2.0 JTS. Mk 2 facelift on a 53 plate, the odo says its done 53k miles but the management thinks its done 80k….Bought from an auction as a runner.
Unfortunately, it isn’t. It’ll start, idle and even rev cleanly, but not for long. It’ll suddenly loose all will to run and splutter to a stop. From cold, best case scenario for this to happen is around 5 minutes.
A friend, Mark, has been given the task of finding the problem.
Mark’s diagnostic kit has identified the problem as low fuel pressure. JTS’s are direct injection. Fuel is pumped from the fuel tank by an Electric Pump at 5.5bar to a high pressure pump that’s driven off the exhaust camshaft. The high pressure (HP) pump then ups the pressure to between 50 and 120 bar depending on engine load and speed.
The pressure isn’t controlled by the pump. There is a pressure sensor and a solenoid that opens and closes a regulator in the HP fuel rail. Excess pressure/fuel is dumped back into the low pressure feed from the electric pump in the tank.
Essentially, the electric pump provides a large volume of fuel under modest pressure. The HP pump is like a pressure washer. It doesn’t increase the volume of fuel going in, in fact the small bore pipes on the HP side limit volume, it just ups the pressure. Its like the difference between running a pressure washer with the end of the lance on (high velocity, will strip paint) and with it off (limp wristed, nothing for the pressure to act against….)
The engine management has set values for pressure that it aims to meet by opening and closeing the fuel pressure regulator. We know this because the diag kit mark has shows the pressure the management is trying to achieve and also has a realtime readout of the actual pressure.
So if you start the engine, everything is fine. Good fuel pressure for a minute or two - At idle it should be around 50bar.
Then over the space of 10seconds or so the pressure drops sharply to 5.5bar and the engine dies.
So far, this is what has been replaced:
Fuel pressure regulator on the HP side
Fuel pressure sender
Electric pump in the fuel tank (LP side)
That lot, by all accounts wasn’t cheap and in the face of mounting costs – parts as well as labour, the car’s owner will not spend any more money on it without knowing it will cure it. Mark is already out of pocket because he’s has to pay for the regulator and he’s spent hours, unpaid, trying to find the problem. I think because its become a challenge, he refuses to give up.
The regulator solenoid acts like a switch, its either open or shut with no leeway in between. Last night we hooked up a switchable 12v feed to it and stated the engine. As you’d expect when it was shut, FP would climb and when it was open FP would drop.
But here’s the weird thing. Even with the regulator wide open and FP down to an indicated 5.5bar, the engine would still idle perfectly normally apparently running on pressure provided solely by the electric pump in tank.
And when it did decide to play games, we tried holding the regulator shut but FP still dropped and the engine died.
So where the hell is the fuel pressure going?!!! There are no obvious leaks on the HP side. It happens more readily when the engine is warm. The plugs are black as the ace of spades but that could be attributed to the engine only being able to run when its cold so it will be being over fueled.
Grasping at straws, on the basis there could be crud in the system between the HP pump and the regulator, I suggested that we take the regulator out and turn it over.
Turning the engine over resulted in fuel going everywhere, obviously, the in tank pump saw to that. But the hole in which the regulator sits had air bubbling out of it for 10minutes or so after we had turned the engine off…. Is this a red herring or is there an air leak?