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[Problemchild]

It's run out of fuel?

The gauges are flakey at the bottom end and can totally misread

Put 1/2 tank in first and then see.

JJ

[Gareth79]

That was my first thought I put in half a can at the roadside (at least a couple of litres) and the gauge increased to 3 bars. I have run it down to the flashing petrol symbol before (with a petrol can in the boot!) so I think it's pretty accurate on mine.

I had half an hour this morning to have a further test - and I have just sneaked some time away at work to check my results.  I found this service manual diagram for a 2008 CZ which has a similar setup:
http://mmc-manuals.ru/manuals/colt/online/Service_Manual/2008/90/html/M190100080556000ENG.HTM

I had 12V across the adjacent yellow-green and black pins (which I now know is 4 and 5, the fuel level sender), but nothing across 2-3 (the pump power) at any time. I will double check that later.  Looking further at that diagram it seems the ignition switches the first relay and then the second is controlled by the ECU, so I should concentrate on monitoring the second one really.  The short running and stalling yesterday (and whine I heard) was curious though, it wasn't doing that this morning.  I might try bypassing that relay entirely - apply 12V directly to the pump when cranking and if the car starts and runs then it's the ECU not powering it for whatever reason.

Also to confirm on the key symbol - it flicks up very briefly when the ignition is turned but doesn't reappear at all, I assume that means all is good.

** edited a couple of times as I did some more reading **

Also a note that I'm not expecting anybody to come up with a solution, this is partly to add information to the 'pool of knowledge' as I figure it out.

[CrazyG]

Are you sure that it is the 'key' symbol and not the 'service spanner'.
If it does vanish shortly after turning the ignition on, then it will be
the service spanner...NOT the unlock/immobiliser key symbol.

I'd still tend to think it could be the crankshaft position sensor....
we'll all have to wait n see.

[Gareth79]

Are you sure that it is the 'key' symbol and not the 'service spanner'.
If it does vanish shortly after turning the ignition on, then it will be
the service spanner...NOT the unlock/immobiliser key symbol.

The key symbol definitely appears for a fraction of a second when turning it on, in the central dot matrix display. I think this is probably normal to show that the immobiliser was looking for and then detected the key.

I'd still tend to think it could be the crankshaft position sensor....
we'll all have to wait n see.

Cutting a very long story short, there is definitely power at the pump, what threw me was that I now assume the ForFour doesn't prime the pump when the key is turned to the 'run' position - it is only powered when the engine is starting, and when the starter is off it's powered off if the sensor data is bad.  I did short the Fuel Pump 2 output pins but stopping and starting it made no repeatable changes, so I am currently assuming that it's probably ok.

Anyway I have a sensor arriving tomorrow and removed the crank angle sensor earlier.  It is somewhat blocked by the air-con compressor and so I figured it was easier to move that to give better access. With shallow torx bits or a flexible drive it would be easier to get out with the pump in place, but the compressor can be moved to the side easily enough.

The camshaft sensor is the same part (MR985041), so if replacing the crank angle sensor doesn't work I'll try it there instead.

[Gareth79]

Ok, so fitted the new sensor in both the crank angle and cam shaft position sensor locations and no change!  The new sensor is Febi Bilstein, absolutely identical to those in the car.

[Problemchild]

i wouldn't have expected that to be the issue if it turns over and runs a little when you used the brake cleaner

also i would expect EML if a sensor was telling the ECU to cut fuel or spark

have you tried the 2nd key?

have you added more fuel anyway??

the only time this happened to me was when the fuel station tank ruptured and i had 3/4 water 1/4 furl in my tank - but my car broke down within 200m

throttle valve not opening properly? can visually inspect

you have ruled out intake issues and battery/electrical by proving it fires with the brake cleaner

it can only be a fuel issue

JJ

[CrazyG]

As  JJ has deduced...it is either fuel or the immobiliser/keyfob.
The immobiliser is a separate circuit tothe alarm/unlock, and
the key symbol should not appear on the dash display at all...
leastways I've never seen it on either of my 2x CDI's or Brabus.
All I have ever seen is the spanner symbol (or 2 spanners) when
it's coming up to service time.

If you have the 2nd key, change the battery in it first (that is if it
has not been changed in a year), and then try that key to see IF
you get EXACTLY the same display and starting problem.

[Gareth79]

The key appears very very briefly, for maybe 1/10th of a second, blink and you'll miss it   I don't have a second key unfortunately, the owner's ex-wife had it and apparently they had an acrimonious divorce (the car was hers but he got it as the settlement, she had his nearly new Mondeo...)

Anyway I have ordered a used fuel pump from a decent breakers on ebay (£24), unfortunately too late for doing it this weekend but I'll get started anyway.  If that fixes it I'll take a view on whether to fit a brand new one (Bosch is ~£125) or just stick with the used one.

Thankfully I still have the Leaf for another month or so!  When that goes back I think I'll definitely get another second car so I can mess around like this without worrying about having transport.  Probably a K12 Micra, I had a K11 and it was pretty decent, albeit a death trap in this day and age.

[cecil]

New keys are available only from Merc- as stated on forum already. You will need a week and about 160

[Gareth79]

Yes, that's why I've not bothered to get a spare.   I did call the local dealer in Guildford for a price on the parts I might need for this problem, but the parts department went home half an hour before the published time on the website.  The receptionist took my details to get them to call me but that was several days ago, so for everything else I think I'll stick to Amazon Prime and eBay for reliability :p

[cecil]

When you have eliminated the impossible, whatever remains, however improbable, must be the truth. You may well end up at Merc in the end. Why not try Brooklands- at least you can have a look round while waiting. They also have experience of sorting mine out!

[Gareth79]

Hurray, we are back running!

I got a replacement fuel pump off ebay (£24, from a 2008 Colt), while fitting it I noticed the fuel level was VERY low indeed - the bottom of the pump sits in a hollow in the tank and it was just reaching the top of that.  I fitted the new one and there was no change. I then went to the petrol station and got another can full and after adding that... it starts and runs fine!  I was planning to do so after it was suggested earlier, for some reason I didn't get around to it.... lesson learned there.

The gauge is now showing half a tank so obviously there is a problem somewhere, I'll trace the wires for the sender and check the connections.  I measured the resistance of the old and new pump senders and they are correct (112 ohms when empty, 11 when full).

One thing about the pump lock ring - my hands are far too weak to turn it by hand and drifting it with wood was unsuccessful (I broke a spline).  I bought this universal tool off ebay and it worked a treat:
http://www.ebay.co.uk/itm/201544783714

I first tried it sitting on top of the lock ring but it wasn't getting enough purchase, then I moved it wider to the sides and it worked fine.

edit: Thinking about it, the resistance increases as the fuel level drops (a fail-safe setup), so a bad connection would show lower/no fuel.  I'll look some more tomorrow!

[CrazyG]

I wish you luck in resolving this last little issue that you have,
but may I suggest you undertake a secondary check; if you have
the time; and for your own satisfaction checkout the fuel gauge
(even though it's not showing the 'known issue' of flashing), and...
as a preventative measure rather than a cure, resolder the
connections shown in the Guide here :-
http://www.forfour.co.uk/forum/index.php?topic=4481.0
that way at least you'll then know you won't have that to fix in the future.

[Gareth79]

Thanks, I'll try that tomorrow it may well be related.

[Gareth79]

Well that was easy!  Resoldered the SMD resistors as instructed, and the gauge seems to read correctly now!  Thanks for the suggestion.  Strange that adding the half-can of petrol on the roadside didn't work.  Lesson learned to always carry a full petrol can.