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Old 09-01-2013, 06:26 PM
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1972 350 Mercedes sl fuel pump issue

Recently, while charging the battery, the fuel pump apparently activated and pumped gas until the engine was full and began leaking - out the rear main seal, I assume. The gas leak led to an explosion in the garage where the car was stored causing some damage. What could have caused this to happen?

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Old 09-01-2013, 07:23 PM
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Originally Posted by donj View Post
Recently, while charging the battery, the fuel pump apparently activated and pumped gas until the engine was full and began leaking - out the rear main seal, I assume. The gas leak led to an explosion in the garage where the car was stored causing some damage. What could have caused this to happen?
There is no way to pump gas until engine is "full". Unless somehow one or more injectors were open and the gas was pouring into one or more cylinders and then into crankcase. Very unlikely.

When fuel pump runs, it pumps gas into the injector rails and then back to the tank via a fuel pressure regulator. It could run for hours that way. There is nothing to "fill". It just recirculates.

However, there are a number of hose clamped connections throughout system including the ones that connect the injectors to the rails. It is quite common for those connections to leak. It is also possible that a hose, like the front connection between the rails, could be weak or damaged.

With gas leaking around the engine and a charger connected to the battery (also under hood?) all you need is one spark.

I am assuming an original pump. If an aftermarket or later model pump was used, it could overpressure the system if the return line is somehow restricted.

I really can't see how the pump would run by itself. It requires the ignition to be turned on but then only runs for a few seconds unless the starter is engaged. Were you trying to start car with charger hooked up?
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Old 09-01-2013, 08:05 PM
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There is no question that gas entered the engine and is now in the crankcase. This was easily verified by checking on the dipstick - both by sight and smell. The gas leak was not caused by poor connections. Also, no one was trying to start the car. It would seem there had to some sort of malfunction, but the question is what.
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Old 09-01-2013, 09:48 PM
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There is no question that gas entered the engine and is now in the crankcase. This was easily verified by checking on the dipstick - both by sight and smell. The gas leak was not caused by poor connections. Also, no one was trying to start the car. It would seem there had to some sort of malfunction, but the question is what.
See my first sentence above. The gas would have to go through the injectors or startup injector into the intake manifold. After that there is no pressure, so it would just drain through whichever valves are open into cylinder heads and down past rings into sump.

If your pump can run on its own, then I guess your injectors could somehow open too. But they should both be closed/off if ignition is off.
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Old 09-02-2013, 02:06 PM
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Sounds like a short somewhere - possibly in the ignition circuit, sending power to the D-Jet system and maybe other systems. The pump runs during the first key-on for a few seconds to pressurize, so an intermittent short could cause that cycle to occur a lot - as well as the opening of the cold-start valve for cold starts - which can cause gas to go into the manifold and then eventually into the engine. Same if the injectors leak or are getting a false signal.

Check your wiring, starting with the ignition switch. Then check the EFI harness. Check all ground connections, also.
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Old 09-02-2013, 08:59 PM
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good points guys, this COULD not happen without power to the pump and CSV, Key left on or ignition switch short
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Old 09-02-2013, 09:59 PM
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good points guys, this COULD not happen without power to the pump and CSV, Key left on or ignition switch short
You need fuel pressure to make any of this happen so the pump would need to be running. Once the pump is shut off ( assuming that it is ) fuel pressure starts to drop slowly if all of the check valves are working, or rather quickly if they're not working. It wouldn't be a lot of fuel if it happened one time but repeated cylcles would add up.
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Old 09-03-2013, 12:15 AM
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Waiting for the NHTSA recall to be issued on 1972 350SLs.

All 24 owners of running cars will have to be notified.


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