Lift Pump Pressure for OM603

[jim54]

Have a 1987 300TD and can't find a spec that calls out the lift pump pressure for the 603...found several references regarding output pressure for the 617's which appears to be around 15 - 19 psi. One thread discussing spring modifications mentioned 603 output at 30 psi but I wasn't sure if that was after tweaking the spring. At 3000 rpm my pump will push-out around 13-14 psi and if I briefly pinch off the outlet it will go up another 1-2 psi. I've replaced all the 'O' rings in the lift pump and figured before I order a new spring from Germany or a new pump I need to nail down just how far away I am from what the pressure should be. Any help, greatly appreciated.

[Diseasel300]

From what I've been able to nail down, 12-15PSI is what you should expect to see. Higher isn't necessarily better, the overflow valve on the 603 is a fixed orifice style, so the overall system pressure is slightly lower than that of a 617. The lift pump is there to fill the fuel rack with fuel, it doesn't require a huge amount of pressure.

[jim54]

Thanks for the quick response Diseasel, looks like I can take the pump off my list of things to potentially replace.

[Diseasel300]

Unless the delivery valve discs in the pump are cracked up or the cylinder worn so much that the piston doesn't fit solidly anymore, there's not much to go wrong with the lift pumps. If it's pumping fuel and delivering pressure and isn't leaking, let 'er rip!

[barry12345]

Bosh injection pump repair shops may have the specification. During the rebuilding process of the injection pump they do check it I suspect. The injection pump must be calibrated at a specific base fuel pressure .

The lift pumps output has to be well above that as a compensation for the pressure differential across the fuel filters increasing with time especially in service. Plus enabling adaquate fuel flow in higher load situations.

Plus the valves in the lift pump act at preventing fuel backflow when the system is off. Plus retaining some amount of pre established pressure if the system ahead is tight. This is important and never discussed. If using the car in cold winters especially. The quicker enough fuel is injected into the cylinders. The quicker the start.

I do not think there has ever been any mention of the pressure regulating spring in the lift pump fatiguing with time. The relief or overflow valve is at the same time subject to spring fatique with time. There have been many reports of build up on the seats on the lift pump valves creating leakage issues over the years.

[Diseasel300]

The 603 fuel system is vastly superior to that of the 61x series engines. The lift pump has a different design for the valves, their only weakness is cracking with age and will show up as drainback when parked or reduced volume.

There is no pressure regulating spring in the overflow valve on the 603 either, it is just a drilled orifice. Much simpler, but the plastic check valve can crumble or warp with age, again causing drainback issues when the car is parked.

Unless you're getting no return fuel flow, the lift pump should be the last thing you suspect as a problem on the 603, there just really isn't anything to go wrong with them.

[Diesel911]

Here is a site that has the flow and pressure tests for the lift pump.

Model 124 Maintenance Manual Index