Direct Injection — Poor Performance

By Dave Hill
www.londonroadgarage.com

A 2003 Vauxhall Signum (same as a Vectra C) arrived at the workshop with a slight misfire and a more general flatness under acceleration. A scan provided the code P0301 (Cylinder 1 Misfire Detected) and a quick inspection confirmed that the insulation had failed on the cylinder 1 coil area.

A new coil pack and set of plugs cured the single cylinder misfire, but still there was a general flatness in performance on the road. When I rescanned it, there was a new code P1191 Unknown DTC.

This is a new system to me and some head-scratching began. This is a direct injection setup where a pump in the tank supplies a high-pressure pump driven by the inlet camshaft. The low-pressure pump delivers approximately 4 bar (64 psi in my case) and the high-pressure pump boosts this pressure to a maximum 120 bar (1740.5 psi).

The fuel rail resembles a common rail diesel setup, with a pressure sensor (3-wire) and a pressure regulator (2-wire). I set about taking a capture of both these components in an effort to understand what was wrong.

Here you can see the duty cycle increase as I floor the throttle but the pressure sensor output doesn’t rise as I expected.

Unfortunately I don’t have any kit that will physically test the pressure in the rail but I needed to prove if the sensor was capable of reacting to a pressure increase, so I used the workshop airline pressure to test it and sure enough I got a reaction, however small.

The ironic thing is that I had had three of these vehicles in recently with various faults and I have missed the opportunity to take captures of various systems to gain the knowledge needed.

A few days later…

A new pump arrived after getting lost in a trolley at Luton for two days! Anyway, it sorted the problem as expected.

Here is the same capture as before, but now the pressure sensor reacts to throttle demand.

I did manage to strip the high-pressure pump (once it was condemned) and the failure was due to a small perforation in one of the three rubber pumping diaphragms.