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Discussion Starter #1
Hi All,

I have a 2003 200K with 80,000 miles.

I have searched and forgive me if I have missed any previous posts.
I am having severe power loss intermittently when the car gets hot. My immediate feeling is that the engine is starved of fuel. I have done the relay solder check and made good suspect joints.
The problem occurs when the engine is at max temp, i.e. when travelling in slow traffic and the fuel tank is 1/4 full.
I have also noticed that when the power loss happens the fuel pump is noisy and the noise sounds as if the pump is changing speed.
I have read elsewhere that the pump is cooled with the fuel in the tank and I can see that with a small amount of fuel in the tank, the exhaust hot under the car and little or no airflow under the car, the return fuel hot from the engine the heat can soon build in the tank.
I suspect if the pump is on it's way out the heat could cause it to partly seize or run slow. Has any one else had pump problems?
 

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Administrator 2009 SLK 55 AMG/Founding Member 2006
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Premium Member 2005 SLK55 AMG; 2005 SLK350
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How long since the fuel filter was replaced?

And it only happens when below the 1/4 mark?

The fuel has to be pumped a little higher as the tank approaches empty, but at these pressures I doubt the height difference would significantly impact fuel pressure.

If there's a small leak in the plumbing inside the tank above the 1/4 mark, that might account for this -- the leak maintains pressure when submerged, but doesn't when above the surface.

I totally don't get the heat connection -- engine temperature and fuel tank temperature should be unrelated.
 

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Discussion Starter #4
Hi guys,

Thanks for your replies.
I can see the link to a plumbing leak but would suspect the problem would show under high load and or speed but that is not the case. I did think about the fuel filter, but this was changed less than 6,000 miles ago. The MAF sensor has also been replaced recently as it was cheap and would have been an easy fix.
My thoughts about the tank getting hot is due to the heat coming up from under the car from the exhaust and fuel returning from the engine. (I assume there is some form of relief return to the tank, I am a diesel man )

The power is cut at slow speed and when trying to pull away, I have not had the problem at high speed when the engine is using more fuel than at idle.

I did not suspect the K40 relay as there has never been any problem starting. (as yet!) I can see that if the relay was weak at low volts it could drop out.

I had one occasion where the radio was off so I could hear clearly that the pump was changing note and was quite noisy. I concur that if the electrical supply to the pump was being interrupted this would happen and I have to admit I jumped to the conclusion that the pump was at fault. I did not suspect the relay after reading the K40 threads and thought the fault was more about a starting fault.

Thank you for your thoughts.

Bob
 

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Premium Member 2005 SLK55 AMG; 2005 SLK350
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Good point... and for full disclosure I'm just guessing/throwing out ideas here with no practical (or even theoretical) experience. What''s puzzling me most is that it's only when below certain fuel level.

Any other symptoms like backfiring or lignts on the dash?

On my '05 350 the pressure regulator and return are all contained within the fuel tank, there's no return from the engine. Not sure if yours is the same (that should be easy to see with a flashlight).

So what could cause the pump to run at different speeds... 1) different electrical input, or 2) internal binding of the pump, or 3) different back-pressure on the output.

If I was a gambling with your money, I'd start with the pressure regulator and pump.
 

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Discussion Starter #6
Thanks Efair, I have had another episode of the cutting out and the engine fault light came on this time with a weak mixture code. I have today replaced the fuel filter and pump. She runs sweet and I hope it stays that way. I did not realize how noisy the pump was.
 
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