'97 Toyota's Crank Pulley Bolt Confuses ECMPosted 8/18/2006
By Brad Davis
Symptom: The engine surges up and down on acceleration. Sometimes the engine would stall and die but would start again right away. It would never set any codes.
The technician had checked the crank and cam sensor and both signals looked perfect. He had replaced the cam and crank sensor but it had no effect and neither did replacing the electronic control module (ECM). When the technician called, we learned that the engine would only surge at high rpm on a road test. All attempts to duplicate the problem in the shop were futile.
During a road test, we had him monitor the throttle position sensor for any cutouts and glitches - we found none. Disconnecting the throttle position sensor did not change the situation either. Next, we had him monitor the crank sensor signal and the cam sensor signal - the patterns were perfect.
The oxygen sensor would go extremely lean when the surging occurred. There was good fuel pressure. I had the tech check the injectors with his noid light. It looked like it was constantly blinking, but when a lab scope was placed to the ground side of an injector, it showed an erratic pattern. The injectors were being turned on and off rapidly by the ECM. On this vehicle, the ECM uses a signal from the igniter called IGF for spark confirmation. The ECM sends a 5-volt square wave signal to the igniter on three different IGT terminals: IGT1, IGT2 and IGT3. Each time an igniter receives a signal from the ECM it must send a confirmation of spark to the ECM. Without this signal, the ECM will not trigger the injectors. Monitoring each IGT signal showed a perfect 5-volt square wave signal and a consistent return signal on IGF. What would cause the ECM to turn the injectors on and off like this?
Thinking back to when I worked at a Toyota dealership, I remembered another Toyota truck with the same symptoms. The injectors would be turned off at 60-65 mph, but it would happen on both high and low rpm. Monitoring the speed sensor signal at the ECM on that vehicle showed a ghost pattern on the speed sensor signal. The ECM thought the vehicle was going 120 mph and it shut off the injectors.
Monitoring the speed sensor input on the current vehicle showed a perfect pattern, and nothing happened when the sensor was disconnected. Since we knew the ECM would shut the injectors off when the vehicle went fast, it made sense that the ECM would shut the injectors off if the engine was also spinning too fast. The ECM uses the crank sensor as the main engine rpm input but we already had a new one, and the pattern looked perfect on the sensor when we looked at it on his lab scope. A scanner showed that the engine rpm was normal but there had to be something the ECM did not like. Discussing the problem in further detail, the tech said that the customer always maintained the vehicle. In fact, the tech had replaced the timing belt a few days before these symptoms started to occur.
A clue? But what could the timing belt have to do with it? Since the technician was close to our dyno facility here in Saint Paul, Minn., I suggested he bring the vehicle in so we could look at it. A recheck of the symptoms showed exactly what we thought: the injectors were being turned on and off and all patterns looked correct. I was monitoring the ignition timing with a timing light when I noticed something. It looked like the crank pulley was slightly out of alignment with the rest of the pulleys, but you would never notice it unless you were staring intently at the setup. It was very slightly out from fully seated. I used an impact wrench to tighten the crankshaft pulley bolt. It felt like the bolt was tight, but I was convinced something was wrong. I took out the crank pulley bolt to remove the assembly so I could check the crank pulley and reluctor wheel assembly. I noticed that the very last thread on the bolt looked a little worn. But at this point, I wanted to check out everything, so I took it apart.
Checking the timing belt alignment and the gear setup showed everything was correct, so I reinstalled the crank pulley, fixed the end of the crank pulley bolt and retightened the bolt the best I could with my impact gun, which it turned out was 120-foot pounds. I then retightened the bolt to 180 foot-pounds of torque, which was not an easy task without the correct pulley holder. A quick look at the crank pulley showed that it was now correctly in line. A test drive verified the problem was fixed.
What was happening was the crank pulley bolt was not tightened to the correct specification, and it backed out just enough to allow the crankshaft reluctor wheel to wiggle or vibrate at high rpm. The ECM thought the engine was over 8,000 rpm, twice as high as it really was, and shut the injectors off.
This was a difficult problem to diagnose because every reading on the scanner including engine rpm and the crank sensor pattern looked perfect. Since this hotline call, I have had numerous calls about the same problem. One thing holds true: you must remove the crank pulley bolt and the crank pulley, reinstall the crank pulley and slam it hard into place a few times and then tighten the bolt to 180 foot-pounds torque. No more, no less!
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