HELP! Intermittent Power Loss

[wpw2511]

Hi all! I bought a rebuilt and restored 1969 280SL a few months back and everything was fine until last week. After filling the gas tank full last week, I took the car down to Malibu and it stalled at a traffic light. I tried starting the car back on, but it wouldn't start. I waited a couple of minutes, and the car started back on and went on driving. I noticed the problem happens when I am in either drive or reverse but don't have my foot on the gas. I started to notice that the car would start to lose power and the RPM needle to start shaking violently, but most of the time not to the point of stalling.  When idling on park and neutral, the car is perfect. In addition, the car also drives perfectly at normal speed.

The mechanic told me that I needed to change my fuel pump. He measured the pressure of my old fuel pump, and it measured at 10 or 11 and he said it should be at least 15. So the mechanic installed a rebuilt fuel pump, changed the fuel filter, inspected the gas tank and washed it with a jet washer. In addition, he changed the rotor and distribution points. Everything was fine until today, when the car stalled again at a traffic light. What could be the problem now?

Thanks in advance!

[Nicolas Aristodemou]

It could be a weak mixture or an inoperative/faulty/misaligned  Venturi switch. Try checking the operation of the switch as per technical manual recommendations and enriching the idle mixture by 1-2 clicks (see linkage tour).

[mdsalemi]

I don't know if my experience will help you, but I had what sounds to be the exact same.  Turned out there was a multitude of issues, and until they were all addressed, the problems remained.

1. Intermittent power to fuel pump, caused by bad electrical connections. The problem was in a crimp-splice; once I eliminated that, that issue was solved.
2. Internal rusting of fuel tank. This was only solved by replacing the fuel tank. Others have salvaged their tanks by having them sealed. Microscopic particles of rust were clogging things. Clean screens and filters restored things for a while, but the real solution was a new tank.
3. A "shift" of the internal wires inside the distributor on an electronic ignition system with optical pickup. The shutter wheel was on occasion contacting wires inside, ceased turning that caused an immediate stall. As the system had worked flawlessly for 5 years prior, my solution was to go with a Pertronix.

So I had the same issue as yours, with a variety of problems contributing to the cause. Once all problems were solved, the issue never reappeared.

[ja17]

Sometimes new ignition points will "settle-in" and the gap will close, causing these kind of problems. The solution might be a simple adjustment of the points. Otherwise, my first check would be a fuel pressure check when the problems is occurring. A fuel pressure and volume check would rule out any fuel delivery problems.

[kampala]

I noticed the problem happens when I am in either drive or reverse but don't have my foot on the gas. I started to notice that the car would start to lose power and the RPM needle to start shaking violently, but most of the time not to the point of stalling.  When idling on park and neutral, the car is perfect. In addition, the car also drives perfectly at normal speed.

Since you seem to be new to this site, I will mention that JA17 is an absolute genius so heed his advise. 

Since you mention that the issue is ONLY while idling and ONLY in Drive/Reverse and no issue while driving - I would suggest you check how your particular car is supposed to INCREASE idle when in Drive or Reverse -- the automatic trans versions of these cars need to compensate idle when in Drive/Reverse - this is accomplished in different ways depending on the exact configuration of your car.   Inspect the AIR throttle area - remove the rubber air intake so you can look inside the throttle body and see the air FLAP - when car is turned off this flap should be fully closed - you can move it manually to see that it settle fully closed.  It should almost stick a little when moving from fully closed to open.   

If you see that your flap is NOT fully closed upon inspection when car is in PARK and running or turned off - you need to do the famed LINKAGE TOUR by JA17 - avail to full members.


IF your flap is fully closed or to do further tests :

Start the car (take safety precautions) , have someone move the gearshift into Drive or Reverse while you watch what happens to the air flap and the linkages to the air flap - the flap typically should crack open just a touch to get the idle back up (depending on config).  If the idle does not move back up or nothing moves in the linkages you know one of your problems is that your idle is not moving up when put into gear.

I repeat:   If you see that your flap is NOT fully closed upon inspection when car is in PARK and running or turned off - you need to do the famed LINKAGE TOUR by JA17.    You may want to do this tour regardless just to check everything in the linkage system.

[Mike Hughes]

Same thing happened to me a few years back.  Check the points first.  The rubbing block on replacement points can wear rather more quickly than one would like and close up, stalling the car.  Feels just like running out of gas.  Open up the points a bit and she should start right up.  Get the car home and replace the points, remembering a teeny dab of dielectric grease to lubricate the cam and rubbing block.

[wpw2511]

Thank you everyone for the advice!

[wayne R]

Would like to just go  a little further here,  when replacing  distributor points, it is best
always in most cases to  replace the condenser as well, you can usually check this
by inspecting the removed points, and looking very close ,and if there are  large burns,
uneven  wear to face of points, thats the sign that the condenser is not doing the job it is designed to do.
Thats the reason there is a  condenser  fitted ,it is there to help stop  spark jumping or arcing across
and destroying the surface of the points.---regards.

[wpw2511]

Hi guys. Since the last time I originally posted this post a few weeks ago, I decided to bring my car to another classic Mercedes specialist instead of my original mechanic. The new mechanic found that there was a short in the idle switch, and went on to replace the switch and solenoid. In addition, I told the new mechanic to check out the distribution points that the original mechanic replaced, and he said everything was ok. I also told him to check out the fuel tank, so he drained the fuel tank and cleaned it as best he could. According to  the original and new mechanics who both drained and cleaned the tank, the fuel tank is supposedly pretty clean.

Today as I was driving, the car stalled again. Sigh. I have officially spent over $3500 with two different shops trying to rectify this issue, but to no avail. My gut feeling is that I have to replace the fuel tank, but what do you guys think?

[ja17]

Its hard for a mechanic to diagnose an intermittent problem when the problem is not occurring. Installing a fuel pressure gage temporarily and driving until the problems occurs would tell me if the problem was fuel delivery (clogged, filter or tank, faulty fuel pump etc.) A fuel pressure drop would occur. Also try unfastening the gas cap for a few seconds if the problem occurs, since a plugged fuel tank vent could cause a vacuum situation and fuel starvation. Don't rule out electrical since a bad ignition coil may begin to fail after it is warmed from running. Make sure the capacitor on the distributor is firmly attached because it must be securely fastened and grounded or it will cause issues.