R129 500SL (1990) - Just won't start

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gcman50

New Member
Joined
Aug 28, 2020
Messages
6
Location
Dublin
Car
1990 R129 500SL
Hi guys,

Would really appreciate a steer here.
1990 R129 500SL parked up for 2 years. Started a week ago but it was in a small garage and because of fumes it was only run for a few seconds. With hindsight maybe not the smartest…
Won’t start now, just turns over. Battery very strong, immobiliser believed to be ok. Fuel tank drained, new plugs. Nothing.
Getting fuel to the Bosch distributor ok, getting pressure out (x7, one was awkward to reach) of the distributor. Dried plugs, nothing.

How much (old?) fuel sits between the distributor and injectors? Lines look very narrow, can’t be that much...
Forums point to Mass airflow sensor (not sure these early cars have them?) or Crankshaft position sensor? Is the second stage of the fuel relay not happy with something? Are there sensors which don't allow it to stay running versus don't allow it to start running? Fuel and spark and no fire a little odd…but perhaps its a sensor somewhere..

Any assistance greatly appreciated.

Thank you
GCMAN50

Note - my R107 and W123s have never given trouble like this !!
 
Hi, thanks for suggestions. Have tried easystart and yes I do have spark..
 
If you have a spark easy start should get it firing.
 
If you have a spark easy start should get it firing.
Thanks. I am going to try new Bosch plugs. Presume Bosch best for these cars? I would normally use ngk but prob not here...
 
I use NGK.

Are you sure you're getting a spark? I've had problems with mb distributor caps and rotor arms.
 
I would be looking at the distributors. Slight damp or cracks ??.
Nothing wrong with NGK plugs, I use them too
 
 
Sure it isn’t just flooded? My 190 has always hated being switched on and straight back off. My E39 was terrible for it too.
 
I bet its the distributor caps and rotors...these earlier 500's are a Bit*ch to start if they have been left standing and even if the caps look ok, for some reason they just seem to suffer if left. Engine will spin over, but won't catch and it seems as if fuel isn't getting to the injectors.

The other thing I can recommend, is don't go for the cheapest set of caps and rotors you can find....I've been down that route and it ends up in disappointment. Bosch always seems to work best for me
 
Hi Graeme, thanks for attaching. Gone through the adjacent thread. Any of the 'worth checking' suggestions have already been covered and slightly different in that it is not starting. It is getting spark and good fuel pressure but no detonation. My mechanic has never seen the likes before:

1) Tank drained, fresh fuel
2) Fuel pump working (stage 1). Stage 2 is only when engine starts so I don't believe to matter.
3) Spark plugs changes - several times
4) Disty caps cleaned
5) Camshaft & Crankshaft position sensors have resistance.
6) All fuses good, inc OVP good.

Getting fuel into Fuel Distributor. Good pressure x 8 out of it.
Car started a week ago.
Not using OEM alarm, Toad unit fitted.
Late 1990 car - non CAT.

Has a 9 pin round and 16 pin (of which 8 are in use) - what appear to be diagnostic terminals.
From my research the 9 pin gives limited value when engine can't be started. It checks CPS which we know to be sound.

Tried WD40 in bores and air drying, (bores look bone dry). Turns over but no bite..

Wits end now...FD.JPG
 

Attachments

  • 9 pin (1).JPG
    9 pin (1).JPG
    1.7 MB · Views: 3
  • 16 pin.JPG
    16 pin.JPG
    44.1 KB · Views: 4
  • Nearside Rear EB.JPG
    Nearside Rear EB.JPG
    1.8 MB · Views: 4
Hi Graeme, thanks for attaching. Gone through the adjacent thread. Any of the 'worth checking' suggestions have already been covered and slightly different in that it is not starting. It is getting spark and good fuel pressure but no detonation. My mechanic has never seen the likes before:

1) Tank drained, fresh fuel
2) Fuel pump working (stage 1). Stage 2 is only when engine starts so I don't believe to matter.
3) Spark plugs changes - several times
4) Disty caps cleaned
5) Camshaft & Crankshaft position sensors have resistance.
6) All fuses good, inc OVP good.

Getting fuel into Fuel Distributor. Good pressure x 8 out of it.
Car started a week ago.
Not using OEM alarm, Toad unit fitted.
Late 1990 car - non CAT.

Has a 9 pin round and 16 pin (of which 8 are in use) - what appear to be diagnostic terminals.
From my research the 9 pin gives limited value when engine can't be started. It checks CPS which we know to be sound.

Tried WD40 in bores and air drying, (bores look bone dry). Turns over but no bite..

Wits end now...View attachment 101623
Forgot to mention - cold start ok too
 
Check the engine earth strap and its connections- high resistance may lead to significant voltage drop on the engine electrics when turning over.
 
Right, problem sorted - glad to say. Thank you all for support.
A specialist MB dealer diagnosed as a faulty component in the immobiliser. It was getting fuel and spark but not the required pulse signals to run. He has bypassed and starts first time every time. Happy days !
 
Right, problem sorted - glad to say. Thank you all for support.
A specialist MB dealer diagnosed as a faulty component in the immobiliser. It was getting fuel and spark but not the required pulse signals to run. He has bypassed and starts first time every time. Happy days !

Right, problem sorted - glad to say. Thank you all for support.
A specialist MB dealer diagnosed as a faulty component in the immobiliser. It was getting fuel and spark but not the required pulse signals to run. He has bypassed and starts first time every time. Happy days !

Right, problem sorted - glad to say. Thank you all for support.
A specialist MB dealer diagnosed as a faulty component in the immobiliser. It was getting fuel and spark but not the required pulse signals to run. He has bypassed and starts first time every time. Happy days !
HI,Just read your post, i am having the same issue as you had on my 91 SL,DO you know how your MB diagnosed your fault component and how did he bypass it? stay safe S-age
 
Well my guess is they used their Star to find the fault and also dial out the immobilliser,suggest you find a good indy hear you.
 

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