Hi all, Car is 1984 W126 380SE The car isn't used regularly, for a year or so it never had hard starting problems, but 2 weeks ago the engine failed to start, and finally started after many tries. However last week it refused to start no matter how many tries, after sitting for a week or so. Put few drops of gas into the air intake flap at the air filter, and it started fine, and runs fine, no more starting problems so far as it was driven for 3 days in a row. My question is, is this a common thing for older vehicles?? If so, what's reason behind it? Or could there be any hidden mechanical/electrical/fuel-supply problems? As far as I know the engine normally starts find cold or hot, and no known engine problems other then misfire in 1 or more cylinders at idle about 50% of the time. Thanks for your help.

Well, you have done a good diagnosis and workaround - I think you can be pretty sure its a fuel delivery problem.

If you have a decayed rubber fuel hose then the car will leak back fuel into the tank over time, and have a hard time pulling it back from the tank. Pulling air instead.

So, you probably should start by inspecting the fuel lines.

Next the function of the fuel pump. Make sure it can push fuel through a temporary line run to a jar.

Then suspect the fuel filter(s).

These are general help items - I'm not familiar with your particular engine.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

Yes, I suspected that, but if that's the case, wouldn't the drops of gas onto the air flap only enough to start and run the engine for half a second before dieing again due to no fuel supply?

I think the fuel pump/filter/accumulator should be fine as they were about 2 years (~10K miles) old. And I think the fuel lines are metal except around the pump area. Not so sure about the condition of the fuel related compponents around the engine though.

When you actually get the engine running, the fuel pump is able to function better than at cranking speed. So whatever is going on, I'd say it is right on the margin of fuel delivery.

I think if you took it into a shop they would put a fuel pressure gauge on it first. In my case I don't have such a gauge, so I have to "wing it" by evaluating the fuel flow at a disconnected hose. At cranking speed fuel should fly outa there rather forcefully.

Also, it would be relatively cheap maintenance to just go ahead and buy some fuel hose to replace all the hose on the system. If its the problem then you will fix it - if its not the problem then at least you've done some fairly inexpensive preventive maintenance.

Just a suggestion, I certainly don't know what's going on.......

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

Fuel delivery is easy to check. Unhook the return line place it in a container, and run the pump for 40 seconds by removing the fuel pump relay and jumping pins 7 & 8. You should get 1 liter of fuel.

From what you describe, though, it sounds like either the cold start valve or the thermo switch that controls it, has gone south.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

I agree with ctaylor. Pulling the fuel pump relay and jumpering the 7&8 pins will tell you a lot more about the situation than you would expect. Another possible problem is with the relay itself. When your car sits for a while, corrosion could create enough of an open circuit on the likely broken solder joints to prevent the pump from running. When you added the fuel to the intake, the engine moved and could have closed the gap in the broken relay solder joints. While the engine is running, the joints could vibrate enough to intermittently close the gap and the subsequent arcing could remove some of the corrosion to allow the car to start fine the next several times.

My money is on the fuel pump relay. Do a search here. I have documented my problem and how to solder it back together in around 10 minutes.

Good point.

Next time the car doesn't start, listen to see if the pump runs. It should come on briefly with the key. If it does not, that would point to the fuel pump relay, or perhaps one of the inputs to it.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Thanks gentlemen for your inputs.

The fuel pump does come on with the key, I will check if the relay is all ok tomorrow.

These are the current symptoms:

1. Rough idle, feels like 1 or more cylinders are not firing, sometimes shakes bad and feels like stalling, but RPM is fairly contant though.
2. Accelerates fine, occasionally hesitates before revving up.
3. Sometimes idles perfectly after a short high-rpm (4-6k) around the block (max 50mph).
4. Starts fine cold or hot, but occasionally crank longer before firing up if "warm". Eg. 45-60min after switching off a hot engine.

Would the fuel distributor or fuel pressure regulator be involved?

Also would the rough idle be related?

Thanks in advance.