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Corsica No StartPosted 4/18/1997By Dave Martin
There is nothing more frustrating after installing a rebuilt engine than being unable to start it up. I got a call on a 1989 Corsica with a 2.0 liter throttle body injection. The engine had consistent injector pulse during cranking, but there was no spark on any cylinder at any time. Usually this would indicate a defective module. Here's why: To trigger the injector, the module creates and sends a 0 to 5 volt digital square wave to the power control module (PCM). The PCM looks at the throttle position sensor (TPS) voltage to see if the engine is in a clear flood mode. If it is not, the PCM then pulses the injector one time for every one square wave received from the module. It only makes sense if the module is able to create this square wave, it must have both power and ground and be receiving a signal from the crankshaft sensor. This technician was very thorough and had conducted a regiment of tests. The resistance and AC voltage output tests of the CP sensor were normal. The module had good connections, power and grounds. He even replaced three modules, two sets of coils, three CP sensors, the PCM and the CP sensor harness, and "nothing made a difference." We retested the power and grounds during cranking and found them well within specifications. Sensor VREF was a steady 5 volts at all times, key on or cranking. After disconnecting the five-wire connector at the module to eliminate any possible interference from the PCM, there was still no spark. This eliminated the computer as a possible problem. Removing the coils and replacing them with a test light still showed no signs of any primary pulse during cranking. I was convinced it was not a problem with parts or a connection. Since the signal generated by the CP sensor is created by slots in the crankshaft, I asked him to remove and inspect the oil pan. He didn't have a lab scope to monitor the actual signal, so he did a visual inspection. The crankshaft slots were clean, no burrs and no damage. At this point I knew the problem must be in the actual signal sent by the CP sensor. I asked the technician to refit the original CP sensor to be closer to the crankshaft (no difference) and then to install the new CP sensor, adding .020 inch to the gap. Instantly upon cranking, there was normal spark on all coils! Apparently, the module was able to use the signal created by the crankshaft sensor to create a reference pulse to the PCM, but was unable to recognize the 10-degree slot to begin sequential coil firing. There was nothing obvious about the block that could explain this kind of problem. The final solution was to add a thin washer between the crankshaft sensor and the engine block, which slightly increased the distance between the crankshaft sensor and the crankshaft. The engine has run fine ever since. The best solution for this Corsica no start would have been to call AutoLine a lot sooner.
Mazda Flatspot At 3500 To 3800 RPMPosted 4/18/1997By Tom Anderson
The owner of a 1988 Mazda RX-7 had complained about a lack of power on hard accel. The technician drove it and found that the engine flattened out about 3500 RPM. When he stepped on it harder, it picked up again and accelerated at a more normal rate. The technician had tried the things a good tech would think of on a multiport injected engine. He swept the throttle position sensor (TPS) and the vane-type airflow meter on a scope. The patterns looked perfect with no dropouts or glitches in the voltage sweep. When the fuel pressure was checked, it stayed right where it should. He called me looking for ideas. There is a shroud of mystery most people feel when dealing with rotary engines. I explained to him that there are four injectors on this engine and they are set up much like a four-barrel carb, with two primary injectors and two secondary injectors that just happen to come on at that 3500 RPM mark. The ECU will cut back the primaries, open the auxiliary port actuators - if needed, since this is a normally closed port - and begin operating the secondary injectors. A check showed the vacuum sensor was OK. (Who knows why Mazda calls it a booster sensor?) The O2 sensor voltage dropped from .7 volts to 0 volts immediately, the auxiliary port actuators opened and the secondary injectors began pulsing. In this case, the ECU was trying to do its job, but the engine just wasn't getting what it needed. Either the injectors were plugged and not spraying fuel or they were spraying, but the fuel wasn't getting to the engine as it should. I advised him to pull out the secondary injector rail and inspect the mixing plates in the holes below the injectors. This piece has several holes in it that the fuel has to spray through to get into the intake runners. The holes were blocked with a carbon-like substance, so the tech cleaned and reinstalled the rail. He also checked the injector spray pattern at this time by securing the injectors in the fuel rail to keep them from popping out from the fuel pressure. He pressurized the rail and operated the injectors manually. An injector "pulser" works very nice here. The injectors sprayed a fine cone pattern. The reason it drove the way it did was that when the ECU began operation of the secondary injectors, the fuel hit the plugged mixing plates and the fuel puddled there. These large drops of fuel weren't burned in the engine completely so the vehicle lost power. The ECU received a lean O2 signal from the oxygen sensor. Remember, the O2 sensor measures oxygen, not fuel. Because of the large amount of oxygen, it brought the primary injectors up again and the power would improve. The car now runs fine!
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