Full Tank of Premium and Guage reads "Empty" - PeachParts Mercedes-Benz Forum

#1 05-10-2005, 10:48 AM

I did use the search function and discovered many issues similer but not exactly like mine. As stated i filled up my tank and while driving noticed the fuel indicator move from full to empty gradually over 2-7 minutes. When i parked the car, left it for about 20 minutes and restarted i had a full tank again with the same process bringing me to empty. Any feedback would be greatly appreciated. Posted this in the Tech forum as well

Thanks in advance gentleman

2001 ML55

mbdoc · #2 05-10-2005, 11:09 AM

The fuel tank sending unit is part of the fuel pump assembly.
It can be tested at the wiring harness going to that assembly.
Resistance in OHMs is 200 empty & 1 full, of course +- 2 ohms at either end.

#3 05-10-2005, 11:19 AM

Can i purchase the sending unit separatly, perhaps a DIY?

Where is the access to the unit located

Tks Doc

#4 05-10-2005, 12:01 PM

MB DOC - Do you know Hanz over at Helms, Bayside? Good Fella!

ILUVMILS · #5 05-10-2005, 12:19 PM

Depending on your VIN# the instrument cluster may need to have some of the version coding changed. MB issued a bulletin about this some time ago. Any dealer should be able to tell you if you're a candidate for this fix.

Bill Wood · #6 05-10-2005, 07:05 PM

Quote:

Originally Posted by SilentMethod

Can i purchase the sending unit separatly, perhaps a DIY?

Where is the access to the unit located

Tks Doc

It is under the back seat. Inside the fuel tank. Not an easy fix for a DIY'er.

#7 05-11-2005, 10:10 AM

I opened up the gas cap, and let the car run for a minute or two and the needle went zooming up to full again. Wifey is driving the 55 today so i dont know if it permanantly corrected the problem. Feedback appreciated. Can it be a gas cap issue or lack of pressure?

#8 05-20-2005, 01:46 PM

Problem Fixed - It was the gas cap. Apparently theirs a rubber strip that runs the interior that completes an air tight seal. Changed the cap and problem fixed.

Thanks all with the help

blueeagle289 · #9 05-21-2005, 07:08 AM

I guess it really doesn't matter in the end, but there are certain words we all use in reference to car problems -- that are quite often misspelled .....

One of those is "GAUGE" .... A lot of people spell it "guage", which just ain't right !!! There are many others I see in the posts; some are typos and some are just wrong and I guess if you are not used to using words like "gauge" and "squealing" and many others of that type, it is easy to spell them incorrectly. Maybe we should have a car talk dictionary that people can refer to ??? (I am sure this post will draw a lot of semi-heated comments, indicating that the important thing is getting the message across, but why not write it right??)

Gilly · #10 05-24-2005, 08:14 PM

As long as the message is conveyed, we don't need to pick on typos too much here. "Gauges" of course gets spelled incorrectly alot thanks to GM, who labels them "Gages" right on the car itself!
SilentMethod, do a search on "virtual calculation" to come up with a post regarding inaccurate ML fuel level readings. The observation that the problem was caused by the fuel cap; no way.

Gilly

#11 05-26-2005, 01:59 PM

Gilly - I still have the old gas cap! Your more than welcome to inspect it if you like. I'll cover shipping

Howz mi spelin?

#12 05-26-2005, 02:18 PM

Gilly - I read your post regarding the functionality in question. I don’t believe it addresses my particular issue. As for the solution to my problem, it came directly from the shop manager from L.I. Benz. I told him what the symptoms were, and he told to get a new gas gap. Et Viola

The rubber gasket on the old gas cap was seriously worn and damaged. Why do you feel this has no impact on the air pressure inside the tank?

With all due respect Gilly,

Ben, how am i doing?

Gilly · #13 05-26-2005, 07:30 PM

OK, to expand on this a bit:
Yes the pressure of the air inside the tank will be affected by a bad seal on the gas cap, or a missing one, etc. This is monitored by a pressure sensor which is integral to the fuel level sensor. You also have a level sensor as part of the pump assembly (actually on an ML it's all in one, pump, level sensor and pressure sensor). If you have a leaky cap, a few things will happen, all OBDII related. The first warning sign, say in a real bad leak, like the cap is left off after refuelling, is the low fuel warning lamp flashes. Next you will get a check engine mil. The engine control unit tests the evaporative emission system for gross (large) leaks and also minor ones. A missing cap, or real bad seal, will set a mil for a gross leak. The feds don't want all those hydrocarbons leaking into the air, this is the purpose of OBDII, and why for example in California they require those special recovery nozzles for refuelling.
I would maintain that the function of this monitoring system, or the failure of the fuel tank cap, would have no bearing at all on the operation of the fuel level sensor. Pressure or no pressure in the air space above the fuel won't affect the level or the operation of the sensor. The fuel itself will be at the same level as fuel, being a liquid, cannot be compressed. Further, if the cap really was bad (which I won't debate whether it is or not) you should have had a check engine light, I don't recall you stating the light was on, in reviewing your first post you don't state that it was on.

Gilly

Gilly · #14 05-26-2005, 07:37 PM

The programming change that ILUVMILS mentioned is one where the fuel level sensor ALWAYS reflacts what the info is that's being sent to the instrument cluster. As it is now, it (believe it or not) does NOT. It starts off when the tank is full, reading in this value, but after a few gallons the gauge/gage/guage reading is actually what is referred to as a "virtual calculation", which means it lowers the fuel gauge needle based on the amount of fuel which has been calculated by the engine control module to have been used. I certain circumstances this system can go, shall we say "way out of whack" and display incorrect amounts. Seems the biggest problem is when owners continually top off their tanks before the tank is lowered that first few gallons before "virtual calculation" has begun. So in short if you top off your ML alot, where it only takes a few gallons, this could be the result. But the dealer can turn off this virtual calculation feature so the gauge just shows what the level sensor is reporting and that's it. Just like a normal gage.

Gilly

supradupe · #15 05-26-2005, 10:55 PM

Gilly, does this apply to all model year for the ML? I am having the same situation as you described. The fuel gauge needle drops too soon. I asked the dealer to not using the virtual calculation and they think I am making things up. btw, I have a 98ML and dealer replaced my fuel pump and sending unit last year. the fuel gauge started doing this after the replacement. I brought it back twice and they ended up replacing the instrument cluster, still didn't help. I kind of gave up but your messge got me fired up again...