R129 m113 500sl starting issues

[DrNick]

Yesterday, after just standing for a couple of hours, the SL wouldn’t start.
it would crank, fire and then stop.
after 3 attempts it did actually run and it lasted about 20 seconds ( long enough to move it about 10 feet) before dying. It then tried to fire but died.
Left it overnight and tried it this morning. It fired up, nearly stalled then caught. It idled ok but was lumpy. Tried several starts with both keys and although it did start it was lumpy. It did rev smoothly though. It then started showing the same symptoms on restart (crank, fire, die)
so I’ve now left it alone..

up until then it’s been behaving impeccably.
I changed the crank sensor a couple of hundred miles ago because it was showing slightly different symptoms (occasionally cranked for a long period before firing up and then running ok - no dying) and that seemed to be cured.

The new crank sensor was a Bosch (at least marked as such). I suppose it could be a suspect, but is there anything else that springs to mind? Does it have a cam sensor and would that give these issues?
No check engine light.
r129 sl500 1999 75000 miles

 

[Gbrowncls55]

faulty new crankshaft positioning sensor would be my bet.

 

[alabbasi]

If it's the crank sensor, it would throw codes. You might need to go through the 38pin connector to read them, but they'll be there.

 

[DrNick]

Any idea if a cheap ELM327 would pick this up or does it need something more detailed. I haven't got anything at the moment but only want to spend the money/time once!

 

[Doodle]

I don't believe that the 129 ever received ODB2, so it's going to need something more specific

 

[alabbasi]

96 and later model R129's had OBD2 in the US but it only covered basic codes. The crank sensor might be one or it might not. All my more advanced scan tools will require connection through the 38 pin connector under the bonnet. I believe that there is a cheap foxwell that also has a 38 pin adapter.

 

[Bellow]

Here's the tool you need for diagnostics.

 

[DrNick]

My understanding is that the later ones are OBD2 but with a 38 pin round connector. Trouble is, the definition of "later" is very vague and there doesn't seem to have been a smooth changeover to OBD2 and even then its not clear what codes are actually available. There are low price 38 pin to 16 pin adaptors on Ebay. I could get a cheap ELM327 clone and a cheap 38 to 16 pin adaptor and have a go. You never know, but its worth asking here if anybody has tried this, as there is a fount of knowledge here, but sometimes things are cheap for a reason.
My first stop will still be the crank sensor as I've got one on order now, but as I say, I'm concerned about getting it out easily as the last one was hard and the replacement was tight going in!
Perhaps I'll upgrade to homestart so I can get it moved to a garage!

 

[DrNick]

Here's the tool you need for diagnostics.

A plug spanner? I've got one but I'm not sure what help it will be at the moement. It fires every time then dies. Occasionally it fires and runs and then it runs smoothly. On a first thought, that says to me the plugs and fuel are probably ok. What I'd be interested in knowing is what failed/failing sensor would allow it to fire every time on the first turn of the key but then immediately die? (but then sometimes run ok!). Appreciate it could be many things and eventually I'll end up getting it looked at by someone with experience, but it might be something I can deal with and at the moment it still interesting!

 

[Bellow]

Wet plugs = no spark. (Confirm with check for spark).
Sooty plugs = rich mixture - which has to be a suspicion given it is generally better behaved when cold and worsens as it warms.

It is what is happening (or not happening) in the combustion chambers that matters. The plugs can answer that.

 

[Doodle]

If blink codes are still viable under the later protocol then that's a really cheap (if somewhat more effort) option to read the output.

 

[Wearsthefoxhat]

Hi, yes they do have cam sensors too and quite likely to be the culprit. The cps usually goes open circuit once warm, the car stopping after twenty mins then restarting after ten, when cool.
I don’t think the 38 pin to 16 obd adaptors work. iirc each computer needs to be interrogated individually needing a diagnostic with a multiplexer..

 

[Dickster]

A long shot but make sure your battery is healthy. I had a rough running issue on my 86 911 and rebuilt the MAF sensor, replaced the fuel filter etc. It turned out to be a dud cell in the battery. The motor turned over but the battery only had 10/11 volts. New battery and it runs great now.

 

[DrNick]

I had a poke around near the battery to have a look at the fuel relay. Next to it there is an orange relay, which I think is a overload protection relay.
Its in a bit of a state as you can see from the picture. I obviously need a new one, but what does it do? Would it have any bearing on the current issue?
Its seriously bent and will require a lot of force to put it back in shape so something significant has happened to it, but I don't think recently as I havn't been near the battery for ages!
(It came out like this and the 40A fuse just dropped out)

 

[Will]

My money is on fuel delivery - probably the fuel pump/s sticking or not receiving sufficient power.

You can check fuel pressure on the rail to the injectors easy enough, should be easy to rule out either way

 

[DrNick]

I had a poke around near the battery to have a look at the fuel relay. Next to it there is an orange relay, which I think is a overload protection relay.
Its in a bit of a state as you can see from the picture. I obviously need a new one, but what does it do? Would it have any bearing on the current issue?
Its seriously bent and will require a lot of force to put it back in shape so something significant has happened to it, but I don't think recently as I havn't been near the battery for ages!
(It came out like this and the 40A fuse just dropped out)

Heated rear window, it turns out!

 

[DrNick]

A long shot but make sure your battery is healthy. I had a rough running issue on my 86 911 and rebuilt the MAF sensor, replaced the fuel filter etc. It turned out to be a dud cell in the battery. The motor turned over but the battery only had 10/11 volts. New battery and it runs great now.

I'm charging the battery and will check its voltage after . It seems strong enough but everything is old so everything is under suspicion.

 

[DrNick]

A long shot but make sure your battery is healthy. I had a rough running issue on my 86 911 and rebuilt the MAF sensor, replaced the fuel filter etc. It turned out to be a dud cell in the battery. The motor turned over but the battery only had 10/11 volts. New battery and it runs great now.

I like the sound of this. The battery has gone flat a couple of times over that last year quite quickly in my view (a few weeks of non-use. I've mostly had it on a C-TEK conditioner though and it obviously doesn't go flat then.) and so is likely to be not the best. But it has been fine in everyday use and has shown no typical symptoms of a poor battery. Everything has been perfect until the no start issue.
What could be happening is that there is enough current to kick the engine over (it fires immediately every time) but then the battery voltage drops and all the electronics drop out, and it doesn't recover quickly enough to maintain the start. That would explain that when it does actually start its ok. It also explains why the problem has appeared recently with the advent of the cold weather. Bad time for batteries.
I've got it on charge at the moment so I'll check the voltage drop next time the sun comes out. But I now suspect something internal in the battery. I'll check the date on it as well, but it sounds like it might benefit from a new one anyway. They don't like to go flat but I thought I'd got away with it. I'll report back...

 

[Will]

I don’t think it’s likely to be the battery - if it has enough charge to turn the starter it should keep running - the alternator will be putting out enough to keep the relatively low power required for the fuel pump and ignition etc. By comparison the starter will be far more demanding, you’d notice a weak battery on cranking.

Easy to test that theory - stick a voltmeter on the battery

Have you checked the earth strap?

I’m still thinking fuel delivery - ie fuel pump issue, there should be a schrader type valve on the fuel lines where you can check fuel pressure when this happens, could give some clues.

 

[DrNick]

Charged the battery. It’s a warmer day. Battery voltage started at 13.15v, drops to 10 V on crank but it does start, although not as cleanly as it has done prior to the issue.
I’m monitoring the battery voltage now to see if it drops anyway. Currently at 12.8v after 3 starts.