SLK 230 Facelift returnless fuel system upgrade

Hi !
From what I know about the fuel system on SLK, how it works :

Pre-facelift : fuel tank -> fuel pump -> fuel filter -> fuel rail |-> fuel pressure regulator -> fuel tank (return line)
-------------------------------------------------------------|-> injectors

Facelift : fuel tank -> fuel pump -> fuel filter |-> fuel rail -> injectors
----------------------------------------------|-> fuel tank (return line)

As you can see, on facelift, the fuel pressure regulator is inside the fuel filter. So you can't upgrade it to a higher pressure one (except with a custom filter). Now why ?

OEM fuel system is okay with the OEM hp (193-197), when putting a single pulley, you need to increase your fuel delivery so your engine doesn't run too lean on high rpms.
On pre-facelift, we can easily do it with upgrading the pressure regulator from 3.8bar to 4bar. And even with that, you're near the lean limit while WOT.

On facelift, the fuel delivery is fixed so you're just at the (lean) redline with a single pulley. The only way to increase fuel delivery is by putting bigger injectors, and I've never seen someone do it so maybe compatible injector doesn't even exist. You can ask a specialized shop to make custom one but at what cost !

That's why facelift owners put a pre-fl fuel rail. It's not that hard, it is quite cheap and it open a lot more customisation options. Pre-facelift system have a far higher supply volume and can be fully tweaked to ajust precisely the fuel delivery. Also bigger injectors are identified and cheap.

So with a facelift system you won't go much higher than a single pulley (220-230hp). If you want more you need to retrofit a pre-fl fuel rail.

Dear @capmalo

thanks for this clarification, I investigate since a long time about this subject and your analysis give more details, but I still have few question.

In first I want to confirm that its impossible to find any other injector for the face lift, Maybe injector for Volvo V40 1.9 Turbo (ref 6900371) should fit in size but I think the flow spray is different. I planned to buy one to make test, they provide 440 cc / min on 3 bar,

So to back on the retrofit for fuel return line on a facelift, need few advice from you.

• I should install a fuel rail from pre fl (got it)
• I should install inside a FPR at 4 bar. Do you know if 5 bar exist ?
• I should install a fuel return line from fuel rail to filter, what is the best option. Make by myself a fuel line with stainless steel pipe (which diameter), or buy directly OEM line.
• I should install the fuel filter from a pre FL (without FPR inside)
• Should I change the fuel pump and go to a 5 bar or the OEM is enough ?

When everything will be done, it will not impact nothing, I dont need to remap the ECU ? the car is ready for potential modification.

I spend 2 years to completely rebuilt the car :

• Engine rebuilt.
• Brakes
• Front axle
• Differential
• Rear Axle.
• Gear Box.
• Drive shaft and axle shaft
• Shocks absorber coil.
• Full epoxy paint 3 layers on the chassis.
• Breaking system and antiroll bar from SLK 32 AMG

The only improvement I did for the moment are :

• Porting head gasket.
• Light flying wheel.
• Changing Pignon / ring of the differential to put C180 non S/C set. Ratio 1:3,67 vs 1:3,47 original.
• Small porting on the M45.
• Stainless steel exhaust line (taylor made) with exhaust manifold 4/2/1 supersprint

Now the car is ready and working very well, and I planed to go on the phase 2 which is to improve perf of the engine.

Its for this I need, information o the Fuel return line, and also MAF clamping if you have some advise on the subject.

Dont hesitate to contact me that we can share our experiences.

Regards

Pascal

I'm not a tuner, and don't know much, but all this talk of higher fuel pressure and bigger injectors confuses me - wouldn't the lambda/fuel trim feature of the Motor Electronics cancel all this out?

Or is the assumption that these come with a remap?

@pleroux99

• I should install a fuel rail from pre fl (got it)

-> If you seek for maximum performance yes, if you stay stock and/or put a single crank pulley, no don't bother.

• I should install inside a FPR at 4 bar. Do you know if 5 bar exist ?

-> If you put a pre-fl fuel rail yes do it, 5 bar exist but they are for Porsches so they are very expensive. Best is to put an adjustable one and so you put the pressure that you want/need.

• I should install a fuel return line from fuel rail to filter, what is the best option. Make by myself a fuel line with stainless steel pipe (which diameter), or buy directly OEM line.

-> With a pre-fl fuel rail this is mandatory, and you will need to make a custom only by yourself.

• I should install the fuel filter from a pre FL (without FPR inside)

-> If you put a pre-fl fuel rail yes as you will have an FPR on your fuel rail.

• Should I change the fuel pump and go to a 5 bar or the OEM is enough ?

-> OEM is more than enough to go to at least 260hp