W114 280E uses heaps of petrol - not any more!

[Lance]

Hello all,
I am so close to getting this thing registered even had the RWC booked for tomorrow, so took it for a quick spin to ensure all is well and warm it up for an oil change. But it ran out of petrol. I drove it briefly a previous day and with today have done about 10km all up on 15 litres of petrol. I don't know of any leaks, so I would welcome suggestions of what I should investigate. Could it be the manifold pressure sensor has failed and telling the ECU to give it full fueling all the time? The resistances are spot on at the ECU plug.
Thanks for any assistance you may be able to give,
Regards,
Lance.

[Lance]

Further information;
It is definitely burning it, the plugs are covered in soot.

[oldmercfan]

Hi Lance, I know that 280Es (well, M110s to be specific) are thirsty, but that's ridiculous. Mine is back on the road reliably now and getting about 12 litres per 100 km. Your economy is so bad it makes me think there is some cause in addition to it just running rich as the plugs indicate. One guess is that your return fuel line is not doing its job. If this is blocked you can run out of fuel even with the tank about a quarter full. I have just learned this from practical experience, having had a filthy swirl pot in the bottom of the tank that caused me all kinds of fuel supply problems. The return fuel line feeds into the bottom of the pot (generating the swirl). The pot is effectively a subset of the tank. The top of it is open, so if you fill the tank up past about the quarter-way mark, it will spill over the top of the pot and restore the fuel feed to your engine. Everything will seem hunky dory until the level in tank drops again below the top of the swirl pot. The easiest way to diagnose would be to disconnect the return fuel line hose from the tank (its at the back, right next to the bigger outlet hose that goes to the fuel pump). Put a stopper in the tank, have someone start it up and see if any fuel is coming through the return line hose. That's where I would start anyway based on my experience. There are smarter guys out there who might tell me I am barking up the wrong tree, so i would wait and see what they say.

[Lance]

Thanks for the reply,
Filter in tank is spotless, and I believe that the regulator is doing it's job, tank definitely runs dry.
I am currently removing the injectors. I will try to test them somehow.
The other thought is that the fuel enrichment is on all the time due to a fault in the throttle sensor. Wouldn't think that would cause so much usage but maybe others can advise. I will check the sensor reistances tomorrow.

[TheMadRacoon]

I have about 0.0001% of the knowledge and experience of others here, but I'm wondering if it's worth dropping the oil to see if it's contaminated with fuel. I understand excessive fuel into the cylinders is not good for piston rings or cylinder bore. Petrol in oil may confirm overfuelling at all times? Might also save your engine (?)

$0.00002 worth :-)

[Greg in Oz]

Oldmercfan,
The usual swirl pot issue is not that fuel isn't returning from the engine through the return line. It is more about sediment on the floor of the tank blocking the venturi where the return line enters the swirl pot. This will prevent fuel being drawn from the bottom of the tank into the swirl pot, even if the return line is returning fuel to the tank.

As for this 280E using 15 litres in 10km, there is no way the engine could be burning that amount of fuel. That amounts to 150 litres/100km. My 500SLC only used something like 60 litres/100km when driven flat out on a race circuit. Either it didn't really use 15 litres in 10km or some of that fuel leaked out somewhere else. Fuel consumption that bad would result in worse than just sooty spark plugs. They would be so wet with fuel that they wouldn't fire and the there would be thick smoke behind the car.

[Lance]

So where is it going?
There are no leaks (I would see one that big) and it is not going in the oil. I just changed it last night and it doesn't smell of petrol. Also, the level was not increasing.
The plugs are covered in soot.

[TheMadRacoon]

So where is it going?
There are no leaks (I would see one that big) and it is not going in the oil. I just changed it last night and it doesn't smell of petrol. Also, the level was not increasing.
The plugs are covered in soot.

I thought of this topic viewtopic.php?f=15&t=20263 but you did say there are no leaks.

[Lance]

All that sounds very interesting and may well be the cause, although I haven't seen fuel leaking out. I am going to try to remove the injectors and set them up to see what is happening when the engine is cranked over. This will tell me if the injectors or the ECU is to blame.

[Djenka018]

Are you not measuring distance travelled between the reserve red lights coming up (after refuelling)?

[Lance]

Update,
Fuel leaking out of the rear of the intake manifold at the bottom. Also around the two rear injectors. Will replace injectors anyway as the hoses are perished, and if that doesn't fix the problem then will try another ECU.

[Lance]

Just wanted to update this post as it seems it's not an uncommon problem.
Some months ago I ended up getting another ECU which appeared to help. But at the same time I also checked the cylinder head temperature sensor resistance when the engine was warm. Although it gave the correct reading when cold it went into open circuit when warm, so I replaced it. The engine ran well after I fixed the electronic ignition (related in anther post).
Anyway, last week I re-installed the original ECU which I always wanted to do to confirm if it was the ECU at fault or the temp sender. Interestingly the car ran noticeably better.
Of course the ECU gives extra fueling in proportion to the resistance of the sensor, but there appears to be no upper limit.

[ngruzevs]

Hi Lance, I've got a mate with a similar issue on his W114 280E. I've just sourced a replacement ECU for him and expect it to arrive sometime this week. Will replace it and see how we go.

He's just joined the forum and will be reaching out to you via PM. His name is Rick (not sure of his OzBenz user name ATM).

Nick

[Tony From West Oz]

His login is "Mercman01"
Once he has 5 posts, he will be able to use the PM system.

[Lance]

Hello,
Just sent him a reply, reproduced below for others benefit:

Cylinder head temperature sensor.
When I have time I will take a photo to show where it is. It is simple to check the resistance to see if it is working. I think it is about 2000 Ohms when cold going down to 600 when hot, but you should be able to check the correct figures on the internet. Important to check when the engine is hot as mine read OK when cold then went to open circuit (infinite resistance) when hot. Also check a couple of times as it can read intermittently. Don't forget to check the wires from the ECU plug to the sensor connection. These sit just above the exhaust manifold and the heat causes them to deteriorate. They can break, also causing infinite resistance. The sensors cost about $70 from Bosch.
The sensor gives more fuel when cold just like a choke. My theory is that infinite resistance equals infinite extra fuel, ie injectors constantly open. It seems that there is no limit to the extra fuelling the system will give to the engine. Anyway, just do it, cheap and easy and ignore anyone who says otherwise. ECUs and pressure sensors rarely fail, but this can't be completely ruled out and you never know what previous butchers have done. Pressure sensor resistances can be checked, also suck on suction port and hold your tongue over it, if it holds vacuum and resistances are OK, then it is probably good.
Regards,
Lance.

[ngruzevs]

Is it the Bosch coolant sensor you're referring to

https://www.qp24.de/car-parts/mercedes- ... ure-sensor

[ngruzevs]

MPS part number is 00280100111

[Lance]

Yes, Bosch coolant sensor.