2008 w211 e550 hard start

[Kiwiming]

Hey guys,

Have been having this problem for a couple weeks now but in the last week it has gotten worse and worse.

Usually first start in the morning it starts no worries. But after driving it around and turning it off, when I go to restart it it won't start.

Have checked sparks, coils, fuel pump relay
Replaced cps due to code but the problem still persists.
Had a bunch of misfire codes as well. The new cps seemed to fix the miss fire and very slight miss fire.

Went to go crank it about 5 times and my battery almost completely died. Was on the phone with a tow truck and decided to try one last time and it suddenly started straight away.

Other than checking fuel pressure and pump tomorrow is there anything else I can try?

Battery seemed to die quite quickly after trying to crank it yesterday also.

Car seems to acellerate with no problems and never stalls. Could this just be a dying fuel pump causing intermittent hard starts?

Thanks for your help!

 

[Kiwiming]

Also noticed the rev counter moving when trying to start after replacing sensor which is the first time I've noticed it.

 

[John Laidlaw]

CPS would have been my first thought- could the replacement be faulty?

 

[Botus]

if the battery is worn out, then the voltage under cranking will be woefully low and lots of systems will complain (instruments wobbling is quite a good indicator the voltage is way off what's needed)

if your leaving on its own for longer than 3 weeks expect fun even with a brand new battery.... if the battery is >4 years old its probably had it

 

[Kiwiming]

Forgot to mention after the battery died I put a fully charged battery with same ratings in it and still wouldn't start, so had to get a tow truck

 

[bembo449]

Live data when cranking to see what CPS is reading

 

[sonic]

Forgot to mention after the battery died I put a fully charged battery with same ratings in it and still wouldn't start, so had to get a tow truck

The second battery was fully charged but what was its current capacity?
Before anything else I would put a brand new battery on the car, & try that.

 

[Kiwiming]

Okay awesome, will do both these things today and let you know. Thanks for your help!

 

[Kiwiming]

Turns out battery and fuel pump are fine. The person who has it now suspects it is the ignition key. Could this be correct?

 

[Kiwiming]

I've given them the spare key. The car didn't start first thing this morning. Now it's starting no problem with the first key. They are waiting until it doesn't start again and will try the spare key. Does this sound like the problem ?

 

[Botus]

….not an expert on merc ignition switches and oddly you need to know about these things on these cars they are odd....

if the key won't turn often an ignition switch issue
if the key turns but fails to crank that's another
if the key turns, the car cranks, but its slow, its usually the battery or starter motor
if the key turns off, but the car keeps going that too is quite common...

 

[alexanderfoti]

If it turns over, its not the key. Really needs a good run on diagnostics, to see if you have cam/crank sync, start auth (I would expect so) and a few other bits.

 

[Kiwiming]

If it turns over, its not the key. Really needs a good run on diagnostics, to see if you have cam/crank sync, start auth (I would expect so) and a few other bits.

Would you recommend replacing cam and crank sensors at the same time? Also do these need to be synced using diagnostic software? Thanks bud

 

[alexanderfoti]

No, I wouldn't recommend replacing anything until you have done further diagnostics. No syncing required.

 

[Kiwiming]

So turns out it isn't the fuel pump and the mechanic now suspects the rear sam is faulty. The fuel pump doesn't get any power sometimes on start up. Fuse and relays replaced. What are the odds of a faulty rear sam? No other faults other than hard start