Vacuum in the fuel tank??

[mdsalemi]

Hello Gents,

Yesterday, a gorgeous day in SE lower Michigan, is a day I drove with the top down    to visit "The Good Doctor", Dan Caron and his Benz Barn--but more on that later.

On the return trip I experienced some trouble  :evil: which I might have identified, and want to ask if anyone has had anything similar.  First, I have what appears to be an original gas cap, but it came from a sedan.  It indeed locks down tight.

At the beginning of each "long" trip (several hours driving) I always begin with a full tank of 93 octane fuel.  I drove the 90 miles to Dan's without issue--much of it highway, at 4000 rpm (70mph) or so.  No problems.  The car sat at Dan's for a couple of hours, but for a short test drive.  It was not hot out, 68-72 degrees.

On the return trip, about half way home, with somewhat less then 1/2 tank, I was cruising in the right lane at 70, when I needed to pass a truck.  I put the pedal to the metal, so to speak, and that's when it began to miss BIG TIME.  I was able to continue for about 10-15 miles at about 65 MPH (but only with a very light foot on the throttle), but clearly it was not running right--particularly if I tried to give it any throttle--no response.  Passing was an impossibility.  I got off the highway, pulled into a gas station, and before I even opened the hood to check if a wire had come loose, I decided to fill up with gas.  When I pulled the gas cap off, there was a loud and long hiss--I don't know if it was pressure or vacuum.  This happened the last time I tried to give it some throttle after a long trip and when I was down below half a tank.

I'm suspecting that the vacuum/pressure in the fuel tank is impeding the fuel pump's ability to deliver a quantity of fuel, such as that required for passing when you are already doing 70mph, or really "stomping" it from a start.  I've been able to repeat the problem, but repeating it again is a long drive and half a tank of gas!

BTW Dan was listening and playing with the idle a bit, and indicated "it doesn't get any smoother than this" so I don't think the wires have anything to do with it.  Further, I did nothing to cure the problem but fill the tank with gas, opening the filler in the process.  Once I filled up with gas, I immediately took off, and no problems.  I was not even at the gas station long enough to let anything cool off...

Anybody have any ideas?  Dan will get the car for a little fine tuning in a few weeks, but I'd be interested to hear from anyone with ideas.  I know there has been discussion and issues with fuel tank caps, vapor recovery and ventilation but I don't know if I have any of these issues. :?:


Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored

[Tom]

Michael,

Let me throw out a 2 guesses.

Guess #1-Gas Cap or vapor check valve.  At some point, our cars went from a "vented" system to a "non-vented" system.  The non-vented systems recirculate vapor to the crank case and do create some vacuum.  If you have the little black plastic tank in your trunk, I suspect you have a non-vented car.  Determine which type of car you have and then make sure the gas cap is consistent for that car.  It will say in equivalent German on the gas cap what kind of cap it is-vented or non-vented.

If your car has that plastic tank, your check valve could be bad and may be part of your problem.

Only a guess.

Guess # 2-Bad tank of gas.

Best,

Tom

_____________________________________________

1971 280sl Tunis Beige Metallic (restored & enhanced)
1971 280sl Tobacco Brown (low mileage stock)
_____________________________________________

[Joe]

If you have the vent tube with the funnel-shaped rubber piece on the end, this tube could be clogged. Happened to me. I ran a piece of baling wire into it to clean it, and it has worked fine since.
Joe

[Cees Klumper]

As Tom suggests, I would check the fuel cap (on my car -like yours a '69 280 SL, however without any pollution control devices- there's never any hiss or pressure build-up) and I would check all of the lines to and from the gas tank, particularly the venting ones to and from the overflow canister in the trunk. If it happens again, you could try pulling over and just releasing and re-fitting the cap, without filling up. That way you will know for (pretty) sure that it's pressure/vacuum rather than 'quantity-of-fuel-in-tank' related.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[hands_aus]

Michael,
What is the inside of your fuel tank like? Is it clean and rust free?
The reason I ask is that Dan the Dr. once mentioned a "Flower Pot" (HIS WORDS) inside the fuel tank onto which the return fuel flows.
Apparently fuel is forced through a hole that is half way up the side of these containers. (These Flower Pots are there to keep a small reserve of fuel available for the fuel pump when the fuel level is low and the fuel is sloshing around).
If the small hole is blocked, fuel can not enter when the fuel level gets below half a tank and hence your problem occurs.

Bob (Brisbane,Australia)
RHD,1967 early 250 SL, auto