R129 SL500- It never rains but it pours -Transmission, Injection, Thermostat...

E55BOF

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Well, I fitted the new ignition leads to my 1994 R129 SL500 today. It now has new engine wiring harness, distributor caps & rotors, ignition leads, and plugs. The regular misfire is gone - Hurrah!

HOWEVER... It still runs roughly, and misses, splutters and holds back under throttle - quite badly; it's really not realistically driveable, though it could limp a few miles to be fixed. Additionally, the thermostat appears to have come out in sympathy; it took AGES to warm up, while being driven, not at idle, and never did reach the 80 degrees on the gauge that it should. Even when it did reach something like running temperature, it didn't run much better. Could the two problems be linked? Anything fairly simple I could check?

And there's more... The kickdown (four-speed 'box) doesn't. I presume this is likely to be linkage adjustment? It changes down normally using the stick, and changes automatically on part-throttle, but on full throttle going up a fairly steep hill it just sat there, throttle wide open, farting along at about 45 mph with the speed only increasing very gradually.


AAAARGH! Just b****y AAAARGH!!! :wallbash::wallbash::wallbash:
 

jonnyboy

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I'll give you a tenner for it.

Hard luck Dennis, I think you need specialist help.
 
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E55BOF

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Sold!:D (60% deposit will do for the moment).

I presume you mean the car needs specialist help, though on past evidence some would argue I need specialist help...
 

jonnyboy

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If the cap fits......!!!
I meant both of you.!
 

CAT220

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Well, I fitted the new ignition leads to my 1994 R129 SL500 today. It now has new engine wiring harness, distributor caps & rotors, ignition leads, and plugs. The regular misfire is gone - Hurrah!

HOWEVER... It still runs roughly, and misses, splutters and holds back under throttle - quite badly; it's really not realistically driveable, though it could limp a few miles to be fixed. Additionally, the thermostat appears to have come out in sympathy; it took AGES to warm up, while being driven, not at idle, and never did reach the 80 degrees on the gauge that it should. Even when it did reach something like running temperature, it didn't run much better. Could the two problems be linked? Anything fairly simple I could check?

And there's more... The kickdown (four-speed 'box) doesn't. I presume this is likely to be linkage adjustment? It changes down normally using the stick, and changes automatically on part-throttle, but on full throttle going up a fairly steep hill it just sat there, throttle wide open, farting along at about 45 mph with the speed only increasing very gradually.


AAAARGH! Just b****y AAAARGH!!! :wallbash::wallbash::wallbash:

Sorry to hear you're still having issues Dennis. The rough running, spluttering etc could be a bad coil pack. Do you have a local specialist that could take a look?
 

Charles Morgan

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I doubt coil packs if he has a distributor.

I wonder if it's throttle body issues, or even ECU.
 

jonnyboy

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ECU was my thought too Charles bearing in mind the issues with harness etc.
 

jonnyboy

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Actually, just a thought - if this is an early car, don't they have a vacuum connection on the side of the gearbox? I certainly recall it on my 300-24 anyway. If so, and it's playing up/adrift or whatever, then that could be both issues. no?
 
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E55BOF

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If it helps diagnosis/thoughts, my car has the alloy box full of ECUs on the passenger side rear of the engine compartment - it is post-OVP relay models.
 

Doodle

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Could the two problems be linked? Anything fairly simple I could check?

Yes. Change the temperature sender on the top of the block, it's not expensive.

The ECU relies on this to signal when the engine is up to temperature and thus come out of fast idle enrichment. If it's not reading correctly, the injection system will still be pouring fuel into the motor like a holed bucket which will hamper any further diagnosis.

ECUs are not coded to the car, so very easy to swap if you can find another to test with.
 
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E55BOF

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Good idea; I might as well use the new one I bought, but never fitted, when I was just trying to get the engine to run. Mind you, at present it's NOT getting up to temperature, and I can't think of anything other than a u/s thermostat that would cause that.

Does anybody know which of the ECUs in the alloy box is the engine one, or could anyone find me a part number - the VIN is WDB1290672F103359. There's plenty for sale on FleaBay.
 

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Hi Dennis;
Sorry to hear that you are still having problems. Just a suggestion but, do you not have access to an independent Mercedes specialist near to you that could diagnose the issues with the car? I only ask because you have already thrown a lot of parts/money at this car and have not yet got something that you can use! I know the specialist will probably give you a big bill but they at least have the skills, experience and tools available to diagnose faults. Constantly throwing parts at it is costing you a fortune for little or no return. In your position I think I would contact a good specialist and have the car collected and delivered to them.
I don't mean this to be in any way critical of you and I am not trying to be a smart **** but I think you really need help with this car and I would think that most people on here would like to see you get this sorted.
All the best whatever you decide to do,
Steve.
 
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E55BOF

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Thank you for the thought, but you have somewhat misinterpreted the situation. I bought the car expecting to have to replace the ignition wiring, rotor arms and distributor caps, and engine wiring harness, on an SL500 of this age, and that I have done; I suppose I might have known the battery would be kn******d as well...

Mechanically/electrically, that is all I have done so far. If I can find out the part number, replacement ECUs are cheap from breakers, and worth a try. New thermostats are cheap too.

The car is booked in to Wayne Gates on Monday morning anyway, to at least have the codes read. It will make it that far OK - it runs up to 80 mph without too much drama. Then we'll see; I doubt it will be hugely expensive on a (relatively) simple old bus like this.
 
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E55BOF

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And so far I have been correct:D So far...
 
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E55BOF

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Thanks for that; I've spoken to him before, when I was looking for a good secondhand engine wiring harness. He didn't have any hen's teeth, either...

As you say, he's good.
 

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Just out of interest, when you got the loom done by Si-lec, did you also get the throttle body harness done? I only ask because on mine the TB harness was way worse than the injector harness and could account for the sort of symptoms you describe.
 
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E55BOF

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UPDATE:

The car had to go on the back burner over Christmas, but I'm now getting back to it. I'm getting there.

The gearbox gets better every time I drive it, and I suspect it was just gummed up from lack of use; when I got it in October the car had only done 1200 miles since November 2011, and precisely two miles since it passed its MoT in February 2015. I’ve done 120 miles since, and it’s freeing up as time goes by; manual and part-throttle changes go through OK, if a bit sluggishly, though there’s quite a thump when it changes up at or near full throttle. All that may just be me expecting it to be different – all my recent experience of automatic boxes has been with my three E55s, which all had/have the 722.6 five-speed electronic box. The kickdown definitely doesn’t work, though, but as the four-speeder is a hydraulic box I think it probable that no more than linkage adjustment will be needed. I’ll have the fluid and filter changed anyway.

The engine is running a bit better too, but it's certainly not accelerating like 300 bhp, and there’s still a misfire, especially under load; I’m looking into that. I'll pull the No. 8 plug tomorrow, and see if it shows signs of combustion; if not, it almost certainly has to be the injector - all the ignition components bar the coils are new. STAR should be able to identify the cause if I can’t.

If it is the ECU (which I doubt), Carl at W140 Breakers has a pile of them around the £50 mark; not too pricey!

Still haven't changed the four-pin coolant temperature sensor - I'm waiting for a 22mm box spanner, which should arrive tomorrow. I haven't changed the 'stat yet, either. I presume the thermostat housing is the alloy housing at the engine end of the radiator top hose; if not, please tell me.

Does anybody think a good dousing of carburettor cleaner down the central air intake on to the MAF would be worthwhile?

And finally (for the moment)...the exhaust has now started blowing. If anyone knows of a source for exhausts other than the main dealer, now is the time to tell me!

More anon...
 
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jonnyboy

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Dennis, if you're not 100% bothered about originality, half the time these days its cheaper to have a bespoke stainless exhaust made. Doesn't have to be loud or brash, but may be cheaper and will outlast the car !
 

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