W169 sometimes starts, sometimes not ...

[Tombob]

Hi all,
My 2006 A170 petrol turns over willingly but sometimes will not fire up. When it does start it runs fine but I've not strayed far in case it won't start while away from the house.
I had it towed to my friends garage. They said it had so many faults they just cleared them to see which would come back ... but none have even though it continues to sometimes start, sometimes not. I'm not sure what scanner that use but they're a commercial garage and have serviced Mercedes for many years so I imagine it's reliable.

They're able to start it with Aerostart and it stops after they stop spraying so they figure it's fuel related. They suspect not the fuel pump as that would likely show irregular running so they now suspect the relay.

I've searched the forums and the closest I've found is https://forums.mercedesclub.org.uk/index.php?threads/a200-w169.174643 but this is unresolved so not sure if related.
I don't want to go down the road of guessing with my wallet so I'm wondering if anyone has any thoughts or comments?

 

[ajlsl600]

Check fuel pressures. (would have though code logged if pressures outside spec) Check out any crank sensor and battery. Crank sensors are not 100% reliable, they report start attempt to ecm based on programmed cranking speed if they don't see the the required cranking speed the fuel delivery won't commence.
At least that's some of my previous experience.

 

[Tombob]

Thanks @ajlsl600.
The mechanics haven't had the time to exchange the most likely candidate (the relay). Apparently access to the relay plate is difficult.
I said I'd do it and the car drove back just fine ... but hasn't started since. I've now discovered accessing the plate is an entire forum topic of its own - everything is easy up until moving the carpet aside. I suspect I'll damage the carpet unless I remove the passenger seat and slide the entire carpet "assembly" back to expose the plate. Seems ridiculous ... am I missing something here?

 

[Tombob]

Update.
Removing the passenger seat and the glovebox seems a bit extreme but that gave me clear-ish access to the relay panel. What a PITA!
I removed the fuel relay and bench tested it - it passed. I wondered if it was failing under load (bad contacts?) so I took it to the local auto elec who said it will be fine as the pump load is not great. I guess that leaves me with the crank sensor ...

 

[mattkh]

..... so I took it to the local auto elec who said it will be fine as the pump load is not great.

Did they clean the contacts of the relay for you..?
If you have not pushed the relay back in, please put up a photo showing the flat pins.
Also, what is the mileage..?

 

[Tombob]

Hi @mattkh.
The relay is just a plain 'ol Normally Open with the usual 4 contacts. The elec didn't prise open the relay but said if it powers a 12v tail light bulb then it'll be OK, which it does. I've had the relay back in and (with the carpet pulled back) I can hear the relay and feel it "click" with one hand as I turn on the ignition with the other.
Attached is a pic from ebay of an identical relay, including the same embossed part number (A0025421319). Car is about 90,000Km.

A new crank position sensor will be delivered today. I guess I'll just buy another relay, which I imagine will be cheap and since exposing the relay plate was the real hard work (seriously, that plate was more sealed-in than Tutankhamun).

One thing I've not mentioned is this began after I washed the car. I didn't think this was relevant as there's no dampness under the dash, in the fuse box, on the relay plate or under the carpet. Also, the car has driven after that so any engine bay residual moisture will be long gone ... just the unreliable starting remains ... and no fault codes.

 

[Tombob]

Swapped out the relay. Too early to say but so far it's started 4 times in a row, which is a recent record.
I'm guessing it was bad contacts in the old relay, which @mattkh implied (... and the elec said it wouldn't be!).

I'll update in week or so. Thanks all for help.

 

[Tombob]

I declare the car fixed. A relatively simple issue made very difficult to even test because of where Mercedes hides the relay panel.

Thanks all for the help.

 

[FiveAlive]

Glad you've sorted it. That's the problem with the W168 and W169 - great concept but designed under the assumption that nothing would ever need fixing.