Mercedes-Benz Forum banner

1 - 6 of 6 Posts

·
Registered
450sl 1975
Joined
·
3 Posts
Discussion Starter #1
Hi, I bought a 1975 MB 450sl. looks original, like if nobody do anything on it, mechanical speaking. At the begin the car didn't start, now does but die after a few seconds. I replaced the fuel pump, the injectors, fuel filter, new battery, hoses, sparks, wires, the triggers ..I do not know what to do. The car has 9 injectors, one is for the ignition only, am I right? that is the reason than the car start very well, beautiful sound, but after 3 seconds, the car die. I took a little bottle with gasoline and I put directly on the carburetor throat and the car keep on...if I stop to put gasoline in the carburetor the car turns off. I also removed the injectors, well the line with the injectors, and test it out of place, but normally connected on the car, i turn on the car and the injectors work, it has a good amount of gas for each injector. also I connected the fuel pump directly to the battery to avoid a possible stop of the pump, but the car die anyway.

How can I do??

thanks
 

·
Premium Member
1975 450SL
Joined
·
2,440 Posts
Check if you are getting fuel to the injector rails. You may need a new fuel filter. There may also be sediment in the tank blocking the filter screen.

Check the dwell angle of the points. That was my personal demon for almost a year.

Just a couple of ideas on where to start. Please keep us informed on your progress, we'll be happy to help.

Scott
 

·
Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
Joined
·
10,338 Posts
At the begin the car didn't start, now does but die after a few seconds. I replaced the fuel pump, the injectors, fuel filter, new battery, hoses, sparks, wires, the triggers ..I do not know what to do. The car has 9 injectors, one is for the ignition only, am I right?
It has 8 injectors plus a cold start valve.

that is the reason than the car start very well, beautiful sound, but after 3 seconds, the car die. I took a little bottle with gasoline and I put directly on the carburetor throat and the car keep on...if I stop to put gasoline in the carburetor the car turns off. I also removed the injectors, well the line with the injectors, and test it out of place, but normally connected on the car, i turn on the car and the injectors work, it has a good amount of gas for each injector. also I connected the fuel pump directly to the battery to avoid a possible stop of the pump, but the car die anyway.
It does sound like you have a fuel supply problem, but you have done the right tests and they say that you do have fuel. You must have spark too otherwise engine wouldn't run.

Maybe check the pressure in the fuel rail (the piping that connects to the injectors). It should be 28-30 psig.

Also do a flow test - measure the flow that returns to the tank after the Fuel Pressure regulator and damper (at the back of the engine) The flow should be 1/2litre per 30sec.

If you get gas out of the injectors when just spinning the motor with the starter but don't get any when you try and start the engine, then maybe the ECU is not telling the injectors to open. This could be a faulty ECU, but it could be that the Throttle Position Switch is causing the fuel cutoff. Try starting the car with the plug on the throttle position switch disconnected (That is the one on the throttle body that you called carburetor ;) ). If it then starts, then we go from there!

Good Luck.
 

·
Registered
450sl 1975
Joined
·
3 Posts
Discussion Starter #4
Hi, thanks for the tips and information.. :) I will follow every instruction.
I will let you know what happens ;)
 

·
Registered
450sl 1975
Joined
·
3 Posts
Discussion Starter #5
Found a way to keep the car on

Hi, I try to starting the car with the plug on the throttle position switch disconnected and didn't turn on. I measure the pressure in the fuel rail and is 30 psi. I didn't do the flow test, because the car turn of after 5 seconds...I don't have the 30. I also did another thing..I use a pliers and to 1 cent coins. I put each coin on the return hose..after the damper (when you told me to measure the flow that returns to the tank) I put one coin up and the other down, the I put pressure with the pliers on the coins trying to close the hose...I close it a lot, but I didn't and I leave a little space to let the fuel returns..I start the car and boom....didn't die!! I also press the fuel pedal and the car responded great.
then I remove the pliers and release the hose. I notice the damper has a bolt to regulate something..I mark it and move it and turn on the car each time I move it but nothing happens, I move the bolt in both sides all the way, but nothing...I did that because I believed maybe the regulation on the damper was the problem, but nothing happens, anyway I never replace the damper..I only replace the fuel regulator with an original OEM that I bought at the dealer.
I will appreciate an opinion and an advise.
thank you
 

·
Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
Joined
·
10,338 Posts
Hi, I try to starting the car with the plug on the throttle position switch disconnected and didn't turn on. I measure the pressure in the fuel rail and is 30 psi. I didn't do the flow test, because the car turn of after 5 seconds...I don't have the 30. I also did another thing..I use a pliers and to 1 cent coins. I put each coin on the return hose..after the damper (when you told me to measure the flow that returns to the tank) I put one coin up and the other down, the I put pressure with the pliers on the coins trying to close the hose...I close it a lot, but I didn't and I leave a little space to let the fuel returns..I start the car and boom....didn't die!! I also press the fuel pedal and the car responded great.
then I remove the pliers and release the hose. I notice the damper has a bolt to regulate something..I mark it and move it and turn on the car each time I move it but nothing happens, I move the bolt in both sides all the way, but nothing...I did that because I believed maybe the regulation on the damper was the problem, but nothing happens, anyway I never replace the damper..I only replace the fuel regulator with an original OEM that I bought at the dealer.
I will appreciate an opinion and an advise.
thank you
Sounds like the fuel pressure regulator may be leaking badly. The pressure should stay up at 30psig and only slowly drop to about 20 psig after 1/2 hr or more after you turn key off. But if it is a new regulator, it should be OK, so maybe that is not the problem.

The pressure regulator bolt is used for adjusting fuel pressure, but the damper doesn't do much - just reduces pressure fluctuations. No need to adjust damper. Just to make sure you have right one, here is pic of my engine with them marked:


Perhaps your pump is not delivering enough flow when the engine is running. When engine is off, there is enough flow back to tank through regulator for it to maintain 30psig. But as soon as injectors start firing, there is not enough return flow through regulator and pressure drops.

You might try clamping the line to the start-up valve (9th injector!) - If pressure then holds, then that is what is leaking.

I think you said you bought an aftermarket pump. You should do the flow test and see what you get (test pump with pump running but engine off).

If flow is low, try and get a proper pump for this car. You may be able to buy a rebuilt pump. Or maybe go back and see if you can use the original pump. They can be cleaned and got working again. See this link and especially post #22 that explains why the OE pump is needed.

One other thought - Maybe if flow is low, it is because fuel filter or screen in tank has plugged up. If there was dirt in the tank, even a new filter could plug up.
 
1 - 6 of 6 Posts
Top