YAHOO!! I have found the mystery ticking fault on my car!! (Well I think i have, only time will tell).
DRM159's previous post prompted me to investigate the fuel system problem, my previous thinking having lead me down the path of the variators.
I will describe as best I can what I think is going on, but I have also posted a you tube video which may be more instructive. You can only watch this video by pasting the link into your browser, you cannot get to this video if you search for it in you-tube.
Fuel system problem - YouTube
Here is my description of what I think is happening;
The fuel system has two pumps which are connected. The first is a 'low pressure' pump in the fuel tank that runs at 6 Bar (90Psi), and the second pump is a high pressure pump on the end of the cylinder head that runs at 120 Bar (1750 Psi). So, the pressure in the delivery tube to the engine bay should be 6 Bar, and the pressure in the engine injection manifold should be 120Bar.
Under normal operation, one of these pumps (it doesn’t matter which) generates a characteristic pressure pulse that, when not dampened properly, is the source of the ticking. The pressure pulse causes the tube to pulsate against the mount on the firewall, and this is the sound you here in the cabin. In my car, the tube that that vibrated was the top tube, when located at the point where the tubes come off the firewall and head over to the fuel pump in the engine bay. (again see the youtube video) This is the fuel supply tube.
To dampen the pressure pulses, the high pressure fuel pump on the engine contains a device called a pulsation dampener, I have not opened this up as the alfa engineers thought that allen key bolts should only have 5 sides, not six. However, I have studied a cross section drawing, and it looks to be a rubber diaphragm backed by a heavy spring.
So, on one side of the diaphragm is the low pressure fuel supply at 6 Bar with its pressure pulses, on the other side should be air (the Alfa technical manual says fuel vapour) and of course the spring to balance the 6 Bar. Let’s call this side the spring chamber. When operating correctly the pulsation dampener provides enough ‘capacitance’ in the system to absorb the pressure pulses.
So rather than drain the spring chamber to atmosphere, for some reason it is drained to the fuel tank return line. I can only assume this has been done to 1. keep the wasps out, and also 2. so that in the case of a failed diaphragm, the fuel spill is contained.
The problem with my car was that the spring chamber, that should be full of air (fuel vapour), was indeed full of fuel. Alfa for some reason also put a small orifice in the spring chamber drain line , so if this chamber is full of fuel, not air, the pulsation dampener has virtually no effect.
The pulsation dampener spring chamber drain hose is easy to identify as it the only hose connected to the high pressure fuel pump (the other connections are steel tubes). It has a plastic fitting mid way along its length which I think is a check (non-return) valve.
I removed this hose from the pump, at the pump. To do this I had to destroy the factory hose clamp and replace it with a standard screwed hose clamp. When I removed the hose, it was apparent that the hose and pulsation dampener spring chamber was full of fuel, not air/fuel vapour.
I then did two things, and I must put in a massive disclaimer here that you should do this at your own risk. First I blew into the hose at the point I had disconnected it to ensure free flow into the return line back to tank, and this indeed was the case.
Second, I started the car with the hose disconnected and watched what happened. A squirt of fuel came out of the fitting on the fuel pump and eventually cleared. Whilst the engine was still running I then sealed the fitting with my finger and could perceive a light pulsation under my finger at the same frequency as the ticking sound had been. If you choose to run the car with the hose off, be careful because if your diaphragm is ruptured you will get a gusher of fuel out of this fitting and may burn your car to the ground.
I also noted that the check valve seemed to be working as there was no fuel flowing out of the hose due to back pressure in the fuel return tube.
I reconnected the hose with a new clamp, and hey presto, no more ticking!!!
The remaining question is, how did the chamber fill with fuel? I can only think of two causes, neither of these is the fuel lines themselves, as previous posts report the dealers were pursuing;
1. I have not opened up the pulsation dampener, but if it is a diaphragm as I suspect, then perhaps the fuel is leaking through or around it. At 85 Psi, I suspect this is not the cause as this chamber would rapidly fill with fuel and the problem would not go away after startup.
2. Alternatively, the check valve in the return line to the tank may be faulty. I did not think to suck on the hose to check the function of the check valve, but I suppose it would be good to verify the check valve is working. It may also be that the check valve is not sealing effectively at very low pressures, and fuel/vapour leaks back across it, which subsequently condenses in to fuel over a long period of time. After running for a while the pressure pulses push enough fluid out of the drain line to create a partial vacuum in the spring chamber, restoring function. Again, speculation.
So in summary have not identified the root cause but am very close, and very relived it is not inside the engine!!
Can’t wait to hear from others who have this problem.