1971 280sl stalling when hot
 

I am at my wits end. I have a 1971 280sl that runs great when you first start it and drive down the road. Take it on the road and run it for 5 to 7 miles at 60mph and it will start to stumble, lose power and eventually stall. Pull over, let it sit for a minute and it will start and run ok for a few minutes and do the same thing all over again. It is running rich as the plugs seem to have some black residue on them, but not wet. I have replaced all filters (air, gas, injection pump), standard ignition items (plugs, wires, cap, rotor, condenser, coil), no help.
Any idea's out there? Please let me know if you have any idea's.

Thanks,

P.S. This car has about 50k on it. It is a 6 cyl with a 4 speed. It has been in storage for about 10 years, being started only occassionally with treated gas.

I am not too familiar with your car bur I would check your sensors, wiring harness. I am having the same problem with my 190e 2.6 1991. Check all
electrical sensors if you car has electrical fuel injection.

Thanks. This vehicle has a mechanical fuel injection system.

Your car may seem to be running rich by looking at the plugs, but I'll bet that when it quits running it is starving for gas. Probably a plugged filter or screen. Next time you drive it take along a can of starting fluid, and when it quits, squirt some on the air filter and try turning it over. If it runs for a short time, it's almost certainly got a problem with fuel starvation.

Sounds like a good idea. I have checked all the filters that I know of (fuel filter, injection pump filter air filter), but is there another filter or tank screen somewhere on the vehicle? If there is, do you know where it is and how to get to it?

I'm willing to check anything at this point.

However, it really seems to be heat related. But like I said, I'm checking everything. I am also going to replace the ballast resistors and the injection pump thermostat.

Thanks,

Al

Probably a combination of crappy gas and vapor lock. Insulate the fuel lines and check pressure on fuel pump.

Have the same model and year Pagoda and had the same problem. In my case it was an inch and a half of rusty sediment in the fuel tank. There is a primary screened fuel filter attached to the drain plug for the fuel tank. As the fuel is drawn thru the screen it clogs with crud but will clear after the a short while. The solution is to drop, flush and reseal the fuel tank.

For more detailed info go to http://www.sl113.org/home.asp and search fuel tank rust.

Hope this helps.

Thanks, sounds good. You are the 3rd person to suggest to clean the fuel screen in the tank. I will be doing that this week and let you know how it goes.

No go on cleaning the fuel tank
 

Well, I drained the tank and cleaned it out and there really wasn't much crap in the bottom.

After draining and cleaning the tank, I test drove the car and it still stalls when it warms up and is hot. It takes around 10 miles of highway driving for it to stall. When it stalls, I wait for a few seconds or a couple of minutes and it will start right back up, go down the road for a while and stall again.

Any one have any additional idea's? How about the fuel injection thermal time switch or the fuel injection pump thermostat?

Thanks,

Al

As I found out myself, here on the forum, there is another filter in the fuel pump...I believe it is on the inlet side...a small filter, maybe smaller than your pinkie.
How was the tank screen - dirty? so-so?

Is your injection the kind that has the little dipstick?

When it stalls, unscrew the gas cap, and see if you hear hissing (air being sucked in).

Make sure coolant is getting to the injection thermostat so that it warms up along with the engine.

That chassis has a Swirl Pot fuel system. The swirl pot is at the bottom of the tank [ where the screen screws into] .
A Swirl Pot system needs fuel to come back to the tank from a return line in order to work. It is simply a small tank within a main tank. It is there for cornering so the fuel level is alway adequate to feed the pump. [Stock/Track cars with FI use them for that reason ].
At the very bottom of the SP, there is a small inlet hole to allow gas from the tank to enter the SP..This hole is directly below the fuel return line and it is the venturi action of the fuel from the retun line that sucks the gas into the SP..so, first check is both the fuel return line and the hole in the SP.
I have a few short cut test that you can use to determine where the problem lies.
First one is to take the feed line from tank off the main fuel filter in the engine bay and take the return line off the rear of the MFI injection pump. I then take a clear 3/8" plastic hose we have here and connect the two together..[ hose should be about 3 feet long and use clamps to avoid gas leakage..this line is a pressure gas line of approx. 15psi] ..This creates a loop from the tank. Now, turn On the key to start the electric pump and watch the clear line..you will see fuel running through the line and back to the tank at a good pace , but you want to watch for any air bubbles. If you see air. then the return line is not sucking in fuel into the SP b/c the little hole below it is plugged [ VERY COMMON on these 113 chassis]. This hole can be cleared by removing the sender unit and poking the hole with a bent Brazing Rod..[ the reason for brazing rod is so you do not get a spark].


Before doing any of this, there is a test for SP and return line clogging.
The SP is open at the top , so if you have 1/2 tank of fuel, the SP gets over-flowed form the top. [ at least 5 gals are required to accomplish this]
So, retry your car with 1/2 tank and see what happens..the SP only comes into play the last 5 gallons of gas [ that is why it is there ..with a full tank it is not needed.
The other test is to blow air down the return line at the MFI end with the gas cap off..you will hear the air gurgle from the gas cap filler pipe if the return line is clear.
The last test is a flow test. This is done using the same plastic hose , but this time you just slip it over the feed line bubble flare end at the main filter
and put the other end in a measure container on the ground. You turn On key and watch the fuel go into the container,,you want 1 ltr per 15 secs time..this is a must..When you do this test , you make sure the hose is at the bottom of the container so that as it fills , you can watch for air bubbles here too..you want NO AIR.
What happenes on these SP is the little fill hole I mentioned gets partially plugged b/c it is right at the very bottom of the tank and is just a small hole to feed the SP when the fuel level is low. So, your car will run for a few miles and then quit..but you wait for a while and the SP fill back up and you are on your way..what is actually happening is the SW can nor keep up with engine fuel demand b/c of low flow..cuased by either dirt restriction OR a plugged fuel retun line. Both will cause your exact problem.
Do the flow test first, the 1/2 tank of gas test second, and the plastic loop test third..you will then know your cause of fuel starvation ,if you have one.
These SP are a common complaint on these and that is why many owners know not to let the gas tank get below 1/2 tank b/c the SP problem will only come into play when the fuel level is below the top of the SP.

I will post a pic of a SP and that will help you/others understand how they work..pretty cool device................

Here is a typical Swirl Pot ..this one happens to be from a Ford Focus or Fiesta, but it is basically the same as what the 113 chassis uses .
You can see the screen in the middle and you can see the smaller line fitting [ that would be where the return line goes] and the large line will be the outlet line from tank that feeds your electric fuel pump. You can not see the little SP feed/fill hole in this picture , but it is just above the return line fitting ..They have it above so that crap in the bottom of the tank does not clog it....but yours is at the very bottom and that is why they tend to get clogged..this one would be an improvement over the Benz one for that reason.
You can see how the shape of this pot causes the gas to circulate as the gas comes back via the return line..that is the force that helps suck gas into the SP from the main tank..you can also now see that if one had a 1/2 tank of gas, it would fill the SP over the open top and the SP fill hole is not needed ..which is why you do my 1/2 tank of gas test.
Also , even when the tank is 1/2 full, the return line still is in use/play ,as they want a constanty moving fuel loop from the tank under all circumstances to prevent the possibility of vaper lock while car is running.

http://members.aol.com/ajdalton7/tank-2-768.jpg

Here is another one from the 113 site that is the actual Benz tank one..they call it a flower pot, but that is just a tag they put on it..It is technically a Swirl Pot. Same system, and you can see here what I mean about the fill hole being very low on the pot,below the return line instead of above it, allowing it to be suscepticle to easy blockage on a dirty tank.

http://www.sl113.org/forums/uploaded/ja17/200611475414_fuel%20tank%20with%20fp%20ill..JPG

Probably a combination of crappy gas and vapor lock. Insulate the fuel lines and check pressure on fuel pump.

I couldn't quite tell from the wording in your post, but did you check the little filter in the inlet to the electric fuel pump?