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'90 300SL ,'84 300DT
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Discussion Starter #1
Hello all. For the last 3 or 4 years I've had a problem that confounds me. I drive my '90 300sl about 1500 miles per year but even with New England winters it doesn't stay garaged for real long periods (perhaps, 2-3 weeks at worst) The car always starts quickly and 90% of the time it tends to run very rough for 5 -10 miles and then runs fine. I've changed wires, coil, plugs, rotor, and cap and it hasn't solved the problem. As I seem to notice it more after rainy weather, I've checked the cap and it remains clean and dry. I also use Techron with each tankful of gas as I was thinking of injector issues. Today, after sitting ~ 2 weeks I drove 6 miles to the store and it was choppy under 30 mph. I returned to the car 30 minutes later and also as usual it ran terrifically. Any thoughts? Thanks all....Ron
 

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'03 SL500, '03 SL55, '97 SL320, (2) '91 300SL, (2) '91 500SL, '00 S500 -- all for sale
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When the engine is at operating temperature and idle, I would measure the DC voltage between sockets #2 and #3 of the round connector located on the left-side of the engine compartment. It should fluctuate slightly and average 4-9 volts. The result would suggest what I do next.
 

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'90 300SL ,'84 300DT
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Discussion Starter #3
Hello Bob and it's good to hear from you. I will do as you suggest and will be back to you.
 

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Discussion Starter #4
Good morning Bob, the range fluctuates between the high 5's and mid 7's. Ron
 

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...the range fluctuates between the high 5's and mid 7's.
That range seems unusually wide, Ron, but I don't know what to conclude. I'd be inclined to have the ignition system tested by a technician.
 

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R129 500 SL 1991
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Ron, your readings indicate an off/on ratio (duty cycle) of 38% to 52%. That’s not too bad. Averaging out at roughly 45%, which is marginally low.

The wide range might be caused by the baseline KE mixture setting being slightly out or by not disconnecting and sealing the EGR vacuum line before taking readings.

A good target duty cycle is typically around 48%, but 45% isn’t too bad.

A low duty cycle might be caused by leaking cold start valve or injectors, but this would in turn cause poor starting.
So I suspect that your issue might be the ignition system, typically the distributor cap, given that it’s worse during rainy weather.

What make of cap did you purchase? I recommend Beru every time.

I think, but I’m not certain, that your engine is supposed to run on non-resistor plugs. The resistor is incorporated in to the end cap on the end of the lead. Typically this should be around 1800ohms.
 

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Ron, your readings indicate an off/on ratio (duty cycle) of 38% to 52%. That’s not too bad. Averaging out at roughly 45%, which is marginally low.

The wide range might be caused by the baseline KE mixture setting being slightly out or by not disconnecting and sealing the EGR vacuum line before taking readings.

A good target duty cycle is typically around 48%, but 45% isn’t too bad.

A low duty cycle might be caused by leaking cold start valve or injectors, but this would in turn cause poor starting.
So I suspect that your issue might be the ignition system, typically the distributor cap, given that it’s worse during rainy weather.

What make of cap did you purchase? I recommend Beru every time.

I think, but I’m not certain, that your engine is supposed to run on non-resistor plugs. The resistor is incorporated in to the end cap on the end of the lead. Typically this should be around 1800ohms.
That's all good advice. My only comment concerns using MB cap, rotor and shield. Beru MAY be OK but these items are highly critical in the M104.98x engine. I have Beru HT leads recently replaced for no real reason with NGK. Beru/NGK non-resistor plugs too. I've got some Bosch plugs too but they offer no advantage over Beru/NGK.

Sadly, the MB parts are MUCH more costly than the Beru; it's easy for me to spend your money!

Incidentally, my duty cycle is usually adjusted to around 50-55 %, slightly weak. Seems good though. I also notice no difference in % with or without the sealed EGR vacuum line. Don't know why.

Good luck.

RayH
 

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R129 500 SL 1991
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Thanks for your comments Ray. :)

The advantage of the Beru cap is that it is internally coated with an imide lacquer, which provides a much smoother gloss surface and improved surface hardness.

The gloss surface inhibits coalescence of water droplets on the surface from condensation of moisture from the air.

The surface hardness minimises adsorption (note ‘ad’ not ‘ab’) of moisture on to the surface of the underyling epoxy moulding, which inhibits the formation of electrical tracking.

Beru is a more superior design in my opinion, and ought to cost at least as much... in a fair world, but it doesn’t. So that’s great for all of us.

So it’s not about cost, but more about performance and function.

BUT, whichever make of cap you ultimately choose, ALL will inevitably succumb to the same overall design flaw, which will lead to condensation inside the cap(s).

If the ambient relative humidity exceeds 20-25%, you will get condensation. Fact. :)
 

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I'm not sure that my recently purchased Beru "emergency" cap has such a coating. It certainly doesn't have the coating that the MB rotor arm has - black.

Have you got a photo of the coated Beru product?

R
 

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I also notice no difference in % with or without the sealed EGR vacuum line. Don't know why.
According to a tech on peachparts.com (Stevebfl) the conditions for actuating the EGR should not be met, and he never disconnects the line.
 

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R129 500 SL 1991
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According to a tech on peachparts.com (Stevebfl) the conditions for actuating the EGR should not be met, and he never disconnects the line.
With regard to measuring duty cycle it is important to follow standard MB practice, which is to unplug and temporarily seal up the regen vacuum line ‘A’ at the purge valve before making adjustments to the KE baseline mixture setting (duty cycle) in order to obtain the correct duty cycle reading in the following scenarios: -

at high idle (>800 rpm) and normal operating temperature (70>T<95 deg C) when the regen switchover valve would normally be activated;
at normal idle (680 rpm) and the coolant temperature is greater than (>) 95 deg C when the regen switchover valve would normally be activated;
at normal idle in the case of a defective idle speed switch which is intermittently or permanently open.
at normal idle in the case of a defective purge valve.
 

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Discussion Starter #13
Hello folks and thanks for your replies. I must tell you that on a scale of 1-10 I am a 2 1/2 on a good day! While I was doing some other things today I was thinking that the problem always goes away and the "fix" is driving it for 10-15 minutes. So, today I was thinking about temperature. I ran the car in the garage for perhaps 15 minutes in 62 degree weather. I did the test again with the voltmeter and the reading was ~ 5.9 to 7.1. The car temp was around 80. Tomorrow, I will take it out for a longer ride and then take a new reading as I believe the car temperature is usually around 100 (F). To answer some your questions all the "tune-up" parts that I bought were Bosch and the cap/rotor/plugs/wires have maybe 2K miles on them. The coil I bought late last year and it has maybe 600 miles on it. After a rainy week earlier this Spring/Summer I decided to check the cap before I drove and there wasn't a hint of moisture; totally dry. 90% of the time it starts on the 1st crank and 10% on the second. However, the problem was there with the dry cap.

Thanks again for your efforts and I will be back soon. Ron
 

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R129 500 SL 1991
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Hello folks and thanks for your replies. I must tell you that on a scale of 1-10 I am a 2 1/2 on a good day! While I was doing some other things today I was thinking that the problem always goes away and the "fix" is driving it for 10-15 minutes. So, today I was thinking about temperature. I ran the car in the garage for perhaps 15 minutes in 62 degree weather. I did the test again with the voltmeter and the reading was ~ 5.9 to 7.1. The car temp was around 80. Tomorrow, I will take it out for a longer ride and then take a new reading as I believe the car temperature is usually around 100 (F). To answer some your questions all the "tune-up" parts that I bought were Bosch and the cap/rotor/plugs/wires have maybe 2K miles on them. The coil I bought late last year and it has maybe 600 miles on it. After a rainy week earlier this Spring/Summer I decided to check the cap before I drove and there wasn't a hint of moisture; totally dry. 90% of the time it starts on the 1st crank and 10% on the second. However, the problem was there with the dry cap.

Thanks again for your efforts and I will be back soon. Ron
Did you mean deg F or deg C? Anyway, the duty cycle test is normally performed at 80 deg C.
Have you checked the cap for moisture after a short drive? Typically the cap will be dry when you set off, but wringing wet 5 minutes later. It is caused by the different thermal conductivities of the alloy head and epoxy dizzy cap. The air inside the cap is heated up more quickly by the alloy cylinder head, but the cap lags behind by around 10 deg C for first 10mins of driving. A dew point occurs at the inside surface of the cap and condensation forms. The more short drives, the worse the condition gets because hot air holds more moisture than cold air. So when you go for a short journey, the hot air inside the cap draws more moisture in from the outside ambient air. The moisture concentration of the air inside the cap increases (because warm air can hold more moisture). When the engine cools over night, the moisture condenses on the alloy cylinder head, way back inside the recess for distributor behind the insulator. Next time you start the car, same cycle occurs, except the moisture concentration inside the cap keeps rising. After several journeys, the air inside the cap reaches the saturation limit, like the air at the seaside. But the moisture vapour cannot get out. It is trapped inside the cap because moisture vapour is lighter than air, so it rises to the top of the cap inside. But there are no vents here. No where to go. The only vents are at the bottom of the cap to release Ozone, which is heavier than air.

I had this problem with new Bosch caps after only two weeks of use. Took some serious noodle to work out what was going on. (You would not believe the level of effort that I went to). I eventually solved the problem by cutting additional vents in the cap at the top to let the moisture out. I am not suggesting that you do this right away. I would run the car for a few weeks to build up the moisture inside the cap. Then, instead of checking for moisture before you set off, go for a short drive, maybe 3 or 4 minutes. Then stop. Wait 2 mins and then open the cap. Can you see moisture? Can you post some close ups of the inside of your cap? I would be interested to see if there are any carbon trails.

Happy days! :)
Rob
 

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Discussion Starter #15
Hello again,

Today, after about 3 weeks without use it started on the 1st crank and as usual drove OK on the 2mile round trip. I then looked at the cap and was surprised (although the picture is rather poor) to see a very light "mist" on the inside. If you look hard around 1:00 (just to the right, under the semicircle) you will see the smear of my fingertip. After wiping the cap I then ran some errands covering perhaps 40 miles with a couple of stops. The car ran flawlessly. The temp gauge hovered around 80c. Is this helpful? Thanks once more. Ron
 

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R129 500 SL 1991
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Yeah, I can see the moisture. 99% sure that’s what’s causing the rough running.

So now to the solution. First, how many miles on your engine oil; what colour is it (on a scale of clear tan to black), and when was it last changed? This may come as a surprise, but old oil holds moisture attached to combustion by products in suspension in oil. Clean, tan coloured oil is ok. Dark brown, change it.

So what's happening? As per my previous post, moisture is drawn into cap overnight through vents. More short journeys. More moisture. You have mist after not too many journeys. Some members have reported there caps are wringing wet.

The other path is moisture from crankcase gets pushed through cam shaft journal oil seal into back of distributor, especially when engine is warming up. The moisture can be dissipated on a long run, but sometimes not before carbon trails have been burned into cap between poles. Then rough running becomes continuous and can only be solved by either polishing cap (with Dremel) or buying new one. Your cap is not too bad. Can’t see any trails yet. Good.

So, time to change that oil? Then maybe invest in a Beru. Then belt and braces you could modify your new cap by cutting more slots around the circumference at the top of the cap. Note this invalidates your warranty, so I let you decide last step. It’s worked for me though. Others have smeared silicone dielectric grease on back of insulator and have reported good results. I believe the silicone grease reduces the thermal gradient and eliminates air gap where moisture can condense. Remember it’s only due to the warming cycle and condensation that moisture concentration builds up in cap. If you want ‘DIY’ instructions on mod I can provide a write up. Let me know.
 

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Discussion Starter #17
Hello Robm and thanks for your reply. It is good to see that my issue is not uncommon. The oil is kind of a dark honey but that surely, won't hurt anything. Seemingly, smearing the grease as indicated is an easy approach. Now, my "ignorance" shows...……… I'm not sure what back of insulator refers to. Can you clarify? It's getting to be fairly cool here (USA, MA) so I'll pick a good day to work on it. I truly appreciate your help. Ron
 

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Hello folks and thanks for your replies. I must tell you that on a scale of 1-10 I am a 2 1/2 on a good day! While I was doing some other things today I was thinking that the problem always goes away and the "fix" is driving it for 10-15 minutes. So, today I was thinking about temperature. I ran the car in the garage for perhaps 15 minutes in 62 degree weather. I did the test again with the voltmeter and the reading was ~ 5.9 to 7.1. The car temp was around 80. Tomorrow, I will take it out for a longer ride and then take a new reading as I believe the car temperature is usually around 100 (F). To answer some your questions all the "tune-up" parts that I bought were Bosch and the cap/rotor/plugs/wires have maybe 2K miles on them. The coil I bought late last year and it has maybe 600 miles on it. After a rainy week earlier this Spring/Summer I decided to check the cap before I drove and there wasn't a hint of moisture; totally dry. 90% of the time it starts on the 1st crank and 10% on the second. However, the problem was there with the dry cap.

Thanks again for your efforts and I will be back soon. Ron
old cam seals [behind the rotor adaptor and rear dustcap] will let engine moisture condensation from the valve covers into the distro cap when the engine gets hot, from behind the dust cap behind the rotor & rotor adaptor if the cam seal(s) are old, replacing my cam seals kept my distro caps dry, after finding condensation in them and not being able to figure out how it got there - not even the mechanics at the local MB dealership or other MB mechanics I talked to had ever replaced a cam seal or a backing dustcap - not until I removed the backing dustcap did I find rust and condensation around the cam seal, and the cam seal was so hard and rusted in place, it took nearly 3 hours of gentle persuading and WD40 to get the cam seal out without scratching anything up, and the camseal removal tool was of no help at all. and the new one refused to go in dry as recommended, but a little WD40 a socket of appropriate size and a stack of flat washers and an allen bolt at the end of the cam shaft, to draw it into place took almost another 3 hours to get it to seat well and flat after almost a dozen failed attempts.
As I said - no MB mechanic I talked to had ever replaced a cam seal before or even bothered to look behind the back half dust cap that the front distro-cap pairs with.
also as I was told by an old German MB Mechanic, 1st, always check your over voltage protection relay for a blown fuse, if the fuse is ok give it a wack with the butt handle of a screwdriver to see if the relay is sticking, he wacked mine and the idle smoothed right out. lastly,as he said the MB was originally designed for cooler alpine temps of Europe, the fuel/air mixture is really finicky depending on where you live, even if the duty cycle is right on, depending on where you live, the elevation, temp and humidity of your local environment, [as with older motorcyle carburation plays a role in performance] so just an 1/8th or 1/16th of a turn of the fuel/air mixture allen wrench can straighten out the problem even if the duty cycle is right on.
so, I keep a T-Bar allen wrench handy in my console and after less than a dozen runs after the engine was hot on both cool and hot summer nights, I was finally able to dial it in by doing minor tweeking, as the duty cycle voltage fluctuates too much using a DVM - but using an old analog dwell meter to set points or an O-scope works better, but who of us has or owns either of those anymore?
And as this old German MB mechanic told me, running the engine too lean will cause it to act up, rather than running its mixture slightly rich [using the Allen T Wrench to tweek the adjustment] will cause it to perform better over a wider range of air temp, humidity, & elevations.
It worked for me when everything else failed . i.e. new cam seals replaced, very many new sensors replaced, all new ignition parts, including EZL-ICM, professionally rebuilt Bosch CIS fuel distributor, new fuel injectors, new exhaust system including new cat converters and O2 sensors, all replaced 5 yrs ago, and it still didnt run smooth. But a new over voltage protection relay that was sticking and Tweeking the fuel/air mixture brought it all into line.
 

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Discussion Starter #19
Hello Robm, Bob Terry, and 1992-500sl. Surely, I will take the low tech and "easy" approach by changing the oil/filter and applying the silicone dielectric grease. I looked pretty hard for a Beru cap but couldn't find one for the 300sl. Any suggestions as to where to get one. Also, can you help as to what "back of insulator" specifically refers to? Thanks agin folks...Ron
 
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