A mystery to be solved after many years of ownership.

My '99 CLK Cabriolet with 86,000 original miles, purchased new by me, has developed a strange issue. Everytime the fuel level in the gas tank drops below one-half, the Check Engine light goes on. I fill the tank, drive a few miles, and the light turns itself off. Car is operating beautifully and I enjoy the car each summer since buying it back in 1999. This has been happening all summer long since I took it out of winter storage back in May. Now, with another winter's storage coming up, I thought I'd post it here to see if the collective has any thoughts, ideas, or suggestions as to what the issue may be.

Many thanks in advance for your time.

 

I don't mean to be rude but you should definitely scan your car. You can buy a bluetooth OBD-II reader that connects to your phone for about $10, or, as far as I know, you can still go to Autozone to have them scan your codes.

I would recommend recording what the errors are and reporting back here.

 

#1 - buy a scanner for ~$50 like the Bosch OBD 1150 and let us know what codes are present

#2 - that being said, your fuel filter probably needs replaced (50,000 miles) and fuel pump may have gone bad by now. It could also just be a bad seal on your gas cap and you’ll need to buy a whole new cap. These are just the primary failure items related to the gas tank, and could be affected by the lower fluid pressure in the tank.

 

Just an update for now. My trusted Mercedes mechanic with 35 years experience tells me that he suspects the problem I described could be the fuel sending unit. He surmises that the pressure of a full tank of gas keeps the sending unit working properly, but when the fuel level drops to one-half, the pressure on the fuel sending unit is reduced to the point where it throws a code since it THINKS the pressure on the fuel pump and fuel lines has dropped to an unacceptable level, despite the reality that the engine continues to receive sufficient levels of fuel to operate properly. He tells me he has seen the problem before on Mercedes 3.2 L V-6 engines.

By the way, he also said it's NOT an easy fix to drop the fuel tank, pull the entire fuel pump/fuel sending unit (since it is all one unit) and that the entire unit needs to be replaced (rather than just the fuel sending unit).

Does any of this sound familiar? Opinions would be appreciated. Do I just continue enjoying the car seasonally by keeping the fuel level above one-half and call it a day? Or, swallow hard and spend the money to get this taken car of? Again, the car is a long-term keeper for me.

Many thanks, in advance.

 

You should really get your codes read, otherwise you are another person willing to throw money at a problem when they don't have clear data; but you showed up here, so lets figure it out. I would also say you've mostly answered your own question and that it is not worth it -- *if it in fact* the problem requires the tank being pulled. However, is that really the case? or even the fix?

Whereas the light turns itself off and does not require resetting your OBD system, you've essentially got a "HALF-FULL" light on your car instead of an on-empty light. Some people I know wait until their car is on empty to fill it; other people I know fill it around half full. Then it's also fair to say you can easily get 100+ miles after the light comes on... so it's not to say you *have to* fill the tank at half full, because you don't actually know how much gas you have, but instead your car alerts you once you're on the latter half.

And I should clarify -- does it read accurately below one half or just get stuck at one half with the CEL on?

I think I may also disagree with your mechanic.

There are a LOT of thoughts I have:

If you ignore the problem, you may actually be ignoring a bigger problem, and this 'problem' is actually just a symptom of that bigger problem. Which is potentially not the sender unit and or fuel pump, but the fuel system, and ignoring it would be to accept the causation of future issues.

Have you ever changed your fuel filter? It should have been done around 50k.

If not, I would recommend doing the fuel filter, and buying Chevron Techron and running that appropriately.

I have read of people having issues who have not changed their fuel filter, and who have had success running fuel cleaner through their engine; this however, is an act of masking the problem which is that the fuel filter itself needs to be changed. And you are stressing your fuel pump when the system is dirty and prepping it for unneccessary failure. If you have not done your fuel filter, I highly recommend changing it out(it's not a big job) and appropriately running fuel cleaner through your car. Please do post the codes if you can, so that we can help you. Most auto parts stores should be willing to give your car a scan.

The thing I am least confident about making claims about -- but also a bit skeptical about -- is that the fuel sender unit and pump are one unit, and or that this is the problem. There may be two fuel pumps, one of which is 100% absolutely external and not part of the sender unit, and one of which, if it exists -- is in the tank. But the commonly serviced fuel pump is external, which is typical, and is located right next to the fuel filter, which is also typical. A clog in the fuel system can cause pressurization issues, period.

I have a friend who was a brand-specific mechanic for a solid 10-12 years. I purchased a car, had him do a job on it, and he repeatedly berated me telling me I was an idiot for buying the car and saying the rear differential was shot. He was so confident, I was so stupid, I know nothing. Actually, there was just a bubble on the inner sidewalls of one of the tires. Mechanics tend to know more about cars than non-mechanics -- and anyone else in society. People go to them in desperation and are willing to shell out tons of money. And they have to get stuff done quickly to make money because their employers take the majority of it. This is a recipe for ego-maniacism.