Pump or software?

Frank Massey asks why would a pump run at max pressure?

Published:  07 October, 2013

By Frank Massey

It seemed to me that monitoring the rail pressure sensor and volume control valve duty cycle would be the obvious solution. The Bosch system operates on the principle of a 'fail open, driven closed' valve. When driven closed, the volume reduces and in accordance with Boyle's law, pressure will also reduce. The valve is controlled with a ground switched signal via the powertrain control module (PCM). In our case, it appeared that when the rail pressure sensor voltage reached 4.5 volts, the negative duty was at 90%. This response did not appear normal. Therefore, we decided to use a tool called the 'MM03' - which is a clever device for testing HP pumps while still fitted to the vehicle. The tool allowed us to take direct control of the HP pump. While we were testing, we noted an increase in the return/spill pressure. This is not normal and assuming the application of Boyles law in restricting the spill, we inadvertently increased the volume and therefore increased the pump's outlet pressure.

All programs run on the premise that request control values are received from various sensors then compared to the specified values which form the software parameters. When a control value is not reached, correction or adaption should attempt to rectify the discrepancy. So far, we had concentrated on the physical aspects of high pressure control, priming, inlet control valve, the high pressure pump and injector back leakage.

The evidence did show an inability to control rail pressure and especially the curious response of increasing volume when the pressure went too high, but why? I did previously state this system employed a Bosch CP1h pump with volume, not pressure control. I wondered if perhaps the software had been incorrectly installed for a regular CP1 pump. I decided to drive the vehicle and monitor live data; I had to ensure no rougue request inputs were causing this strange response.

The live data I focussed on included request load values, app input, air mass calculation, rail pressure feedback, brake and clutch input as well as all other possible input variants that had a direct or possible effect on rail pressure control.

Unbelievably, if driven on the open road, throughout the entire load and speed range, the rail pressure exactly matched specified and actual values. With experience, it was possible to reset the error by prompt cycling the key off, even whilst the engine was still rotating!

What happened at the idle load speed range was even more curious, the PCM changed the initial correct control duty of 43% to the inlet metering valve, rapidly reducing it to around 30%. This resulted in maximum system pressure.

? 1 high pressure run away

? 2 DPFs removed

? 3 software validity unknown, especially for the high pressure pump

? 4 replacement volume valves fitted

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