Having your car break down—especially one you or your family are driving—is one of the worst problems you can face in life. I don't know if some people are immune to this feeling, but for me, it is an unbearable frustration, particularly if it happens while you are with your family or in a remote location.

The protagonist of our story today is a Mercedes equipped with the cursed PZEV (Partial Zero Emission Vehicle) design. I will explain this as a story because it was a struggle that lasted for months.

Initially, there were times when the car would cut out and wouldn't restart until a certain period had passed, usually taking 10 to 30 minutes. This happened especially during travel. When scanning the car with a computer, everything appeared normal, except for an error indicating that the fuel pump was not functioning correctly.



I tried to explain the situation to the mechanics to verify any fault other than the pump. Accessing and removing the pump for inspection was impossible because the tank is a single sealed steel piece (the PZEV design). Furthermore, since fuel quality in our country can be poor, everyone immediately blamed the fuel pump.



I didn't initially swap the tank from the steel PZEV to the plastic non-PZEV version because it is expensive and requires modifications to fit. The issue was rare at first, but recently it escalated. The car started cutting out in the middle of the road or on long highways, which could have caused serious accidents. The waiting time for it to restart increased from 10-30 minutes to an hour, two hours, or even a full day.


Due to a lack of time and the high prices at the dealership, I delayed the repair. Eventually, we decided to fix it. We bought a used plastic tank from a parts yard for about $200 instead of a new one. The mechanic who installed it didn't inspect or clean it because it looked shiny from the outside. This was the first trap.


After installation, the car returned to work, and we were overjoyed. Unfortunately, it only lasted a few hours before the same problem returned: engine stuttering, shaking, and then shutting off.

I then went to one of the best mechanics in the area. We decided to move the car to him. He opened the tank, and we discovered the first trap: the tank was full of water and dirt that hadn't been cleaned. We bore the cost of cleaning it, replaced the pump, and changed the pump seal rubber as it was worn out.

We started the car, and it worked. However, as soon as we left the workshop, the problem returned. This was the second trap.



The mechanic checked the electricity going to the pump and discovered that no current was reaching it. He checked the fuse box base, but when he pulled the relay and used a jumper wire (direct connection), electricity reached the pump, and it worked perfectly. This was promising, and the next target was to repair the fuse box base.

The next day, upon thoroughly checking the base and the current before it entered, we discovered the root cause: the ECU (Engine Control Unit) was not providing the ground signal for the pump to operate, even though the fuse base was fine. The funny thing is, when we tapped on the ECU, the car started!

At that moment, we realized the previous PZEV tank was likely fine and the problem was never the tank, but rather the ECU. We replaced the ECU with another one, programmed it, and the "beast" returned to work more efficiently than before.


Conclusion & Advice:

Before jumping to conclusions, follow proper diagnostic steps and tests. It is preferable to go to an experienced mechanic who has the time to dedicate to your car, as many rush their judgments.


Follow these steps:

1. Check the Power: If the tank is a PZEV type, check the incoming electricity, especially when the car cuts out. If power is reaching it, check if fuel is reaching the engine. If fuel isn't reaching the engine, the problem is the pump (this means you need a new tank, preferably the plastic non-PZEV type for future ease).
   
2. Check the Relay: In case of weak current, check the relay or the ignition switch and ensure there are no power distribution issues.
   
3. Check the Fuse Box: If there is no current, review the previous step or move to the fuse box base and ensure it is functioning.
   
4. Check the ECU: If the problem persists, move to the power input coming from the ECU. If the issue is with the positive current, check the battery and its connections. If the issue is with the ground, the problem is the ECU. You will need to repair it if possible or replace it.
   

The solution might be right in front of you from the start, but haste, jumping to conclusions, or lack of experience can hide it. I hope mechanics listen to their customers—while some customers may not know much, mechanics should still do their duty perfectly or delegate it to someone capable.



I apologize for the length, but I wanted you to benefit from this experience.



Best regards.

Thank you for sharing the details. Even if we don't have the same exact problem or symptoms, this is a good lesson that we need to troubleshoot all the way to the root cause instead of guessing and spending a lot of time and money on the wrong solution.