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Fuel Flow Measurement

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If I disconnect a fuel line going to a NEW injector and crank the starter, I get a timed "drip" of fuel. Seems like it should have more force. So I refer to the Haynes Repair Manual on page 77-79 for a diagram of the Exhaust Emission control for a Fuel Injected engine with Automatic transmission. The discussion here is about a Fuel Cut-off Solenoid - the bottom one on the rear of the Injection Pump. The discussion of this system is also covered in the Service Manual on page 07-14/9 which covers the testing of the Starting Device. So here is the question - should I be reading 12 volts between earth and the green lead on this solenoid when I am cranking the motor? The one kicker here is the statement on paragraph 2 in the Haynes Bible that says that below 62 degrees the system is inoperative. I take this to mean that with a cold engine the solenoid will not see the 12 volts. If that is true, I can disconnect one of the wires going to the Coolant Temperature Switch and forget about this whole circuit. But if I need 12 volts can I just "hot wire" it from the fuse panel and get the darn thing started ?

Frank Mallory says: first, after removing the injector nozzle from the engine and re-connecting it to the injection line, you need to bleed it (i.e., connect it to the line loosely & crank the engine until fuel emerges from the fitting, showing that all of the air is out) before you can expect to get a good squirt from it. Second, the lower solenoid (the only one, on a US 280 SL) should only be activated when the car is in 3rd or 4th gear and coasting (i.e., throttle closed). This is for smog control and has nothing to do with starting - it simply cuts off the pump when the engine doesn't need to be producing power. The top solenoid, if you have one, should operate during cold starting. This solenoid was deleted from later cars, because improved ball check valves made it unnecessary. "Hot wiring" is not a good idea unless you know exactly what you are doing. However, lots of these cars have had a button or switch added by the owner, to give manual control of the cold start valve and solenoid.

Will Samples says: do what you want with the solenoid, but do not leave it wired and hot full time, it will overheat. More importantly, your motor should run just fine WITHOUT it. It is most likely not the problem. To see if it is a fuel problem, spray Starting Fluid into the intake and crank the engine. If it wants to fire, then it is fuel starvation. If it fires but sounds or feels rough or backfires, it is timing. If it does nothing, it is electrical. Be careful with spraying the fluid. It is very hard on the engine when it fires using this fluid. Just use it for testing, not for sustained running. The meager drip out of the injection line is about right. There is no great gush of fuel.

Someone wanted test numbers for the fuel pump by the gas tank. The delivery pressure with the ignition on (minimum terminal voltage at the pump of 11 volts) at a point "behind the fuel filter" should be 0.8 to 1.1 atm. I would check it on each side of the filter just to see what the difference is. Discharge pressure (with a dummy plug on the fuel line) behind the fuel overflow valve on the injection pump should be a minimum of 1.3 atm. The output rate at this same location should be 1 liter in a minimum of 15 seconds. That would be pretty easy to measure. I assume you would need a special tool to measure the pressure in the fuel lines. Hopefully, if you get one, you can convert between psi and atm. This info is from "Service Manual - Maintenance, Tuning, and Unit Replacement - Passenger Cars starting August 1959". It's a great manual and I strongly suggest that anyone with intentions of doing any of their own work purchase one appropriate for their car.

One month ago I wrote you about "vapor lock" problems in my 230 SL (the original message is below). After some suggestions (thanks Bob from Brisbane !), I checked the fuel return line with my mechanic, and we discovered that it was very few fuel returning from the injection pump: there's a valve united to the injection pump (I think 250 and 280 SL have a different design) to adjust the fuel pressure in the system. This valve was nearly closed, and it sent too much pressure to the system: 1,3 - 1,4 kg instead of 0,8 - 1,1 kg. Once adjusted to correct pressure, this valve allow fuel to return to the tank (cooling it) in more or less correct values (about 2,8 liters / minute) - Data book: 3 liters / minute -. With the clogged return line, during low consumption moments, I suppose fuel pump was beating all time the same fuel until it was transformed to vapor. The problem was smaller when engine consumed more fuel. Now I can hear a fuel circulation noise on the return line near to the injection pump (with engine stopped and ignition on). My mechanic told me that function of "Fuel pressure damper" is to reduce this noise (my car has not the "fuel pressure damper" because its engine number is 2164, and this part was introduced after engine number about 3.000).
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