Hesitating at full throttle ?

[marcjero]

Ok the pump is out. It's a Walbro GSS284. Does anyone know the specs of this one ?

[EmilysDad]

Lower tank sufficiently to get enough clearance.

According to the book you have to, but in practice you can remove it without doing so .... just! I replaced the large O ring from under the pump & managed to do so without moving the tank.

[tolsen]

Any idea in order to get the tank nut out of the car ?

Lower tank sufficiently to get enough clearance.

[marcjero]

Any idea in order to get the tank nut out of the car ?

[Thrumbleux]

Well I guess that the injectors at 6000 rpm and full boost should be at about 60-80% duty cycle. I don't see the 2.5 factor here. I think the flow can be 40% higher in the very worst case. I don't see the standard injectors able to produce 200hp even at 100% duty cycle.

I based that on an injection duration of 8ms vs 20ms for the cycle at 6k rpm - hence the 2.5 factor.
Admittedly, the 8ms was a random figure I assumed typical of injector duration. Maybe for sequential systems it is.

[marcjero]

As per Buckster, 'signal' to the FPR?

I will check this one again

Softening of wire, expansion, etc. A cold engine will accept (needs) more fuel.


And reduce the torque reaction....

Not a lot of torque reaction when the car is parked. In fact I was considering an injector fault not the harness contact theory.

At say, 6000 rpm, and 100% duty cycle, the pump would have to be delivering circa 2.5 as much fuel as normal. Which would equate to 2.5 x 80hp = 200hp. The pumps are reckoned to be good for 130hp.

Well I guess that the injectors at 6000 rpm and full boost should be at about 60-80% duty cycle. I don't see the 2.5 factor here. I think the flow can be 40% higher in the very worst case. I don't see the standard injectors able to produce 200hp even at 100% duty cycle.

[Thrumbleux]

I already replaced the fpr. I installed a 4bar model.

As per Buckster, 'signal' to the FPR?

Well the fuel pressure is even dropping under 1 bar. I don't think injectors are the issue. Wiring harness contact is unlikely the issue else the problem would occur even when the engine is cold.

Softening of wire, expansion, etc. A cold engine will accept (needs) more fuel.

If the injector stay stuck open (faulty injectors), they should not resume normal operation as soon as we release the throttle.

And reduce the torque reaction....

Moreover even with close to 100% duty cycle fuel pressure should not drop so much.

At say, 6000 rpm, and 100% duty cycle, the pump would have to be delivering circa 2.5 as much fuel as normal. Which would equate to 2.5 x 80hp = 200hp. The pumps are reckoned to be good for 130hp.

[marcjero]

Fuel pressure regulator?

Another possibility is one or more injectors remaining open.

If fuel pressure should be around 4 bar above manifold pressure, then a drop to 1 bar is barely enough to overcome manifold pressure when the turbo is boosting and reducing fuelling to such an extent would make the mixture so lean as to not ignite at all - flame out.

I already replaced the fpr. I installed a 4bar model.

Well the fuel pressure is even dropping under 1 bar. I don't think injectors are the issue. Wiring harness contact is unlikely the issue else the problem would occur even when the engine is cold. If the injector stay stuck open (faulty injectors), they should not resume normal operation as soon as we release the throttle. Moreover even with close to 100% duty cycle fuel pressure should not drop so much.

[marcjero]

Quick and easy on a Roadster as fuel sender unit is accessible from the top. Two days job on a 450. Arm yourself with a fuel sender unit nut removal tool and a kettle to make piping hot water. Pour piping hot water over the large nut to ease its removal. I prefer the three legged removal tools. Mine is a Laser 4768.

Actually the large plastic nut is unbolt. But I can't find a way to get it out of the car because it's too large.

[Buckster]

I used to get a hesitation that went away after I fitted a spring clip to the vacuum hose that went to the fuel regulator, it may be worth checking that hose.

[Thrumbleux]

Another possibility is one or more injectors remaining open.

If fuel pressure should be around 4 bar above manifold pressure, then a drop to 1 bar is barely enough to overcome manifold pressure when the turbo is boosting and reducing fuelling to such an extent would make the mixture so lean as to not ignite at all - flame out.

If however the pressure drop is due to injectors open so long that the pump cannot keep up, then over fuelling would be the result (and an overheated cat).

If you can shut down the motor ASAP after a misfire event and pull the plugs, then dry or soaking/black plug or plug(s) should tell you which. Or, if the fuel pump slows (faulty pump) or quickens (open injectors, reduced load on pump).

A persistently open injector could be caused by a short in the wiring (they open on being grounded) and if batch fired, a single short would affect all three injectors. The engine moving on its mounts due to torque reaction during full throttle running may be bringing a part of the wiring harness into contact with a potential earth path (as occurred with early 964 Porsches when their ignition system shorted to the ABS and caused both a misfire and the ABS warning light to illuminate).

[Thrumbleux]

Fuel pressure drops to about 1 bar when misfire occurs.

Fuel pressure regulator?

Fuel pump noise is different too when this happens.

Faster? Slower?

[tolsen]

Quick and easy on a Roadster as fuel sender unit is accessible from the top. Two days job on a 450. Arm yourself with a fuel sender unit nut removal tool and a kettle to make piping hot water. Pour piping hot water over the large nut to ease its removal. I prefer the three legged removal tools. Mine is a Laser 4768.

[marcjero]

So the fuel pump voltage is fine. And the fuel filter was changed few miles ago. So I think it's time to change the fuel pump. Is it possible to do this job without removing the fuel tank from the car ? I wasn't able to get the ring out from the boot until now. Thanks guys !

[marcjero]

I think I know. Fuel pump could be damaged and overheating : fuel pressure starts dropping, engine runs lean and just stalls when boost is coming. Running lean increases EGT. I will hook a pressure gauge to the injection rail.

Yes confirmed ! Fuel pressure drops to about 1 bar when misfire occurs. Fuel pump noise is different too when this happens. Blocked fuel filter (recently changed), power supply issue, or just a weak pump ? We will check the pump voltage during a run but I'm pretty sure the pump is dead. What brand of pump would you recommend ? An ethanol compatible pump would be great.