P0340 on my mother's Aveo

[Ray Vanlandingham]

Hi.

My mom has a 2008 Chevy Aveo, purchased used from CarMax about a year and a half ago.. the car is in the 70-80k range. It is an LT, with the 1.6L engine.

Several months ago, she began having a check engine light. The code returned was P0340 (Camshaft Position Sensor 'A' Circuit Malfunction). Not having a o-scope to actually 'test' the output of the sensor with, and able to get one (OEM) for about $25 on Amazon, I replaced the sensor and wiped the code.

About a week later, another MIL. Checking showed not only the same P0340, but a P1346 (Intake Camshaft Position (CMP) Sensor System Performance). As it seemed that a possible cause at this point was a timing problem, I ordered a timing belt kit and dived in. Once disassembled, it was clear that the belt was a bit stretched... I was able to easily wiggle the belt, on the forward side, more than it's thickness, and thus move the crank pulley about half a tooth. It was clear from checking the marks that the timing belt had not skipped (and the engine has never indicated a miss) but I went ahead and put in a new timing belt, tensioner, and idler pulley. I did not replace the water pump, but did spin it to check... it's 'stiff' but does not grind, and I intended to pull this back apart at 100k and replace it. Again wiped the codes, drove the car a few miles, and no light. Yay, I thought.

About a week later, the P0340 returned (but not the P1346). When driving the car, she reports that after starting, and at the first few stoplights (when cold) the engine seems to 'lag' at giving power when the throttle is opened, but that once it's warmed up it seems to work fine.

I'm at a bit of a loss here, as I'm rather unsure what to do other than start playing the 'keep changing parts' game.... it seems possible that either the crank sensor is bad, or that it's some electrical or PCM issue.

-- Ray

[avguy]

Did you compare the new timing belt to the old one? On the three belts I've done on our Aveos, there was absolutely no stretch on any of them - zero difference between the old and new belts when stacked. So, unless you observed a difference in comparing them, IMO it's highly unlikely the timing belt loop is the reason for those codes.

Have you checked the reference voltage at pin #1, and ground at pin #2 of the cam position sensor? And also how about the condition of the connector, and all the visible wiring? If all of that is ok, then I suggest you bring it to a shop that has a scope, in order to check the waveform of the cam sensor (and crank position sensor, if the cam signal is good).

[Ray Vanlandingham]

Yes, before removing the old belt I placed reference marks on the belt, cam pulley, and both cam pulleys with a Sharpie, and then transferred those marks to the new belt by counting teeth. When matched at one end and pulled out together, the loop made by the original belt was slightly longer, by a hair less than the thickness of the teeth. It was not very stretched (it's not like the new belt fit inside it), but it definitely was a little bit longer and had what seemed to me to be excessive play before I removed it. Also, you are probably aware there at little 'indicator arrows' on the tensioner, that you use to remove the slack and adjust the tension of the new belt. Looking at the photo I took before removing it, the two arrows were probably a quarter of an inch apart before I messed with anything (they are supposed to be in the same spot when the tension is correct). Based on the markings on the original belt, it was the factory original, as would be expected at this age.

Just to clarify, since replacing the belt the P1346 has not returned, after several hundred miles of driving, just the P0340.

All of the wiring harnesses and the connector seem fine by visual inspection... even the tape wrapped around the wiring harness tubing seems original. I did not actually test the circuit (I did not have a copy of the pinout and specs at the time), since I already had the new sensor on hand and it seemed to fix the issue for the first couple of hundred miles. I guess the next step is take my multimeter to it, to verify the voltages, and if they are okay have a shop actually put it on the test machine.