W7dc is the correct heat range for that engine. W9dc are only used when the car has bad rings or valve seals [ an un-healthy engine]. That is why I mentioned the diffent heat ranges for the NGK plugs .
On Bosch, As the # increases , the heat increases. So a hotter plug will not oil foul, but if the engine is in good shape , the W7 is correct and factory spec. plug heat range.
On the NGK, the opposite is true..as the # increases, the heat decreases..so a #5 would be a hotter plug than a #6. [ BP5ES/BP6ES, etc].

If your plugs where stark white , the mixture is lean and the most common cause of that condition on MFI is the air slide on the pump gets stuck in the open position. This happens quite regularly on a car that has been sitting/stored for a period of time .. They usualy free up after a few good runs.
There is a simple test to verify this problem:
On the top/rear of the injection pump you will see a small, round air filter canister . This allows outside air to be drawn into the engine at cold starts
to compensate for the cold start enrichment. Once the car is up to temp, this intake filter should closed off air intake [ via a slide valve in the pump].
So, here is the test:
.. you take a piece of vac hose and put it to your ear and the other end at the filter intake and you will hear the air being drawn into the filter when the engine is running cold..as the coolant temp rises, the valve slowly shuts off the intake air at this filter and once the car is up to temp, there shoud be NO AIR entering this filter [ you will no longer hear this at the tube in your ear]..if it still allows air to pass when warm, you can take the top off the pump slide device and clean it so it closes all the way. If the air flow ceases when warm, then it is OK and is working correctly.
The other possible is a vac leak ..a common place on that engine is right at the brake booster hose connection . Check there .
An easy test to know you have a vac leak is you should be able to stall the engine at idle by screwing in the idle air bleed screw [ right/front of engine]. If the engine still runs when warm after you screw the idle air in [ closing off idle air] , then the engine is getting air from elsewhere...so you know you have a vac leak .. [ this is just another quick test]
These test only take a couple of minutes to perform and will tell you a lot about where your MFI mixture stands.
As said ,there are some little tricks on these MFI systems, but don't let anybody scare you away ..each of the pieces all come together quite sensibly when they are explained and there are quite a few of us here that know them quite well . They have gone through a few changes and modifications , but those are all known to me and others here who will be more than willing to share that info..that is what this site is all about................
Try those test and if they are cool, I can explain further.

Just curious to see anyways.....Pics? Sounds like a cool coupe..

here's a cool link with a lot of pictures of the injection pump if you're interested. pretty good rundown on operating theory as well.




http://www.sl113.org/forums/topic.asp?TOPIC_ID=790&SearchTerms=injection,pump,tour

I had a freaky gremlin like this...turned out the ignition circuit wire from the fuse block to the coil had completely disintegrated. Replaced it and the problem was solved!

__________________
Carmody
Wichita, Kansas USA
Air Capital of the World


1968 220 (my first MB!)

Arthur, Thaks for that info. I CAN stall the engine by turning in the air control valave - so I'm half way home I guess. I switched to NGKs this weekend, right before doing my valve adjustment and cam cover gasket replacement. I'll "listen" at the injector pump intake as you suggest - and let you know the result.

Very good..
You may also want to check that site Qzawa posted , as those 113 chassis use the same style pump and there is some good info there on the linkage adjustments and such.
These Bosch MFI are very cool mechanisms and they are very robust .. .
I have been working on these pump/systems since their introduction in the '60s and have a passion for them..once tweaked , they run terrific and hold there setting well.
An added note ... the MFI pump gets incorrectly blamed for a high percentage of fuel related problems on these chassis , when the main problems usually turns out to be fuel feed from the electric pump and filters..there are 3 filters [ tabk/pump/engine] and one should always do a fuel flow test before any other fuel adjustments..
You are looking for a flow volume of 1 ltr per 15 secs time at the main filter, engine not running, key ON for electric pump. If more guys would do this simple test, they would not wind up in so much trouble with their diagnosis of these system.........

Arthur - my hiss noise at the pump goes away when the engine warms up - I have a general "feel" that all is well with the mixture. I have a smooth idle, and plenty of power to accelerate. I DO have some exhaust popping with a trailing throttle - which is probably more apparent in my car because it's a manual gearbox - and I use it to slow down. I'm able to blip the throttle to change down - and it's a nice snappy response. I should qualify the fact that my plugs were still white at the porcelain by stating that they probably had less than 400 miles or so on them. I've kept them in the boxes (I think Bosch ND7s actually fit a lot of British cars too, and I'm always dragging home an MG that someone has given up on). The car did actually seem to start easier (faster) with the NGKs - but I may be hearing things by now.

I'll be replacing my brake booster vacuum line anyway - because it snapped when I tried to take the cam cover off for the valve adjustment this weekend (insert expletive/s here). It let out a nice "hiss" when it broke - so it was obviously holding vacuum.

I just know my intermittent miss is going to turn out to be something really silly. As I stated before - the cam cover gasket was leaking pretty badly, especially along the plug side of the head. I have a hunch that I may have inadvertently fixed my misfire by adjusting my valves - since the new gasket seems to be keeping everything nice and dry now - I know oil isn't a great conductor - but I'm guessing enough of it in the plug hole would short the high tension circuit? Who knows at this point. Weather too lousy to drive today - maybe tomorrow. Thanks again!

Tom W.

try this. buy a can of carb cleaner and spray around the injectors the intakemanafold and any vaccume lines. if the engine hesitates or increases rpm you found a leak.

<< I DO have some exhaust popping with a trailing throttle - which is probably more apparent in my car because it's a manual gearbox - and I use it to slow down. I'm able to blip the throttle to change down - and it's a nice snappy response>>

That is Normal on the early style pumps.[which you have] The later versions added a solinoid on the back of the pump to cut fuel on decelleration. It was basically early emmissions mandates that made this change , but many prefer the trailing throttle with manual shift and they disconnect the wire to that added solinoid on the later version [ meaning I would not worry about it and I actually prefer the earlier design you have]
You may have had a valve lash problem, or the air slide may have been hanging up..just watch the plugs color. On the misfire, I would also check the boots at the plugs these are actualy resistors and do break down and can cause a mis-fire.

Anyway...a couple of simple tech tricks to check for lean/rich conditions on MFI system is as follows:

With a warmed up to operating temp engine---At idle , pop the ball socket off the link rod from the pump at the cross arm. [This is the socket next to the brake booster where the pump linkage meets the cross arm, driver side.] Now , at idle , increase the fuel manually by pushing down slightly on this arm to the pump [ which is now disconnected from the other linkage] As you add this manual fuel, the car should start to die out from an over-rich condition. BUT, if the car rpm starts to increase as you enrichen the fuel, you know you have too much air being metered into the engine [ meaning a lean condition].
You can do the same on the other link at the air intake flapper..If you disconnect that link and start to open the air flap [ throttle plate] and the engine starts to climb in rpm, you know that the pumps settings are too rich
for the normal position of the air flap. [ if this were the condition,it is usually be coupled with sooty plugs]
Being mechanically linked , these two units [ pump/air throtle plate] must be set to perfection b/c there is no fuel feedback as in a new car..it is strictly mechanical. [ which is why you always look for good, tight linkage sockets and adjustment.They are critical] Just consider it as a mechanical typewriter..the stuff has to line up in order to work correctly.
Another common problem that causes a lean condition [ aside from the obvious vac leak] is when the throttle plate is mal-adjusted at idle. The plate should be completely closed at idle [ to the point of actually being able to feel it click shut ] This happens when guys who are unfimiliar with these MFI systems attemp to change/adjust the idle with the set-screw at the throttle plate.
This should never be done as the idle is correctly set on these with a combination setting of the air bleed screw at the intake for air and a fuel metering adjustment at the knurled screw adjuster for fuel at the back of the pump. [ I can show you that,if need be..a little tricky, but once expalined, easy to do]
Anyway, a quick/easy tech trick to make sure your air flapper plate is correctly seating/closing at idle position is to simply take off the air intake hose between the air filter and intake so you have acces to the intake throat and , at idle/warm engine, place a piece of cardboard or such over the intake throat to block the air..the engine should not change rpm [ or very little] because at idle ALL the air reguired for the proper mixture goes through the AIR BLEED adjuster screw...get it??? You will notice that the air bleed screws hose is BEFORE the throttle plate. So, if the throttle plate is not completely shut at idle, the mixture can be lean regardless of the screw adjustment [ which is why I mentioned that one should be able to stall the engine by turning the bleed screw in, thus cuting off idle air..if it does not , then you know the engine is getting its air side of the equation from some other source, which is no good]
If the engine changed/dropped rpm when you blocked of the air intake, you know the throttle plate is not closing correctly/fully. This plate adjustment is also critical to the timing of the engine b/c these use a vac RETARD spark system, where the spark is retarded at the distributor at idle . As soon as the flapper is opened [ losing the vac to the distributor] the spark timing advances instantly...a very good system. But if the flapper is not closing all the way, you can see how it also effects the engines timing. [ and thusly, the performance]..just a side note to be aware of. This retard is easily checked by disconnecting the vac hose at distributor while engine is running at idle..there shoud be a very pronounced increase in engine rpm when this vac line is disconnected and return to normal idle when it is replaced...
These tricks are not in any repair manuals and are only an accummulation of techniques we older/vet techs use to do a quicky ,general diagnosis when a MFI repair was presented.. they are an aid in narrowing down root causes and help put you on the right diagnosis track....just some time saving tips of the trade.......
If this is of value to you , I will post ..if not, that's Ok . Some would prefer not to get into the actual workings..

Have fun.............

Arthur, many thanks (again) - believe me, it's ALL of interest. I'm actually becoming a big fan of this injection system. I think it's a work of art - and just shows how far ahead Mercedes was in the 50's & 60's. To be honest, I wanted one of these cars for years but was put-off by some of the horror stories of the system - probably by people who wouldn't take the time to understand them. It's not unlike people who hate British cars because of the multiple S.U. carbs - which are also nice pieces of engineering as long as they don't get messed up by people guessing about how to adjust or rebuild them.

The flap at the front of the plenum seats nicely. As an aside, if I pull my vacuum line off the intake, the RPMs increase dramatically - the only way I knew I had a vacuum retard dizzy when all this began. The small rubber portion of my vacuum line was so brittle it shattered when I tried to check it for sealing - and so it has been replaced.

One more note - my air control valve is at the far rear of the engine - there is nothing resembling it at or near the front. Odd?

Ok..111 chassis. I am thinkng 113.

Valves adjusted, cam cover gasket replaced, wire-wheeled the cam cover, vacuum booster line replaced, NGK BP6ES spark plugs installed (Bosch wireset and other ignition components were nearly new), PCV hose assembly replaced, disassembled injection linkage, cleaned & polished, oiled & replaced, checked timing & dwell (again) - waiting for throttle dampener assy to arrive from za fatherland. Cleaned the exterior of the engine (still more to do). It's been running perfectly and idling like a six-cylinder sewing machine for three days - touch wood! Probably hasn't had this much attention since it left Stuttgart...Maybe NOW it's fixed?

thanks for the write up on the fuel injection and related controls.
i got off my can after reading your post and looked at a 72 280se 4.5,a fellow came by the shop the other day and wanted to sell his old car,450 $ later i rolled it into a parking spot and called it number 99(going to buy one more then fix and have a huge sale)
car did start and actually sounded pretty good then about a minute later it would start choking down and run real rich,so decided to take the air filter off to have a look,well the thing cleared up with the filter in the trunk,nah that cant be,filter looked clean not new but clean,put the filer back on and she runs rich,left the filter off and drove around the block,will retest tomorrow after it cools down.
i did close the air screw and from experience not a rpm meter it idled down to probably 100-200 rpm, so i have to find the reason,it looks like some one took a screw driver to the air horn(contains the throttle flapper)so am thinking the thing got tweekwd so it aint closing all the way.
thanks again,you inspired me to fix something
larry perkins lou ky

thanks in advance.
authur,on my 1972 280se 4.5, i have a situation where i can move the connector at the distributor,the one with 5 wires at the bottom of the dist, the engine is really running rough and as i wiggle the connector the engine wants to smooth out but never perfect.
i disconnected the female connector,cleaned with contact cleaner and a small pointed tool,measured the ohm value thru the plug and got zero ohms. the male reciever contacts looked shinny,but the male black plastic part that goes into the dist seems a bit loose.
any help is appreciated.
larry perkins lou ky