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  #1  
Old 06-27-2003, 02:11 AM
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Cold starting problem on '72 3.5 280SE

My "new" 108 performs really nicely, but it has a real problem at start up in the morning. It starts on first turn of the key, but once moving, misses and has a lack of power until it reaches 175F on the temperature dial. Then it runs like a smooth swiss clock!

I have tested this by not driving it and letting it warm up on the driveway and it is a smooth take off if I leave it until it reaches normal operating temp (175F).

Is there a cold start device here (that I am not familiar with because I am new to 108's) that may be playing up?

Any advice appreciated.

Richard Zammit


Last edited by zambo; 07-04-2003 at 07:35 AM.
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  #2  
Old 06-27-2003, 10:39 AM
jobber
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It sounds too obvious but try changing the cold start injector valve. Its easy but expensive. The only one I could find was a through an MB dealer and cost $173(Aust). Its a Bosch part so you could try a bosch dealer for a better deal.

Its located right on top of the front of the engine has a fuel line from the injectors and a wiring connection. Two allen bolts and its done, mine was leaking. You could try seeing if it works by pulling it off the manifold, leaving it connected to the fuel line and starting the car from cold. If fuel sprays out it works I guess, although it should shut of when the engine is warm. Obviously be carefull atomised fuel can put your cigarette out.

There is also a supplementary air valve that supplys additional air during cold starting. You can take this off and test it. It should be fully open at -20c and fully closed at +65c. Its pretty close to the starting valve and has a hose in from the air cleaner and out to the manifold. The Sensor sits in the coolant line.

John
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  #3  
Old 06-27-2003, 12:11 PM
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Not an expert, but I believe that the cold start valve supplies raw fuel only for a very short time so it would not affect warm-up to 175. Think the air valve is a more likelyl suspect.
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'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #4  
Old 06-27-2003, 07:34 PM
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Sounds more like a bad temp "feeler" (sensor) in the coolant crossover in the intake manifold. It seems that it isn't responding to engine temp.

Could be a bad wire, too, check. You must remove the air filter housing, it's on the passenger side, the connetor with a rectagular plug like the one in the air filter housing.

You may also have oil in the trigger points -- this will cause stagger and miss until the oil works off at higher temps. They are in the lower part of the distributor -- you must pull it to clean them.

Peter
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  #5  
Old 07-01-2003, 05:53 AM
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Update

Thanks for the replies. Before I rushed off and started replacing bits, I decided to watch the problem over the last few days.

Interestingly, the missing when cold only happens when dead cold from overnight. We are in winter here in the southern hemisphere, but as I live in the subtropics, our overnights are only down to maybe 7-11 degrees Celsius - not cold at all.

So whilst the morning mis-firing continues until the car is warm, if I leave the car parked for 8-10 hours at work and the temp gauge reads at it's coldest, I can start the car at the days end and drive off without a single mis-fire.

Does this narrow any of your previous views on what the cause may be, down at all.

Thanks in advance.

Richard Zammit

Last edited by zambo; 07-01-2003 at 07:10 PM.
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  #6  
Old 07-01-2003, 10:42 AM
jobber
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Your right about the cold start injector, at -20c it only lasts 12 sec, decreasing as the start temp increases to zero at 35c. This is controlled by a thermostatic time switch(sensor), I think thats the one peter says could be a problem. Although there are several other sensors reading air and coolant temp that could also be confusing the ECU. Well worth pulling of the air cleaner and checking all the sensor wiring plugs for bad or shorted connections, there often frayed or corroded.

I'm guessing and assuming days are pretty mild in Brisy, even in winter, the air valve is not working correctly. When the engine is not so cold the lack of extra air is not a problem. Try waiting for a really cold day, and see if you get the problem in the afternoon as well. Also try driving off at various temps in the morning to see if there's a consistent point it smooths out. This is a fairly complex engine and these are only the most obvious possible solutions, as peter points out there are plenty of other possibilities.

good luck
John
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  #7  
Old 07-01-2003, 07:08 PM
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Thanks Jobber.

The smoothing out happens just about spot on at 175F - prior to that it misses (when put under load - idles fine) and refuses to accelerate to full power - after 175F, it's smooth as silk!

I have tried pulling away at various temps below the 175F mark and it makes little difference to the "rough" behaviour.

I'll do the various checks that have been suggested and see how I go - thanks to all.

Richard Zammit

Last edited by zambo; 07-04-2003 at 07:34 AM.
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  #8  
Old 03-05-2004, 12:21 AM
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Trigger Point misfire

I had the same symptoms on my 280 SEL 4.5. Due to a bad distributor shaft seal ( not 0-ring ) oil entered the trigger point assembly and fouled the contacts. The distributor was rebuilt although it can be expensive,and I replaced the trigger unit and now the car runs like a clock.
Try getting a used or junkyard unit, you might get lucky. Also check for shaft wear and play.
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  #9  
Old 03-05-2004, 02:05 AM
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Thanks Oldspeed!

I thought that mine was starting to get the same symptoms again just recently. It had been 7 months since a clean of the trigger points cleared everything up.

As it turned out, whatever it was that was causing the short term misfire from cold went away. I figure it could have been a bit of crap in the injector system or something else - either way it disappeared.

Even if it was a bit of "blow by" causing the trigger points to gunk up, the removal of the distributor is pretty simple and not a chore to clean every service (i.e. 6 months or so).

But if it gets worse, I will take your advice and look for a new unit.

All the best

Richard Zammit
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  #10  
Old 03-05-2004, 12:12 PM
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Quote:
Originally posted by zambo
Thanks Jobber.

The smoothing out happens just about spot on at 175F - prior to that it misses (when put under load - idles fine) and refuses to accelerate to full power - after 175F, it's smooth as silk!

I have tried pulling away at various temps below the 175F mark and it makes little difference to the "rough" behaviour.

I'll do the various checks that have been suggested and see how I go - thanks to all.

Richard Zammit
maybe running lean? due to a fault in the coolant temp sensor/wiring? or maybe the air temp? its an easy check

mike
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  #11  
Old 03-05-2004, 01:09 PM
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hey zambo...

Off topic here, but, I have a picture of your car on my wall!! I just purchased Mark Barrett's old euro headlights and he sent me a pic of your auto before he sold it to you (and since the headlamps are actually the ones originally in your ride.) I happened to mention seeing a thread on the M-100 site about rev counter problems in a 3.5 in OZ and he gave me the history/connection, etc. Tiny world, eh wot?

On the topic -- my 4.5 has reverted to its balky warm idlle and it sounds as if I may have some of the issues as do you. Wish mine would just clear up of its own accord. I just redid the plugs and wires as well as replaced the leaking fuel pressure regulator and it seemed quite better for a few weeks. Still must pull the distributor and eyeball/clean the injection trigger points, figure it couldn't hurt (unless I put the distributor back in wrong!)

Take care of that good looking ride...

kevin
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  #12  
Old 03-05-2004, 02:42 PM
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If it's any consultation, my 4.5 does the same thing. Although the rough running appears to be from being to rich until the temp hits the magic 175. Car starts instantly, idle starts low with black smoke, slowly builds up and then drops off to normal as it appoaches the 175 mark. Then all is well until it cools off again.
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  #13  
Old 03-05-2004, 10:51 PM
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Seriously, is there a 3.5/4.5 that DOESNT blow black smoke under heavy/full throttle until the engine reaches 175/80? Doubt it!

I'd suggest cleaning the trigger points as well. Mine did the same thing until it got warm and cleaning them out worked like magic - ran like a top when cold!
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  #14  
Old 03-06-2004, 04:11 AM
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Thanks Kevin for the heads-up on my Euro headlights - I was wondering when Mark was going to send them to me! Just kidding (LOL), Euro's are standard here and cheap as chips to buy from a wrecking yard, so I hope Mark did a good deal on them for you. FYI - stacked headlights as per North American standard issue are expensive here in Oz - maybe a grand to twelve hundred for a good set - our dollar is about 78c to the US$ so you can figure the math.

Mark talks online to people about the car he sold me quite a lot - I am beginning to wonder if he regrets selling it ... though he has a cracker of a 3.5 himself - better than mine.

As to all the other bits and pieces on this re-invigorated topic, I am pretty convinced that Tom's call is right - these funny warm up issues can be a range of things, but the trigger points seem to clear up a lot of the cases I have heard of (and experienced once!).


All the best to all

Richard
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  #15  
Old 03-06-2004, 11:44 PM
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The best thing you can do for your 3.5 is buy the benz service manual. There is a complete and concise rundown for diagnosing the EFI system. Either pick up the whole engine/body manual (the 108/109/111/113 "starting 1968" book) or get the 3.5/4.5 engine manual, which covers the cast iron block v8s to 1977 or so.

Either book can be had on ebay for under 50$US if you look carefully. They also sell for more than that to the folks in a hurry.

The specifics of just the EFI system can be found in the Bosch service books that cover the D-jetronic (d-jet) system.

-CTH

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