Hesitating at full throttle ?

[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.

[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.

[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]

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]

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]

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

[tolsen]

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

Lower tank sufficiently to get enough clearance.

[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.

[marcjero]

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

[tolsen]

The Walbro GSS284 pump is also used in the diesel 450 but then only deliver 2.5 bar. Perhaps there is a regulator built into fuel sender unit. Fuel pump as fitted in sender unit for the Roadster appears to be rated at 4.9 bar.

[marcjero]

Yes that's not clear. Some sites are stating a flow of 65 lph. I found ones stating 120 lph. Could be replaced with a gss340 I think. But it's overkilling. gss242 could be a better match.

[EmilysDad]

How much is an OE pump? I've found the MB aren't that badly priced & you'll know you have the correct part ....
mine cost me over £400 but that included diagnostics and 4 hours labour (MB) charge for 1.4hrs

[tolsen]

Cheapest complete fuel sender unit is £116.90 on misterauto.

[marcjero]

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.

Yes. The tip is you have to pull the fuel sender unit and clock it. Then you can remove the ring from the car.

[marcjero]

Cheapest complete fuel sender unit is £116.90 on misterauto.

Yes I found it. But no clear specs again. I plan to run e85 and tune a bit in the future. The problem is the Smart engine is running high pressure fuel flow : 4 bar + 1 to 1.5 bar boost so we have to cope with a total of 5.5 bar (80psi). Most of the time turbo cars run a 3 bar fpr only.

http://www.autoperformanceengineerin...l/fpspecs.html shows some interesting data about the most famous Walbro pump. If you consider the smallest 190 lph model (gss242) you see that the flow drops a lot at after 80 psi. So the standard flow rate of a fuel pump doesn't matter a lot. What is the important is the flow rate at full boost. I see that gs242 could still flow a large amount of fuel at 80psi. Would be interesting to compare with gss284 figures. I guess that gss284 is flowing 110 lph at low pressure and 65 lph at 4.9 bar. gss242 could feed about 100lph at 4.9 bar. This is the reason I consider this model. It could support 120 bhp with E85 without any problem.