Caddy: Jerky Engine, G28 Sensor, Low Rail Pressure

Hello everyone,

I'm experiencing the following problem with a Caddy, manufactured in 2014, engine code CFHC, 103 kW:

- Erratic engine running (misfires) occurring regularly between 3000-4000 RPM, and occasionally even starting as low as 1500 RPM.

- Intermittent jerking during driving, with a direct error code P0322: Crankshaft Position Sensor (G28) - No signal - accompanied by the engine check light.

- If the jerking continues, it will trigger further errors, including P0087, which indicates low rail pressure.

Replacing the G28 trigger module, the engine wiring harness, and the control unit did not improve the situation. Furthermore, I observe a drop in the 5-volt supply voltage when the error occurs, as seen on the oscilloscope. Due to the error, I also measured the rail pressure sensor. In the attached image, you can see the 5V supply voltage of the rail pressure sensor at the top and the signal at the bottom. It is noticeable that the rail pressure sensor signal has significant peaks upwards, then the 5V voltage collapses, which also disables the G28.

The pre-injection pressure is within acceptable limits, both from the pre-injection pump and the auxiliary fuel pump.

The error persists even when the following components are temporarily disconnected for testing purposes:
AGR (Exhaust Gas Recirculation), Intake Manifold Control, Turbo Position Sensor, Fuel Temperature Sensor, Differential Pressure Sensor.

If the engine is running only with the camshaft sensor (with the G28 disconnected), it will also exhibit this jerking, and eventually the fuel rail pressure will drop, or other components within that sensor circuit will go outside their operating limits and trigger error codes.

Are there any experiences with this type of error?
Thank you in advance!

vehicle maintenance

Hi,

It looks suspiciously like it's been chewed on by a rodent or has had its cable insulation stripped.

Best regards, Rainer.

Hi,

Thank you for the quick response.

"Yes, a chafed cable was also our initial suspicion, as all the +5V and ground lines of the affected sensor circuit are connected with solder connectors within the engine wiring harness from the factory, and we've heard reports that the wiring harness often chafes in the area under the battery. We haven't found any evidence of rodent damage or gnawing on other components either."

However, the fault persists even after replacing the engine wiring harness. Furthermore, checking the voltage at terminals 30, 15, and ground on the control unit using a different connector with a load applied showed no abnormalities, nor did testing for cable movement.

"It's strange that the rail pressure sensor is outputting these peaks directly at its output. My suspicion is that the +5V power supply is collapsing when these peaks occur frequently, and as a result, the G28 will immediately trigger an error."

Currently, we are still working on testing the fuel system. Unfortunately, we cannot obtain raw data readings from the rail without a rail pressure sensor connected to the engine control unit, as this is necessary to eliminate interference caused by wiring or the control unit. The engine is currently in a limp-home mode and does not display the error. Therefore, we are currently testing the fuel pressure at the inlet and outlet under various load conditions.

If there's any new information on that topic, I'll let you know right away!

Greetings,
vehicle maintenance

Hello,

It's possible that the rail pressure sensor is internally defective, which could be causing this issue.
Can you temporarily use one from a different vehicle for testing purposes?

Best regards, Rainer.

Good morning everyone.

Okay, so we've now installed a different rail pressure sensor. However, the error symptoms remain the same.

I was able to observe something unusual while measuring the fuel pressure.

Fuel pressure after the auxiliary fuel pump: approximately 4 bar.
Return pressure before the fuel filter: approximately 2 bar.

When the error occurs, the fuel pressure fluctuates by approximately 1 bar in both lines. Even when fueling the vehicle with an external pump connected to a canister, the same pressure anomaly is observed.

Surely, such pressure fluctuations cannot be of electrical origin and, for example, originate from the flow control valve of a high-pressure pump?

Greetings,
vehicle maintenance