W126 300se fuel pump running continuously

[Def_Fid]

My fuel pump has started running continuously until it overheats and stops. Its now very noisy. It was renewed less than 2000 miles ago along with the fuel filter and fuel pump relay. So far as I know, it should cut as soon as feed pressure to the fuel injection pump is reached. Is there a sensor somewhere in the line, or is the pump meant ot automatically cut out at a pre-set pressure?

Puzzled.

 

[power-steering]

hmm.. curious to see the responses to this myself.

 

[pikeydave]

Need a bit more info.
Are you saying it's running continuously whilst the engine is stopped?

 

[television]

The fuel goes into the distributor, and comes out to the pressure regulator, then back to the tank. If the fuel pressure regulator was faulty, the car would not run very well

If the fuel pump relay had a short this could do it, the way to test it is to pull it out and check that no short circuit exist between pins 7 and 8
http://www.detali.ru/cat/oem_mb2.as...&L=765.706&CT=F&cat=568&SID=47&SGR=045&SGN=03

 

[Def_Fid]

More info..

The pump comes on (now noisily) with the ignition. The car starts and runs perfectly, although the pump can be heard running continuously. Running at 60-70 mph the car runs sweetly, presumably the pump is being cooled by the fuel running through it. However when in stop-start crawl, the pump gets very noisy then cuts out altogether, and of course so does the engine. Leave the car for half an hour, the pump works again, engine starts until the pump overheats again....

I'll have a look at the contacts on the pump relay.


Cheers

 

[Cosworth]

I would replace the fue pumps as they could be the cause of fuel pump relay problems.

Replace the pumps and fuel relay and hat should cure your problems.

 

[television]

I would replace the fue pumps as they could be the cause of fuel pump relay problems.

Replace the pumps and fuel relay and hat should cure your problems.

Interesting bit is that this has never come up before here or anywhere that I can recall

 

[roofless]

it will run till it builds up pressure when the ignition is turned on then cut off till you start the engine & then it runs continously by sending excess fuel back to the tank. i would start by reading this http://www.scribd.com/doc/3299223/Bosch-KJetronic-Fuel-Injection-Manual-

 

[television]

it will run till it builds up pressure when the ignition is turned on then cut off till you start the engine & then it runs continously by sending excess fuel back to the tank. i would start by reading this http://www.scribd.com/doc/3299223/Bosch-KJetronic-Fuel-Injection-Manual-

Indeed it does flow back to the tank, the hard bit is we know that the pump runs for a few seconds to charge it then it cuts,until the engine is started

 

[roofless]

Indeed it does flow back to the tank, the hard bit is we know that the pump runs for a few seconds to charge it then it cuts,until the engine is started

the op is right in the fact the pump is cooled by the fuel & as the car runs well it's hard to find a fault uther than the pump noise the only thing i can think of is the pump or poss the fuel accumulator as it also helps dull the sound of the pump even the fuel filter could be in a poor state

 

[Def_Fid]

Fuel pump and filter less than a year old, the pump gets very hot, like its pumping against back pressure, so I don't think the problem is fuel flow- when the pump is running, the engine works perfectly. I'll look for a short in the FPR.

Thanks all,

 

[GEORGEROV]

There's a fuel strainer in the tank itself. Could this possibly be getting blocked?.

 

[Number_Cruncher]

Upon turning the ignition on, the pump should run for a set time. This pump running time is to allow the accumulator to be recharged, and the system pressure to build - there is no pressure sensor which shuts the pump off.

During running, the pump builds pressure to overcome the primary pressure valve in a K jetronic system or the fuel pressure regulator in a Motronic system. In either case, the pump should run continuously, with fuel being returned to the tank. As has been mentioned, the continuous flow provides cooling.

If the main fuel filter becomes blocked, then the fuel flow will become restricted, and yes, the pump could overheat.

 

[television]

Upon turning the ignition on, the pump should run for a set time. This pump running time is to allow the accumulator to be recharged, and the system pressure to build - there is no pressure sensor which shuts the pump off.

During running, the pump builds pressure to overcome the primary pressure valve in a K jetronic system or the fuel pressure regulator in a Motronic system. In either case, the pump should run continuously, with fuel being returned to the tank. As has been mentioned, the continuous flow provides cooling.




If the main fuel filter becomes blocked, then the fuel flow will become restricted, and yes, the pump could overheat.

I cant see that as when you have an injection fault and you are testing, on cars with a fuel pump relay you bridge pins 7 and 8 to make the pump run all of the time, otherwise it will cut

 

[Number_Cruncher]

>>otherwise it will cut

Yes, it cuts because, typically, the ECU earths one side of the relay coil to switch on the fuel pump. The ECU does this for a set period after switch on, and while running it does this when there's some evidence that the engine is still running - whether from the crank angle sensor on more modern cars, or from the Hall sensor on the ignition of older cars.

If the car is in an accident, and the pressurised fuel feed pipe ruptures, the engine stops, and so, the ignition / crank angle signal stops, and the ECU switches the fuel pump relay, and hence the fuel pump off.

This is why, when testing, you have to bridge the relay - if you could inject a simulated ignition pulse (or crank angle signal as appropriate) to the ECU, the relay would remain energised.

The other safety method which was used on some cars did not use the ignition sensing logic, but instead, relied upon an inertia switch to cut the fuel pump in the event of an accident.

 

[television]

>>otherwise it will cut

Yes, it cuts because, typically, the ECU earths one side of the relay coil to switch on the fuel pump. The ECU does this for a set period after switch on, and while running it does this when there's some evidence that the engine is still running - whether from the crank angle sensor on more modern cars, or from the Hall sensor on the ignition of older cars.

If the car is in an accident, and the pressurised fuel feed pipe ruptures, the engine stops, and so, the ignition / crank angle signal stops, and the ECU switches the fuel pump relay, and hence the fuel pump off.

This is why, when testing, you have to bridge the relay - if you could inject a simulated ignition pulse (or crank angle signal as appropriate) to the ECU, the relay would remain energised.

The other safety method which was used on some cars did not use the ignition sensing logic, but instead, relied upon an inertia switch to cut the fuel pump in the event of an accident.

You are correct just had a peep at a the KE and LH and that is the way that they are wired, since the return to tank is connected from the pressure sensor, does this fit into the equation as this is also fed from the ECU.

KE on pin 13 is also the cut off.

Thank you mr Number cruncher.

I am off to bed good night

 

[Number_Cruncher]

>>does this fit into the equation

I'm not sure - I'm not particularly au fait with KE jetronic, but, I didn't think there was a fuel pressure sensor in the system. Is this right?

One other method of fuel pump switching which I had forgotten was in some L jetronic systems, and Motronic systems which used flap air flow meters, the air flap could operate a micro-switch which would shut off the fuel pump if he air flap returned to the rest position. Perhaps I might be forgiven - this method is much more a BMW thing than a system which you might find on a MB!

 

[television]

There is a pressure regulator, Just checking on another a 126 fault I see the the Tach,, KE, AC,and fuel pump relay are connected to the TDC connection.

I willl do some more investigating later

 

[television]

You are spot on Number cruncher, the fuel pump cuts 1 second after receiving the last pulse from the TDC

 

[Number_Cruncher]

Thanks for checking up - Most of the stuff I post is relating to work I did on cars 20 years ago, and it's more than likely that I will mis-remember quite a bit!