G500 Sudden Power Loss
Guys,
Sad moment today watching the G500 getting towed onto a recovery truck.
It had been running sweetly without any problems or any sign of problems then suddenly, today, about a mile from my house it just died. It was a cold start, I drove around a mile at 50mph then on a slight incline in the road it felt like it went into limp mode. There was no power but the engine was running, I coasted into a lay by thinking that switching off and restarting would take it out of limp mode however on restarting it was idling really lumpy. Following turning it on and off a few times the lumpyness increased to a point where it would cut out after 10-15secs or if the accelerator was depressed even slightly. It had to be winched onto a rescue truck as it wouldn't move at all under its own steam.
Any ideas of what could have gone wrong?
Rescue mechanic thinks only one side of the engine is running and it may be coil packs.
It does seem like there is simply no fuel getting to the engine, could it be air getting into the fuel lines or a fuel pump failure? It has recently had new filters and plugs. It has had a new cam shaft position sensor last year.
Any help is much appreciated, mechanic will take a proper look at it tomorrow.
Thanks,
Ian
Could be ignition coil modules or mass-flow sensor, or cam-shaft sensor, or fuel-flow problem. But diagnostics should help pinpoint the cause.
What year? How many miles?
These engines are full of ecu controlled sensors which will protect the engine if a component is faulty or sensed as being faulty. A MB code reader may provide useful information and a place to start. Unfortunately, the fault code can often be wrong and components can be changed which are not at fault.
The engine (M113?) has two plugs per cylinder and I think it unlikely that your fault is caused by a single power pack failure. The engine will go into limp home mode if the fault is severe enough or safety if compromised. But that will still provide 10% engine power and won't be lumpy.
Let us know if you are given any fault codes or update on the engine performance.
When I had similar symptoms on my G500 a while back, it turned out to be a faulty mass air flow sensor; it caused severe loss of power, lumpy running and inability to rev beyond about 3000 rpm. As others have advised, please tell us what you find!
Avery similar problems with a 270 CDi. Problem sorted by replacing the air mass censor. Engine diagnostics did not reveal the fault. even at the Brooklands MB centre.
Anorher possible source could be a blocked fuel filter in the tank or fuel pump that is about to expire.
Sorry to hear the news on your 500. When you say 'limp mode' do you mean a loss of power when driving? one min full on 'warp' factor 10' the all of a sudden , no matter how hard you press the accelerator, its like a very very slow buildup of power?
I would look for another mechanic if he thinks a V-configured engine is running on one bank; IMPOSSIBLE!
The V engine has a split sparking sequence so that the engine can keep on running even if half the spark plugs were not to work as the non work half will be evenly split on each bank keeping the engine in effect, in balance. I would say it very extreme for all banks ignition packs to fail - maybe one or two.
You are more like to suffer an air mass sensor, crank shaft sensor or one of all others mentioned by Arnie above. Best place to start is getting a fault code reading first.
well, that also tells you something:
It's probably fuel-supply or MAF, and you've done some tests on the MAF, so...
It's easy to listen to the fuel pump running. If it sounds noisy or not smooth that would indicate a potential problem. Also it's easy to check the fuel pressure on the engine.
I had a very similar problem years ago in a C220. At first it was thought to be the MAF. After much wailing and gnashing of teeth it transpired to be a faulty ECU.
My first choice would be to replace the fuel filter if it has not been replaced recently (within the last 30-50,000 miles). When it gets clogged it creates symptoms similar to those you described.
The fuel filter is located on the left hand side of the G attached to the chassis rail about level with the rear passenger door. The filter operates on the high pressure side of the pump so if it has been clogged for a while it will have made the pump work harder than it should which can damage the pump. As others have said it might be worth checking the pump as well.
Hi
No expert but if all one side of engine is dead, does not the fuel pump supply both sides??
I bet is a wire on one bank of cylinders or plug for coil pack wiring not perfect. A lot of sensors cant pick up a fault like this as they only know if something is not working but connected if you follow
Let me know how it goes i doubt its a major thing
G
I would check the simplest thing first.....
are the battery terminals tight enough? is the battery good?
engine earth lead
gearbox earth lead
these tend to rot and can cause other apparent faults.... including a weak spark...
Guys,
The fault has been traced back to the fuel tank. When the fuel tank is removed and the fuel lines are placed in a tub of petrol the G starts on the button and runs as sweet as ever. The fuel is also returned via the return line with no issue and the pump pressure is good.
I have attached a couple of pictures of the tank. The white 'bell' inside the tank won't fill with fuel meaning the pump just pumps air. As far as can be seen no lines are blocked and everything is clean with no obvious debris.
Can the large 'disk' on top of the tank that has the hole for the sender be removed to allow access to the tank? It doesn't come off easily an we don't want to force it and break something.
Is there a schematic of the fuel tank/ fuel system available as we are pretty much at a loss as to why it appears that the tank is creating a permanent air lock.
Just to clarify the G was running very sweetly and this was a sudden loss of power while driving, no coughing or spluttering or gradual decrease in performance in the lead up, just going then not. Is there something in the fuel tank that could have failed?
Thanks in advance,
Ian
There is a fuel-return line inside the tank that is supposed to keep the bell-shaped thing full. This internal line has a habit of coming off and is difficult / impossible to replace. If you fill your tank more than 1/4 the 'bell' will fill from the top.
There is also a strainer inside this 'bell' and good time now to check that it clean.
long term solution is either new tank or to keep it filled above 1/4
(I was going to suggest this initially as a possible cause, but never known it to cause rough idling and loss of power: the engine usually just cuts out.)
(You can read-up on 1/4 tank stalling. Its a relatively common issue.)
The top does not come off....with out cutting it out and then plastic welding it back in, never easy...and careful with petrol...
I have done just that and cut one and actually removed the bell completely, all OK for standard road use after. as a cheap fix, probably not for a 500!!
There will be a white pipe the same as the bell floating around in there somewhere which has snapped off and unless you have a handy 2 year old with mechanical skills its one of Arnie's two suggestions..new or run 1/4 full...
A new tank on its own it enough but not stupid money now i think around £600
Only doubles plus in price with all the attachments etc to convert from steel to plastic on early models
Ian, Gav just answered that !
I never quite figured out how the system is supposed to work. Fuel is taken from the bottom of the 'bell', inverted flower-pot thing; it's then pumped to the engine and the excess is returned into the 'bell'. So, eventually the 'bell' will empty, unless it has another way for fuel to get in. One other way is through the hole in the top, but the tank level must be more than 1/4 full for fuel to get in. There must be another way too?
As far as access to the tank, I think there is only the hole at the top, as you've opened, and the fuel exit plate at the bottom, which is also removable.
I thought my G400 had developed this problem a couple of months ago, when I had less than 1/4 tank and it wouldn't start and there were bubbles visible in the fuel lines. After filling to over 1/4 it was fine. The funny thing is that it's since been (cautiously) to less than 1/4 - nearly empty, in fact - and it's been fine.
I had a very similar problem with an old Daimler some time ago. When running with the tank half full to full everything was fine , once the level dropped to a 1/4 full, the engine will remain idling but the whole system was on limp home mode. The problem was traced in the fuel tank. In this case , the fuel pump is submerged in the tank and it has a coarse filter at the suction . so it was quite simple to draw the pump out and clean the filter.
It will also be worth checking if the breather in the fuel tank is not blocked and the tank is not under vacuum when the engine is running. Might be worthwhile to run with the fuel cap off and see what happens.
Looks like a new tank 
£725 for a little bit of broken plastic.
Thanks for all the help!
Cheers,
Ian
£725 for a little bit of broken plastic.
Ouch....
Have you located the pipe and where it has become disconnected? I was thinking that with an endoscope and some long-reach pliers, it might be possible to operate.
(You can get video-camera endoscopes with displays for little money on ebay.)
Alternatively, to feed another pipe in from the return-feed connection on the tank and get it into the bell ?
I am not sure what makes the pipe come-off in the first-place, but I have found the problem on both old and nearly-new vehicles. I was thinking that if it gets blocked, the return, fuel-pressure might force it to come off.
The pipe on the ones i have seen is made of the same material as the bell and moulded / formed in one piece with the bell
It is not clipped or fastened on in any way and usually snaps of flush with the bell so there is nothing to sleeve over
If you could sleeve inside i would think that the bore would be too small to get any flow
I think it is around 8mm OD from memory
I guess you could plastic weld it but i don't think it would hold
Assuming they crack with age / brittleness and vibration?
It seems to be a very hard plastic
Gav
.....It is not clipped or fastened on in any way and usually snaps of flush with the bell so there is nothing to sleeve over.....
Gav
Ah, I thought that the pipe broke off at the other end, where the fuel return line attaches.
If there was more access, a stainless swagelok pipe might make a nice replacement.
If you cannot reach inside the tank... And are going you junk it anyway.. Can you not cut a hand sized access hole in the top big enough for you to reach in and repair the broken bit... Then make an inspection plate from ally and a gasket to cover the opening you made.. With washers on the fixing holes.. To close it back up... Would be considerably cheeper than a new tank..... Seems like a sensible first step.... Before resorting to replacing the tank?
Man maths: Good news you can now go and blow all that cash on a new toy....
That sounds like a great success. Did you record any images so we can see exactly where the problem (and repair) is?
Check the tank straps as they rust and have been known to fail.
The images here show how the plastic bell or 'swirl-pot' works. The return fuel is used to create a Venturi effect which draws additional fuel into the pot, when tank is less than 1/4
Google images " fuel tank swirl pot "
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