly well grossly out of adjustment, yet it is very sensitive to proper adjustment for emissions considerations. If the engine will not set up, one of the following four problems may exist:
The in-line fuse for the emissions control unit could be blown. This fuse is attached directly to the control unit, located behind the left kick panel.
The thermal reactor may be bad. In this case, exhaust will leak from a hole burned through the side. If you can't hear a leak, don't look any further.
The air pump may be worn. You can check this by placing your thumb over the inlet and increasing the engine speed. If the pump is good, you will immediately want to remove your thumb due to the suction.
The air control valve may be bad. This is generally accompanied by a severe popping during deceleration or between gears when shifting. To verify this, check the air control valve by removing the hose (air pump to air control valve) at the air control valve and check to see that no air is leaking from the inlet of the air control valve at idle. (However, if the car is not properly set up, this will also result in severe popping.)
Problems Common To '81 To '85 Models [go to top]
Troubleshooting the '81 to '85 model years is basically the same even though there were some differences in emissions controls. Changes include the number of catalytic converters and some of their design features, and a larger, fuel-injected engine in the upper end GSL-SE models '84 through '85.
Again, proper set up is very important. These carburetors have only one mixture screw which operates in a similar fashion to what most technicians are accustomed. Proper mixture can be attained by running the engine at 2,500 RPM for three minutes, then setting the CO at 2.0% at idle with the air pump hose (air cleaner to air pump) disconnected and plugged at the air cleaner end. This disables the air management system. Ensure that both leading and trailing ignitions are operating.
Beginning in 1980, the RX-7 had an electronic ignition. The car runs so well without the trailing ignition, which is used primarily to clean up emissions, that it may be difficult to detect a problem. Simply remove one of the "T1 or T2" wires from the distributor cap and check for spark while cranking. Hint, if the tach is inoperative, the trailing igniter is probably bad, which will greatly affect emissions (a very common problem on RX-7s).
In '84 and '85 models, the fuel pump was included in the trailing ignition circuitry. If the trailing igniter were bad, the fuel pump wouldn't run and neither would the car. The next most common problem are the converters.
The front cat is a monolithic type which tends to break down and plug up the exhaust flow. High emissions and low power output, accompanied by a blown exhaust gasket, are the symptoms here.
The rear cat is a pellet type and frequently wears out. The casings generally will start to leak with wear. For the '82 models, a recall was issued to replace the pellets. Part of the recall included installing an orifice in the hose (air cleaner to air pump) that was about 12mm wide. If the '82 you're servicing has this orifice, the recall has been accomplished. However, not all cars that got the recall got the orifice. Your local dealer should be able to tell you by referencing the VIN. Most dealers should still perform the recall under warranty, if it hasn't been done already. Most converter problems begin beyond 90,000 miles.
The air control valves are subject to failure on '81 to '85 models. Typically, the valve burns through internally, causing exhaust gases to be present at the inlet side of the valve if the hose (air control valve to air cleaner housing) is removed at idle. If exhaust is present, verify that the valve is bad by checking the control system.
Inspect the in-line fuse attached to the emissions control unit under the left kick panel (on '81 and '82 models only). The fuse has a yellow wire leading into it and an orange wire coming out. Next, check the blue colored vacuum solenoid valve located in a row of different colored solenoid valves on top of the motor. If the solenoid valve is supplying manifold vacuum to the steel pipe, then the air control valve is bad. If not, replace the blue solenoid valve.
Shutter Valve Operation [go to top]
Another common problem is the shutter valve operation. If the shutter valve is bad, a loud howling noise will be heard coming from the air cleaner area. It will be accompanied by a very rough idle and very high emissions. The shutter valve is located in the intake manifold directly below the carburetor. Remove the carb and replace the valve, paying particular attention to which hole in the valve the carb butterfly linkage is occupying.
The GSL-SE models made their appearance in '84 and offered a fuel-injected engine which was also 10 C.I. larger than the standard 70 C.I. 12A engine. Designated the 13B, this engine has similar emissions controls as later models ('86 to '92), including three catalytic converters, a throttle position sensor, and a variable resistor to control injector duration.
Air control valves and the converters are the primary suspects when emissions problems arise. However, with the introduction of fuel injection, the injectors are very susceptible to heat deterioration since there are no valves to protect them from the combustion chamber heat. After time, usually beyond 60,000 miles, the pintle and seat in the injectors wear and relax. As the heat rises in the engine, usually a hot soak situation, the fuel behind the injector boils and leaks by the weakened seat, causing a hot restart problem and excessive wear on the converters. Flushing the injectors will clean up their performance a bit, but in time the injectors may need to be replaced.
Troubleshooting The '86 To '92 Models [go to top]
Model year 1986 brought the advent of self-diagnosis and an emissions checking procedure. From '86 through '92, this procedure consists of verifying that the inputs for the control unit are set up properly. The first and most important, and often overlooked, item is the throttle position sensor.
Mazda has a special tool available, Part #49 F018-001, for checking the TPS. This tool can easily be made from a couple #1156 bulbs. Simply plug it into the green three-pole connector located near the right front shock tower (in turbo models, it's near the left front corner of the intercooler). Turn the key to the "on" position with the engine warmed up but not running. One lamp should illuminate. If neither lamp illuminates, adjust the throttle sensor by turning the screw clockwise. If both lamps illuminate turn the screw counterclockwise until a point is reached halfway between where they both illuminate and only one illuminates. If neither lamp illuminates or both illuminate all the time, then the TPS is bad.
Next set the idle speed. The initial set coupler must be jumped in order to accomplish this. The connector is a two-pole green connector located near the battery. Verify the initial ignition timing while the jumper is in place, 2º after TDC to 2º BTDC. Remove the jumper and hold the engine at 2,000 RPM. Slightly depress the TPS and the engine RPM should drop. If the RPM does not change, the TPS is bad.
The air control valve incorporates a split air solenoid and a port air solenoid valve along with an anti-afterburn valve. When any of these are malfunctioning, emissions-related problems will result. Check the anti-afterburn valve by removing the hose (air pump to air control valve) at the air pump. Place a finger over the opening and raise the engine speed to 3,000 RPM, then close the throttle rapidly. Check that air is sucked in for a few seconds while decelerating and that no air leaks at idle.
Another reason for failure is the relief solenoid valve. This is not due to the valve itself, but rather the two wires leading to the water temperature switch in the lower left hand corner of the radiator, which control the operation of the relief valve. These wires break near the sending unit leaving the control unit in open loop or causing the relief valve to malfunction. Repair the wires if they are found to be broken.
Mazda recommends replacement of the oxygen sensor at 60,000 miles. If it hasn't been replaced by 80,000, it probably should be. To check the O2 sensor output, warm the engine to operating temperature and connect a volt meter between the O2 sensor and ground. Accelerate and decelerate the engine suddenly several times. Verify that the voltage is between .5 V and 1.0 V during acceleration, and between 0 V and -.4 V during deceleration. If not, replace the O2 sensor.
The least likely of the more common problems affecting the '86 to '92 RX-7s is the water thermo sensor located in the back of the water pump. If faulty, it will send a false signal to the control unit, leaving the system in open loop or cold engine condition. The control unit will then enrichen the mixture.
Check the sensor by removing it from the water pump and inspect the resistance across the two terminals. At 70ºF, it should have approximately 2.5 ohms resistance. In a heated container of about 175ºF, it should have about .3 ohms resistance. Replace the sensor if readings are outside of these values.
The 1986 and later models have an on-board self- diagnostic feature. However, a special tool is required to pull the codes from the CPU. The Mazda "self diagnosis checker" p/n 49-H018-9A1 is available for this purpose. Since the code numbers for any given malfunction change as often as the model year, working on the RX-7 really requires that you have the specific shop manual for the year in question.
If you don't have access to the right manual, try calling the local
dealer. Most of the Mazda guys will go out of their way for you. Most '93 and newer models
are still covered under the factory warranty. We'll see more of these as time goes on.
Jon Wedeman is an ASE Master Technician, a Mazda Master Technician and a member of the Mazda Service Guild. He has 15 years of experience servicing and repairing Mazdas, and has been a technician at Rotary Specialists, Englewood, CO, for 8 years.