[esean19]

Hi,
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.

 

[wallyp]

If you have a mechanic's stethoscope, put it on, hold the sensing rod on a fuel injector, and have a helper try to crank the engine. You should hear the injector clicking - no clicking no fuel, no run.

If you don't have a stethoscope, you can try it by holding a metal or wood rod against the injector and your best ear, but a mechanic's stethoscope is $20, and will be useful many times. https://www.amazon.com/Lisle-52500-...&hvlocphy=&hvtargid=pla-4584138871914356&th=1

 

[esean19]

Appreciate your quick response Wally. I do have a stethoscope but no helper. I sure can find somebody to help. Meanwhile, what will make all injectors stop working? And when they do, do they totally block the fuel? There is zero sign of fuel getting into the engine. That's why I rulled out bothering checking fuel pressure at the engine. I will defenitely follow your advice as soon as I can get some help. I though still think the pump is not coming up. I don't hear the initial humming sound when turning the key. Is there anything else that commands the fuel pump? Like any security feature? Thanks again.

 

[wallyp]

There are fuses and relays involved, both for the pump and for the injectors. There should be a chart for the fuses and relays in your truck - if not, search using Duck-Duck-Go, Google, or your favored search engine to find the information.

The fuel pump just supplies fuel at 55psig to the injector rails. Absolutely no fuel enters the intake or the combustion chambers other than by being injected by computer-controlled fuel injectors.

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.

 

[esean19]

Thanks again Wally. I have spotted and checked the fuse and relay for the fuel pump. I did not know there is a fuse or relay for the injectors. I will look it up. I will do the recommended test to see if any fuel will come out at that port. Thanks a million.

 

[esean19]

Hi Wally,
Checked the fuel at the little cap in engine comoartment while a helper cranked the engine. Zero fuel came out (center needle pressed in). Not even a drop. Also listened to the injectors and defenitely has the clicking sound. Both my helper and I trief and confurmed they are clicking. Also I had him totally close to top of the fuel pump and no humming noise when turning the key. So I beieve there is no power to the pump. I have not checked the voltage at fuse box yet. The fuel pump fuse looks good and the relay is new. I swaped three relays to make sure. Unfortunately the car nor the manual does not have the fuse/relay chart. I get different charts from internet for my make and year model. Don't know what else to check. Thank you.

 

[wallyp]

The correct fuse/relay chart should be in the tool kit in the spare tire well.

 

[Witek_M]

Hi Wally,
Checked the fuel at the little cap in engine comoartment while a helper cranked the engine. Zero fuel came out (center needle pressed in). Not even a drop. Also listened to the injectors and defenitely has the clicking sound. Both my helper and I trief and confurmed they are clicking. Also I had him totally close to top of the fuel pump and no humming noise when turning the key. So I beieve there is no power to the pump. I have not checked the voltage at fuse box yet. The fuel pump fuse looks good and the relay is new. I swaped three relays to make sure. Unfortunately the car nor the manual does not have the fuse/relay chart. I get different charts from internet for my make and year model. Don't know what else to check. Thank you.

Do you have volt meter, test light, fabricated jumper for relays?

 

[esean19]

Thanks again Wally. I have spotted and checked the fuse and relay for the fuel pump. I did not know there is a fuse or relay for the injectors. I will look it up. I will do the recommended test to see if any fuel will come out at that port. Thanks a million.

 

[esean19]

Yes I have a voltmeter and a MBII scanner. I bypassed the fuel pump relay and the pump started running. Then I was able to start the car and took it for a very short drive. Tested the solenoid points on the fuse box for the relay and there was no voltage. One was exact zero volt and the other showed a very low voltage in milivolts. So I think I have a good ground but nothing from ECU. If that sounds correct, is there any other fuse between ECU and the fuel pump relay. Need to find the wiring diagram. Read on another old post that a signal from the key is also required to complete the command. Unfirtunately I have one key only. Now that the engine was running I hope I'll get some codes. Thank you all.

 

[Steven Keaton]

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.

 

[esean19]

Thank you Steve for your input. Seems like you did not read the previous comments. I have already done what you suggested. I have replaced the pump and the filter which I shouldn't have. Got the pump runinig with bypassing the relay. The issue is that ECU is not communicating with the relay.
Thank you though for your input. Truely aporeciate you taking the time and trying to help.

 

[Steven Keaton]

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.

 

[Steven Keaton]

Also, don't sweat replacing the fuel pump. Likely it had already consumed most of its lifetime. Consider it preventive maintenance.

 

[Steven Keaton]

Fuse 105 is in the engine compartment fuse block.

 

[esean19]

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.

Thanks again Steve. I have already checked fuse 105 and even swapped with a similar one last night. If I'm not mistaken I had 12v plus on both sides. I will check again and go thru all the steps you've suggested. Also I am comfortable with the terms that you use. I am an electrical engineer close to retirement 😔. 🙏

 

[esean19]

Thanks again Steve. I have already checked fuse 105 and even swapped with a similar one last night. If I'm not mistaken I had 12v plus on both sides. I will check again and go thru all the steps you've suggested. Also I am comfortable with the terms that you use. I am an electrical engineer close to retirement 😔. 🙏

The voltages mentioned in my previous post were between the relay coil pin 1 and ground and between pin 2 of the relay and the ground. One was. Absolute 0V and the other just few milivolts. Pin 3 and 4 were connected with the jumper which turned the pump on. At this point I don't know which of pins 1 and 2 of the relay is circuit 85 and which one is 86.

 

[Witek_M]

The voltages mentioned in my previous post were between the relay coil pin 1 and ground and between pin 2 of the relay and the ground. One was. Absolute 0V and the other just few milivolts. Pin 3 and 4 were connected with the jumper which turned the pump on. At this point I don't know which of pins 1 and 2 of the relay is circuit 85 and which one is 86.

Here is a snippet of fuel pump activation circuit.
Relay coil F4/kO/86 gets power from fuse F58f105. Ground comes from ME N3/10 to 85.

 

[esean19]

Thanks again Steve. I have already checked fuse 105 and even swapped with a similar one last night. If I'm not mistaken I had 12v plus on both sides. I will check again and go thru all the steps you've suggested. Also I am comfortable with the terms that you use. I am an electrical engineer close to retirement 😔. 🙏

So I got to the fuse box in the luggage compartment again to check the voltages at the fuel pump relay as advised. Before doing that, took the fuse 57 out which I believe I didn't before mistakenly took out fuse 50 thinking it was 57 and even replaced it. It was dark that night. Then put it back. I gave it a try and the pump came up. I tried few more times and took the car for a ride. Everything seems to be normal. When at the fuse box I reached to the back of the box and the insulation foam there felt a bit wet and it was touching the back of the fuse box. However didn't feel like there was any connectors there. I pushed the foam away anyways. By the way the day that this happened we had a long unusual raoning for few days. The contacts on the fuse looked clean and shiny. So mixed feelings here. I am glad the car is running. I don't know how I can trust the car when I don't know where the problem really was. I feel bad for the wrong sequence of my trouble shooting and hurting my back and my wallet replacing the fuel pump and the filter for no justified reason. And above all I feel great for the support I received here from each and every one of you. Thank you all for your time and your priceless advice and guidance. I think I know where from the water gets in. Will seal soon. 🙏🙏🙏🙏🙏

 

[Steven Keaton]

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.