Toyota’s, like all modern diesel engines utilise a common rail fuel injection system, of which, a Suction Control valve (SCV for short) is a commonly worn and problematic part. Lets dive in:
As an analogy, consider the SCV as the throttle of the fuel pump. Its purpose is to regulate the fuel pressure, a critical aspect to how the ecu maps and tunes your motor.
Unlike older petrol engines that used an electric pump, they simply pumped 100% all the time and returned to the tank what it did not use. Given that 1KD engines will operate with a fuel pressure as high as 1,800 Bar, or 26,000psi in the old money, it physically takes a lot of power/energy to drive the pump to deliver this amount of pressure.
To employ a similar system would be far too wasteful and have a massive impact on fuel economy, so they use these SCV to ‘throttle the pump’ so it supplies just enough.
An scv is essentially a volume metering device that acts on the low pressure side of the pump (called the suction side - hence the name). They move up and down the cylinder, allowing fuel past this flow metering point.
It's important to understand how the 1KD ECU calculates fuel quantity and how it delivers it. Firstly, based on the driver and various sensor inputs, it will determine a quantity of fuel that needs to be injected. This quantity is a factor of injector flow rate (which is fixed and the ECU is instructed what this is by the injector compensation code), rail pressure and injector opening time.
As rail pressure, even in a very well operating system, is VERY dynamic, so they utilise a Target Rail Pressure for the quantity calculation. Relying on a separate closed loop function of the ecu to marry up the actual and target, moving the SCV and therefore the Pump output to do it.
The issue is that any variation between the Target and Actual rail pressures will see an error in the amount of fuel ACTUALLY injected into the engine.
So in a well operating system, you might see variations of +/-200psi from the targeted rail pressure. When the SCV is worn, it's common to see this number +/-500psi.
Lets use the Toyota Hilux 1kd as an example. Warm, idle conditions will see target rail pressure around 5,100psi and Injection quantity will be around 6mm cubic / stk. So Injection opening time will be around 330 micro-seconds, where the ECU has calculated this on the 5100PSI targeted rail pressure.
In a good system, with little fluctuation, say 4950-5250, the engine will run smooth. In a bad system you’ll see values from 4600-5600 (or worse). This means that the shot to shot consistency is very erratic as we’re over fueling when the rail pressure is high, and under fueling when low. Important to note, this actually has a far greater impact on Pilot injection quantities than main.
Usually this will result in the engine being quite rattly, rough running / not smooth, increase in fuel consumption, and in really bad cases, random engine stalling and so on.
What happens to the SCV is that the piston and the bore will start to score, and the SCVs movement will be irregular and ‘sticky’. The ECU will ‘learn’ and drive each SCV a little different. This learning becomes important later.
Many vehicles still fitted with the original SCV will note that it is short version. These have been largely phased out in favour of ‘long versions’ SCV. Why?
Basically a longer piston means the internal load factors are distributed over a larger surface area and therefore the SCVs performance is more consistent, reliable and lasts much longer.
Toyota had long versions in some of their Common Rail pumps from factory. Those were 04226-0L010. If you have a toyota 1KD fitted with a short SCV, you should replace this unit with a long version replacement kit setup, Please use part number 04226-0L020 to find this kit. These are a much better unit.
Tools required:
5mm allen key, O-ring grease - AND probably a scan tool to remove the ‘learning’ the ECU has done on your terrible old SCV.
A full installation video will be linked on our partner website -www.DIYdiesel.com.au
If you feel like this process is best handled by a professional, please click here to see your nearest DieselGeek affiliated workshop.
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