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Marty
Marty, ASE Certified Technician
Category: Mercedes
Satisfied Customers: 53168
Experience:  Bosch Certified Master Technician with over 35 years experience in the automotive industry.
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Hi, I have a 1989 Mercedes 300 se which stopped and refused

Customer Question

Hi, I have a 1989 Mercedes 300 se which stopped and refused to start. I have replaced the fuel pump relay, and now the fuel pump distribution unit. The petrol pump is working fine but no fuel getting to the plug. good pressure at the distribution unit inlet, spark is fine. What is the significance of the black electronic box on the distribution unit and is it connected to the ECU. i HAVE NO HAIR SO i CANT TEAR IT OUT
Submitted: 3 years ago.
Category: Mercedes
Expert:  Marty replied 3 years ago.

eurotec :

Hi, and welcome. My name is Marty. The black box is the electro hydraulic actuator. It is basically a fuel pressure regulator. It will rarely ever cause the car to quit running and not start. I would suggest that you remove the air cleaner and try pouring a little fuel into the intake or using a little starting spray and see if the motor will fire momentarily. Let me know what happen.

JACUSTOMER-wgvaa23q- :

This enlightning piece of advice has cost me €32. is this a joke.I have already told you the spark plugs were working fine. The air cleaner has been off the car for 6 weeks. We have tried all the simple things. Can I get my money back? I cant believe I fell for this scam

eurotec :

Hi, I assure you that this is not a scam. I just need to know some things before I can help you.

eurotec :

I need to know if the motor will run on ether or when you add fuel to the intake.

JACUSTOMER-wgvaa23q- :

I cant check until tomorrow. Can I come back to you. I am beginning to think it might be the ECU

eurotec :

It's not the ECU. The car will run with the ECU removed from the car.

eurotec :

I'll be glad to help you but you need to calm down.

eurotec :

I'm here to help and to have you jump all over me does not help.

JACUSTOMER-wgvaa23q- :

Sorry, it is the first time I have used this site. I am quite sceptical of sites I dont know, possibly paranoid but quite calm. I will check in the morning and come back.

eurotec :

I've been working on Mercedes for 35 years. I'm not here to scam you I'm here to help. Let me know what you find out and we'll go from there. If it will fire on ether or fuel in the intake we will want to bypass the fuel pump relay and do a flow test on the injectors. I'll explain how to do that after you tell me the outcome of the test.

JACUSTOMER-wgvaa23q- :

Ok Thanks.

eurotec :

You're quite welcome.

JACUSTOMER-wgvaa23q- :

Hi Marty,

JACUSTOMER-wgvaa23q- :

We have tried putting ether into the intake but it makes no attempt to fire.There is good pressure on the fuel intake . There is also fuel in the return pipe. If you open the stud on the opposite side of the distribution head there is fuel coming out under high pressure but if you take the plugs out they are dry. there is a good spark.The fuel seems to be blocked somewhere between the distribution head and the combustion chamber

eurotec :

Good morning, the reason that I wanted you to perform this test is because it is very common for the distributor cap and rotor to short out and cause this problem. Even though you have spark the spark will be random and jumping around inside the cap. When you pour a little fuel into the intake the motor should at least try to fire for a second or two. At this point you can either replace the cap and rotor or pursue the fuel issue. In my opinion I think the cap and rotor may be the way to go first. My other thought is that you have a large vacuum leak below the air flow meter. There is a large boot that can split and there are some hoses that run from the idle control valve to the intake that can split and give you a large enough leak that you won't pull the air flow sensor plate down when cranking the motor. You could try manually pushing down on the plate and see if it tries to fire. To perform the nozzle test you need to remove the fuel pump relay and install a fused jumper lead from terminal 7 to 8 (30 & 87) in the relay connector so the fuel pump runs continuously. Next you can press all the way down on the air flow sensor plate and listen for the sound of fuel screeching through the injectors. If you hear them screeching you know you are getting fuel to the cylinders. If you don't hear anything at all then I would remove one injector from the motor and reconnect it to the metal fuel line outside of the motor and perform the test again. You will need to bend the fuel line a little to reconnect the injector. If you get no fuel we will need to do some checks at the fuel distributor but I have a hunch you will have fuel and the problem is either the cap and rotor or an intake leak below the air flow meter.

eurotec :

Full Size Image

eurotec :

Let me know what you find out. I'll be away from my computer until about 9 AM EST. I'll reply back asap.

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