Darkmann

2001 E430: In the manual, you are warned not to top off when refuelling (as if anyone really doensn't top off). I can't see how this will hurt anything in the refueling, vapor eecovery or evaporative emmisions systems. Is it just because of possible high pressure "blow out" effect that may douse the filler with fuel? or is there a more technical reason for this warning?

young

heh, i don't top off what's the point? i don't use cash anymore for gas (gotta earn those points) and i don't think getting that extra bit of gas is worth it.

i am curious about possible consequences though of topping off the gas.

Darkmann

I am curious to find out too. I used to design stuff for the gas companies and the main reason for not topping off is that the gas pumps have a "vapor recovery function" for clean air purposes. Basically, a vacuum pump in the pump sucks in the refueling vapors as you refuel, so you don't breathe them. All of CA, all airports, practically all big cities and many many localities mandate these systems. As of a couple years ago, CARB also required that new cars have a system in the vehicle to suck in vapors while refueling - this will cover those areas that don't have it mandated. The recovered vapors go into the reciculative system where they are burnes as fuel. I don't see how topping off will hurt the vehicle or system in any way. However, I can see a build up of negative pressure in a particular vehicle recovery system such that as soon as you pull out the nozzle, a splach of gasoline will shoot out right in your face. There are EPA regulations (long ones too) attached to that scenario. That's the only problem I see with topping off.

Richard

In Europe we dont have the same vapour recovery laws as the USA so we dont have pumps with vapour recovery.

If you top-off, you often get some spillage , and this evaporates from the floor, from where your nice vapour recovery systems wont suck it back out of the atmosphere ..

For us, fuel is just so damn expensive that we make sure we dont spill any

Drew

For the good of the environment ... sounds like a good reason for the 'don't top off' suggestion in the manual.

I 'round-off' when I pump ... is that the same thing?

young

rounding up is what i meant about the cash vs credit card. rounding off w/ a credit card doesn't make any sense. but w/ cash, i can see someone trying to get it so its not 16cents

Drew

I hear ya bro ... for accounting/tax records ... I've always used a credit card ... even back in the old days when you had to walk inside to pay!

Even though it doesn't make sense to you (nor my wife BTW) I have always had this thing about 'un-even' charges ... whether its for gas or dinner, I always want to round up (and with dinner, then figure the tip) ... so at the pump i'm always trying to hit the next dollar. Makes no sense as you said Young ... but then again its me ...

Brian Yee

I have topped off every car I've ever owned, warning or not, since I began driving in 1974. The automobiles included BMW, Porsche, MBZ,(Including my CLK55 and a E 320) Volvo, Acura, etc, etc, with absolutely NO negative results. I spoke to my buddy who is an engineer with the SCAQMD (Los Angeles area) about the topping off issue. He says it's to control "airborn aromatic pollutants", aka evaporative pollution. Quite a few people don't know how to top off properly, thus leaving $.50 worth on the ground that makes it's way to the atmosphere. He quoted some incredible number as to the amount in tons that gets into the air from gas spillage due to topping off.

JamE55

HHhmmm interesting... i thought this has something to do with the fuel sensor that if you top it off you'll get some sort of error?? But anyways good info!

Bora20

There is a problem with topping off. I forget what it does, but it essentially can cause the car not to start. It has to do with the fuel system being under a vacuum I believe.

I know that when my father-in-laws 99 E430 was topped off, it had to go to the dealer to be started.

Darkmann

Your Dad's car must've had an emissions related problem. It's purely environmental. And, BTW, topping off and rounding off are same thing. Like I said earlier, I was an engineer with the oil companies. The main problem for me is that there are lots of bad nozzles out there that will prematurely shut off, making you think that the tank is full. In that case, you can't help but top off. So, please if you encounter any bad nozzles, make sure to report it to the attendant - they will change it as soon as they know. It's the law in practically every municipality in the US.

linh

It does not matter !!! Mercedes is the only manufacture that have this problem with their's cars!!!! I had never hear a corrola,civic or even a Ford Pinto had this problem !!!! This is clearly a problem with Mercedes "QAULITY" !!!

Darkmann

Yep, it's a good thing to not top off, if anything, not to unecessarily spill gasoline which requires vry specific EPA cleanup procedures. Also, when you round off AFTER THE NOZZLE HAS CLICKED OFF, it's same thing as topping off. Just don't spill.

Flashman

My brand new 1996 Chevy Blazer would give engine errors (turn on ENGINE light) when you topped off fuel tank. A quick call to dealer revealed this was a common problem. He told us to stop topping it off, it messes with fuel sensors. We followed his instructions and the ENGINE light never came on again.

Rick

tompink

Well I was fool enought to pump in an extra gallon or two in my E-320 tank after the gas pump shut itself off. The E-320 does not have a vent around the gas intake like my Lumina 95 to let out the excess air pressure.

Well as soon as I took the gas hose out of the car, about half a gallon of gas shot out at me. Fortunately most of it missed me but I smelled of petrol all the way home.

Then the car started bucking, missing, so forth. Went dead once at idle. After about 35 miles (enought to burn at least a gallon of fuel) I pulled off the road and I took off the gas cap. The tank let out a burp like you would not believe! After the gas tank pressure was equalized to air pressure, the car ran and continues to run perfectly.

It is OK if the pump stops at $7.01 to run it up to $7.05 to avoid the pennies but don't try to top it up like the Lumina. That beast, a good car BTY, will take all the fuel you can put into it until it runs out the opening and still it does not seem to hurt it.

QQQ

I went to a MB clinic, where technicians refamiliarize new owners with new features and answer any questions, tour of the workshop, etc. The technician specifically points out that you should NOT top up the tank as it could flood the canister in the vapor recovery system. Then the car will not start. While the canister is not expensive, the job takes several hours so you get a hefty service bill.

rdniii

Mine had to go to the dealer twice for this problem.

Two things happen. The way the Mercedes tech expert explained it to me was:

1) The gas will continue to go in, then "blow back out" spraying gas all over your freshly waxed paint.

2) Your car will start running rough, sputter, stall, turn on the check engine light, and then refuse to start again, unless you drive it for 20 minutes at rpm above 3500. If it won't start, it has to go to the dealer.

What just happened:
The tank is designed to have an air pocket on top of the fuel, that the vapor recovery system uses to suck the fumes off the tank with. If you fill it too full, it begins to suck raw fuel and dumps it into your intake. The car's computer gets weird readings, makes adjustments to the engine, which don't jibe with the other readings. It finally gives up and turns on the check engine light. It has messed things up so bad that the car won't run unless the dealer resets everything.


To avoid #2:
If you get the "petrol shower", siphon off about 1/2 gallon (2 liters) and the problem goes away.