0

I had a faulty camshaft position sensor on a Chevrolet Spark (aka matiz) with a 3 cylinder engine (F8CV engine) that was causing it to stall intermittently and being hard to start. When I replaced the sensor both issues are gone except that now the engine stutters when accelerating (uneven acceleration). When I unplug the sensor, the car accelerates fine (bit low on power but even and smooth acceleration).

So I plugged a OBD2 scanner and ran the same stretch of road with and without the camshaft position sensor. Recorded throttle position, engine RPM and time advancing. I got this:

With sensor:

With sensor

Without sensor:

Without sensor

I replaced the fuel pump, fuel injectors, intake manifold seals and the map sensor. That didn't help.

I ran the car with the sensor for several days (to allow it to relearn) and that didn't help. I also disconnected the battery terminals for few hours to reset the ECU but nothing has changed.

This car doesn't have an EGR and always runs on open loop mode. (no MAF sensor or O2 sensor).

The car idles and revs (in neutral) fine.

I'm suspecting I have a faulty knock sensor and that the timing advancing only occurs when the camshaft position sensor is plugged for some reason. But before I tear apart the engine looking for the sensor, I would like to know what you guys think and if I'm missing anything.

4
  • Why doesn’t it have an o2 sensor?
    – HandyHowie
    Oct 27, 2021 at 12:46
  • @HandyHowie It came from the factory that way. To cut cost I guess? My country has very loose emission regulations. Oct 27, 2021 at 12:51
  • Is it possible that the cam position control valve is defective?
    – jwh20
    Oct 28, 2021 at 10:16
  • @jwh20 The car starts fine and never stalls so I'm thinking no. Plus as per a similar engine's service manual, the camshaft position sensor is only responsible for the firing sequence of the injectors and doesn't affect the timing. Oct 28, 2021 at 11:42

1 Answer 1

1

So after a lot of inspection and diagnosing. I sensed that one of the cylinders isn't firing correctly (engine vibrates, uneven). By disconnecting injectors I found out that the cylinder in question is #3 (disconnecting the injector doesn't make any difference). So I swapped injectors with cylinder 2 and now cylinder 2 isn't firing. It was a bad injector even though I replaced all three just recently (that's why it took me so long to suspect there's something wrong with the injectors). So I replaced the faulty one with a new OEM part and the problem is gone!

It turns out my car doesn't have a knock sensor and it must be using the CKP sensor as a way to detect knock. It's possible that it was detecting knock on the misfiring (or not firing) cylinder and that's why it was advancing ignition timing. Disconnecting the CMP sensor makes the car somehow ignore that signal and therefore not advancing the timing which made the car run smoother even with a cylinder not firing at all.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .