Sensing a commercial issue

Blue Print technical consultant Jim Gilmour tries out the new G-Scan 2 on an Isuzu N series

Published:  13 March, 2014

By Jim Gilmour

The complaint was that when driven, the engine would run for only a short time before losing power and coming to a halt - at idle the engine would sometimes surge then stall. The company had asked a third party to code read the vehicle and they had retrieved some relating to exhaust gas recirculation (EGR).

EGR is monitored in a number of ways, one of which is the air mass meter. Diesel engines do not use a throttle to reduce the pressure in the manifold so the intake air mass is a function of engine capacity, intake air temperature, air pressure and engine speed. The engine control module (ECM), calculates the air mass reading based on this information. EGR fills the cylinder with as much as 60% exhaust gas which by-passes the air mass meter, so increasing EGR will show as a reduction in air mass intake.

There are 48 parameters on this engine, so to quickly get a picture of what was going on, the G-Scan 2 was set to record all parameters and the engine was started. Fortunately, I did not have to wait too long - after about 30 seconds of normal idle, the engine began to surge then faltered and stalled.

I navigated to where the engine started to falter and the flurry of data changes drew my attention to the fuel rail pressure; I then selected the data that I wanted to look at in graph form Figure 1.

Measuring delivery pressure requires installing a pressure gauge which is intrusive and time consuming; to try and avoid this, I analysed the data in more detail. Looking at the relationship between desired and actual fuel rail pressure, I noticed that at times the actual pressure exceeded desired levels. This data is unlikely to be caused by a reduced low pressure delivery volume so my next step was to repeat the test and monitor the control of the SCV.

The G-Scan 2 has a built-in oscilloscope and multimeter, so I back probed the SCV solenoid. The engine control module controls the SCV by varying the duty ratio ('on time' as a percentage of the period), the SCV is a normally open type meaning that when disconnected it is fully open, maximum delivery is achieved and pressure is increased. The on-time of the duty cycle controls the closing of the valve so the longer the on-time the lower the fuel pressure. The scope is connected to the switched ground wire; consequently the on-time is when the solenoid is switched to ground. Figure 2 shows the duty ratio when the rail pressure was maintained at a normal idle pressure of 300 bars. Figure 3 shows the duty ratio commanded to increase to try and compensate for loss of fuel pressure, causing the engine to die. The two images also show the SCV control signal and the ground to be good. The SCV windings were checked for resistance and insulation and were within specification.

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