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1626 Views 23 Replies 4 Participants Last post by  esean19
Came out of the store and no start. Kept cranking with no sign of firing up. Towed home. Replaced fuel pump relay, checked fuses, no luck. Removed air filter housing, poured some gas into air intake and it fired up and running for few seconds smoothly. Repeated several times and fired up every time running normally then died. Concluded not getting fuel. Ordered and replaced both fuel pump and fuel filter breaking my back with no results. No codes. No history of not firing up. Have had the car for about 5 years. 160K miles. Both batteries new. I thought because it fires up when I pour gas in the intake it shouldn't be any of the sensors or the key not communicating. What else should I look into? I don't have any advance tools or software just a basic MB code reader. If aware of any previous discussions here, please guide me to. Please help. Thank you.
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There is at least one fuel cap on the injector rails. Slightly loosen - do not remove - the cap, then turn on the ignition very, very briefly. If the fuel pump works, you will be leaking or spraying gasoline into the engine compartment, which is very dangerous. A quarter of a second is plenty of time to see if fuel is being delivered to the rails.
There is a Schrader valve on the fuel rail. Loosening the cap does no more than loosening the cap on your tire stem.

Take the cap off and turn on the ignition. With a rag in place, press a small screwdriver on the valve button, just like when you release air from a tire. Don't hold it down or you'll hose yourself and the engine bay with gasoline, and you had better get out of there in a hurry.

I am only suggesting this because I strongly suspect your fuel pump is dead and there is no pressure on the fuel rail. You should hear the fuel pump whirring when you turn on the ignition.

Those fuel pumps don't last forever. Fortunately, changing it, while laborious because you have to remove the seats, is not that big of a job. Just keep a fire extinguisher at the ready and work in a well ventilated area.
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Oopsie - sorry, the previous comments were a bit of a mess. I was hung up on the fuel pressure stuff.

Since I feel bad for telling you stuff you already know, I looked at the wiring diagram.

On the activation voltage for the relay: Are you measuring the voltage across the relay coil terminals? Or each relative to ground?

Pin 85 on the relay goes to the ECU. Pin 86 gets voltage (12V?) via fuse 105 (15A), which looks like it supplies multiple items. So check to see if, when you have ignition on, you have voltage to ground at 86. I think these are 12V relays but have not confirmed. Pin 85 should be 0V to ground.

The ECU completes the circuit, presumably to ground. Be very careful about providing too much current to the ECU; there is a 15A fuse on that line, but I bet the chips on the ECU won't be happy about carrying 15 amps. MB is not good about protecting the electrical system. Be careful that you are only using the meter in volts mode, or you risk passing through current. When you hook up the jumper to bypass the relay, be super careful to not connect it to the activation terminals!

So this is possibly as simple as fuse 105 being blown or resistive.

Are you comfortable using a meter? If so, you should test pin 86 to ground (should be 12V, probably) with the ignition on; should be 0V with the ignition off. Pin 85 to ground should be zero with the ignition on; pin 85 should have continuity to ground with the ignition on and no continuity with the ignition off. If you are not comfortable with these terms, speak up.

One odd aspect to MB electrical is they like to complete circuits to ground, not energize the supply.
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Also, don't sweat replacing the fuel pump. Likely it had already consumed most of its lifetime. Consider it preventive maintenance.
Fuse 105 is in the engine compartment fuse block.
Relay coil F4/kO/86 gets power from fuse F58f105. Ground comes from ME N3/10 to 85.
"ME" = Motor Electronics, aka ME9.7, aka the Bosch computer with heat sink fins sitting top center on the motor.
I have already checked fuse 105
Curious how you knew to check 105.

At this point I don't know which of pins 1 and 2 of the relay is circuit 85 and which one is 86.
I think they are numbered on the fuse-and-relay block
fuse 57 ... the pump came up ... By the way the day that this happened we had a long unusual raoning for few days. I think I know where from the water gets in. Will seal soon. 🙏🙏🙏🙏🙏
Is 57 the fuse on the line supplied to the pump? I am troubled because if that fuse was the issue, your bypassing the relay should not have solved the problem.

The next step in your troubleshooting would have been to check the functioning of the relay. They do fail.

It is worth it to patiently do diagnosis. Your fuel pump, costly as it was to your back, is a minor expenditure in the scheme of things. I wasted about $1300 on an ECU and throttle body before I figured out what the root cause was, and if I had been more methodical I would have avoided that pain. When you go to the doctor, they (should) do a bunch of tests before replacing parts.

Water intrusion is a terrible weak spot on the GL, rather ironic for a supposed all-terrain vehicle. It penetrates at the roof rack mounts, the rear pana roof, the sunroof, and the taillight lenses. If you are in a rainy climate and store the vehicle outside, you are living on borrowed time ... as are we all 🙏
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