156 TS 1.6 Hesitatant Acceleration from Idle
I've been having this issue with my Alfa for the last 60k km, but it was probably there for even longer. I've tried a lot of things but I'm out of ideas, so it would be great if you can help me with this diagnostic. I'll try to describe everything in detail, along with my tests so far.
When at idle, if pressing the pedal hard, the engine would hesitate to accelerate. The rev's drop a little, and then (after 1 second or so) it rev's up normally.
If I accelerate slowly to 2000rpm , and then press hard on the pedal, it does not show the hesitation. When driving normally there is no problem.
The tests done and facts
Using an ELM 327 BT device, I've started checking various things:
- Oxygen sensors work fine (all 3 of them)
- MAF shows a reading of 3-4 g/s at idle, and the intake air temp of about 40C (which seems normal). While accelerating, it goes up to ~100g/s and the graph looks normal to me. Without the MAF connected it shows the same symptoms.
- Throttle angle is fine
- Throttle pedal is fine
- Engine coolant sensor is fine
- Fuel consumption is normal (low)
- If pressing hard on the pedal from idle, it misfires (specially when cold)
- No fault codes in the ECU (Bosch Motronic 7.3.1)
- Disconnected injectors one by one, and all of them are functioning (I can hear the difference in the engine sound).
- The issue is more observable when cold (during winter it has to warm up for a few minutes, otherwise is not drivable).
- The Idle is smooth and correct, at about 850 RPM
- Variator is fine, and the problem is when accelerating from idle.
Now, the most interesting thing:
- If I disconnect the temp sensor on the thermostat, the engine runs FINE and the problem goes away (for a few minutes, until the ECU compensates I think). But it does rev up without hesitation without that sensor, which kind of proves it is not a mechanical issue.
The car has been taken to AR representatives while in warranty several times and they were not able to fix this so is a bit more complex I guess.
I would be grateful if you can share your theory on this.
Alfa 156 1.6 TS
Alfa 75 2.0 TS