You're leaving out a lot of details that could be helpful. You said, "It starts right up, but then stalls". Now you said you drove it, so we know it's not in theft mode. What did you have to do to keep it running?
If it runs rough or has low power, I'd suspect the new timing chain is off by one tooth. If the power seems normal but it's just idling too slowly and stalling when you release the gas pedal, that still points to the need to relearn minimum throttle. If coasting for seven seconds didn't work, the next suspect would be the brake light switch. There's three parts to it. One part can have a poor contact or the entire assembly can be out of adjustment and not affect the brake lights. Do the coasting procedure again while pulling up on the brake pedal with your toes. Also, be sure the engine is up to normal temperature and you coast for at least seven seconds. The Engine Computer needs to see high intake manifold vacuum for seven seconds to trigger the relearn. You can get high vacuum by snapping the throttle and releasing it, but that will just last one or two seconds. The only way to get high vacuum for seven seconds is by coasting. At that point the computer knows you have to have your foot off the gas pedal and it can take a reading from the throttle position sensor. From then on, any time it sees that same voltage, it will know your foot is off the pedal and it has to control idle speed.
It is also possible for road vibration to bounce the throttle position sensor while you're coasting and that would cancel the relearn because the computer would interpret that as you moving the pedal. Just repeat the procedure until it works. When we do test-drives after performing any service that required disconnecting the battery, we don't really pay attention to idle speed. We're checking to make sure other repairs were successful. By the time we return to the shop, idle speed is normal.
The next thing, assuming the only problem is the idle speed is too low, is to have a mechanic connect a scanner to view live data. Look at the "AIS step". That's the automatic idle speed motor. That is not a motor like you would normally think of. The Engine Computer pulses that assembly to turn the armature to slowly run a threaded shaft in or out. That varies the size of an air passage around the throttle blade to control idle speed. The computer will set that motor to one of 256 "steps".
For a properly running, warmed up engine, step 32 is typical. If one cylinder is misfiring, you'll find it at around step 50. If you see it at step 0 and idle speed is too high, the computer is trying to reduce idle speed and is not having success. That's almost always due to a vacuum leak. When you have step 0 and idle speed is too low, minimum throttle hasn't been relearned yet and the computer is not paying attention to idle speed or trying to control it.
If you see the AIS is on a step considerably higher than step 32 but idle speed is too low, that proves minimum throttle has been learned and the computer is trying to raise engine speed. There's three possible causes for the speed to not increase. The AIS motor could be defective. That is not real common, but when they DO fail they usually get tight and won't turn freely. That will not set a diagnostic fault code because it's not an electrical problem that can be detected. It's a mechanical problem. Second, there could be an electrical problem. That is usually with the four wires going to the motor or there's a stretched terminal in the connector. Stretched terminals are caused mostly by someone probing in them to take a measurement, and they damage the terminal. The motor could have an electrical problem but that is real uncommon because there's eight wires on four terminals. Two of them on the same terminal would have to be broken to have the computer detect it and that's not likely to happen. It's more common for just one of the four wires going to the motor to be cut or corroded, (or that stretched terminal in the connector), and that WILL be detected and a fault code will be set.
The third thing is the air passage is plugged with carbon. That was REAL common on the 3.0L engines in the early '90s, but with better fuel additives that hardly ever occurs anymore. That didn't seem to be a problem on the other engines.
Thursday, October 3rd, 2013 AT 12:30 AM