What is P0172? http://www.obd-codes.com/p0172
- Irregular behavior
- Fuel system
- Excessive fuel pressure
- Leaking or contaminated fuel injectors
- Leaking fuel pressure regulator
- Low fuel pressure or running out of fuel
- Vapor recovery system
- Air leaks after the MAF
- Vacuum leaks
- Improper seated engine oil dipstick
- Stuck EGR valve
- Oil overfill
- Cam timing
- Cylinder compression
- Exhaust leaks before or near HO2Ss.
My trouble shooting regime
Observation: The gear change was jerky from 1st to 2nd. Thereafter CEL appeared.
The OBD scanner reads: P0172 confirmed.
I thought it was the transmission since this vehicle had its transmission serviced before.
The transmission fluid was drained. Its color is clear red which was good.
There was some very very fine metal waste on the magnetic drain plug.
The fluid was passed through coffee filters and no deposits which is good.
Jerking persist after refill of transmission fluid
Finally caught the problem.
After investigation, it was a result of a leaky gasket on the valve cover.
That compromised the seal integrity resulting in vacuum leaks.
Now the engine computer calculates everything. The leak in vacuum threw the fuel calculation into disarray resulting in jerks.
The final fix
Most of you will not experience this, but for mine, the engine was lifted out of the body during the transmission service by a third party workshop. This is a service provided as "insurance" by a used car dealer. The valve train cover was found to be assembled but the seating is not aligned.
The workshop personnel used silicon liquid gasket to patch this imperfection. Because of this misalignment, the rubber gasket on the valve cover cannot seal perfectly, resulting in vacuum leak.
I finally zoomed on the oil spray which was usually absent in a well maintained compartment.
This is unusual because there should not be oil leaking out at all.
From here, i had to replace a new valve cover gasket and patch the imperfect seating with new silicon gasket.
For my case, i waited about 2 hours before usage of the vehicle.
A full cure would take about 1 day.
After this replacement, the P0172 went away.