Hi Jacobandnickolas. Sorry to butt in. Your reply wasn't there when I started typing.
There's no reason to be looking for the crankshaft position sensor if you have a MAP sensor code. The Engine Computer needs the signals from the crankshaft and camshaft sensors to turn the automatic shutdown (ASD) relay on. That relay powers the ignition coil pack and injectors, among other things, so if a signal is missing from either of those sensors, the engine won't run at all. It is possible for the crank sensor to cause rough running, but it is extremely rare, and the defect that would cause that is very rare.
The timing belt is a good suspect too. If it jumps one tooth, the Engine Computer will turn on the Check Engine light and the diagnostic fault code will be "cam and crank sync". At two teeth off, the computer will shut the engine down by turning off the ASD relay to protect the valves. At three teeth off, the pistons will hit the valves and bend them.
There was a related problem that has probably been addressed with the single-camshaft engine where the dowel pin between the camshaft and its sprocket would shear off, and the sprocket would turn a little. That made the camshaft late, and since the camshaft position sensor is on the other end, on the driver's side of the engine, it looked the same to the computer as if the timing belt jumped a tooth. There is a point in between being the equivalent of one and two teeth off where the engine may or may not start intermittently, and power will be down. I never heard anyone list running rough as a symptom.
This MAP sensor code is an uncommon one and points to the sensor just starting to fail. Usually they went from working fine to total failure within a day. One of the common observations, and the way we drove the vehicles into the shop, was you had to keep moving the accelerator pedal. As soon as you tried to hold it steady at any speed, the engine would stall. The MAP sensor has the biggest say in how much fuel enters the engine. The problem is it will only set a fault code when the signal voltage goes outside of 0.5 to 4.5 volts. As long as the voltage stays within that range, the Engine Computer will react accordingly, but the sensor can report the wrong voltage within that range. That sensor is pretty sensitive to very tiny changes in vacuum, so the wrong voltage, even by just a little, has a big affect on fuel metering. My suspicion is the rough running is due to an incorrect amount of fuel leading to lean cylinder misfires, and the fault code for barometric pressure is related. On older engines incorrect readings were usually caused by a cracked vacuum hose going to that sensor, but that was improved by plugging the sensor right into the intake manifold. Since you replaced the sensor already, and since there's no vacuum hose to it, first check the other hoses for a vacuum leak, then measure the voltage on the sensor's signal wire. Check on the ground wire too. If there is a corroded splice or connector terminal, that is going to result in a higher voltage on that ground wire, and that will translate into a signal voltage that is consistently too high.
Normally you will find 0.2 volts on the ground wire. The feed wire has 5.0 volts. With the engine not running and the ignition switch on, typically you'll find around 4.2 volts on the signal wire. That represents barometric pressure. With the engine running, higher manifold vacuum results in a lower signal voltage, usually around 1.2 volts.
Measuring the signal voltage will only tell you if the sensor appears to be doing something. Beyond that you need a scanner to view live data. It will show the same signal voltage but it will also show the amount of vacuum that represents. Normal at idle is around 18 or 19 Hg, and it will go down when under load or accelerating. If there's a wiring problem, you're not going to have a signal voltage that you have to wonder if it's right or close. It's going to show a vacuum reading that is easy to spot that it's wrong.
Sunday, January 5th, 2014 AT 12:49 AM