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Mixture screw adjustment...scary stuff

Stringer

Active Member
Joined
Sep 16, 2012
Messages
144
Location
Lebanon
Car
300E 1987
Hi
I own an old 300E in Lebanon, which is a bit of a project which i have rescued. When i drove it away, i noticed it ran smooth when cold, but the moment it got up to operating temp, it became v sluggish and shook upon acceleration.

So, i am in the process of looking at all the usual suspects. Have changed to date

plugs
Air filter
Fuel filter x 2


But i think the car was 'tuned' to accomodate its previous incorrect settings and is consequently running too rich (black plugs and high fuel consumption)

I am about to install an 02 sensor, which the previous owner disconnected as he couldn't be ****d to get a hole drilled out for it when he changed the exhaust. I suspect this is drastically effecting the mixture.

But here's my question. Would you change the o2 sensor THEN look at mixture adjustment. OR, could i leave the sensor and just LEAN up the fuel distributor with the infamous 3mill alan key. And if so HOW FAR down should i push before i know that the key has engaged (am very scared of pushing too far and messing it up)

Also, is it worth changing the coolant temp sensor in case this is sending wrong info to the (is it the EHA?)


Any help would be enormously appreciated.

thanks
 
Poor running 300 and 300-24 models crop up regularly - have a search and see what has been recommended before. Also see:

http://www.mbclub.co.uk/forums/engi...l-economy-intermittent-tick-over-problem.html

If the car had an oxygen sensor I would refit one but I would start with the distributor cap & rotor arm. Later cars had a closed-loop system with a Lambda probe & Catalyst - I don't know if the early cars had this or whether cars in your territory would be the same as UK cars

I would think the only way to get the mixture spot-on would be to fix all the faults then set it using a CO meter

Welcome to the forum, BTW!

Nick Froome
 
If the car had an oxygen sensor I would refit one but I would start with the distributor cap & rotor arm. Later cars had a closed-loop system with a Lambda probe & Catalyst - I don't know if the early cars had this or whether cars in your territory would be the same as UK cars

If it has a Bosh K system it doesn't have closed loop, if it's KE then it does. The difference is the EHA, which controls the fine mixture control.

I would suspect an air leak into the lower part of the fuel distributor or intake manifold being the root problem, as I suspect this car never had a Lambda sensor.
 
hey DM
thanks for your help, mate. In fact, i think the car DID have an o2 sensor originally as the wiring is all there nicely curled up in the plastic box under the passenger seat.

ALL mercs in this territory are from Europe, btw.

Damn, i wish i knew which type of fuel distributor i had. Is there an obvious visual way of establishing which one i have just from having a butchers? If a closed loop system means that there is a flow back pipe straight to my tank, then it is closed loop. The thing i noticed also was the the EHA has no vacuum tube on it (i mean, there is no nipple for one even...?)

thanks to both of you

M

ps here's a pic of mine
 

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thanks for that. But please put me out of my misery and tell me what the SECOND vacuum tube (slightly lower) is, on the top left hand corner of the photo (i.e. right next to the dipstick). And a crate of guiness if you can tell me how far to push down on the mixture setting without *****ing it up and leaving me in tears.
best
M in beirut
 
The EHA is electrical, so no vacuum required.

You don't push down on the adjustment screw, you turn it with the allen key. A quarter turn makes a significant difference.
 
sure, but don't you push it down ever so slightly to engage the real screw underneath, so to speak, before turning?
 
Not the ones I've done, just engage the allen key and turn.
 
You do have to push down to engage... i wouldnt mess with the screw unless you have a reliable gas analyser...

later cars do have lambda but it really doesnt do that much... mine was also disconnected when I bought it, wiring it up got rid of the fault code but made no difference to how it drove.


I would look for an air leak
 
or perhaps a vacuum tube missing? The second tube coming from the rocker box at the top left of my photo is a mystery to me. Do you know where it's supposed to go to as mine is missing?
thanks
 
or perhaps a vacuum tube missing? The second tube coming from the rocker box at the top left of my photo is a mystery to me. Do you know where it's supposed to go to as mine is missing?
thanks

In that case plug the other one as the clear one is the feed to the thermostatic valve, the other one is the feed to the modulator in use.
 
or perhaps a vacuum tube missing? The second tube coming from the rocker box at the top left of my photo is a mystery to me. Do you know where it's supposed to go to as mine is missing?
thanks

If you have a vacuum pipe missing, you have an air leak causing your rough running problem. Do not play with the mixture screw untill you have checked everything else.
 
i'm pretty sure that the second one - transparent - is the one which is missing. But could you not just tell me where those two components are? thermostatic valve and modulator?
 
The thermostatic valve is the device shown in the picture with the two pipes plugged in, the modulator is whatever the black pipe runs to.
 
I think the clear pipe goes to the throttle body, but cant be sure.

Look under the inlet manifold, it may just be hanging there.
 
If you are talking about the thermo vacuum switch screwed in the head just to the right of dipstick then its likely one vacuum line runs to somewhere on the inlet manifold as a vacuum source and the other to the charcoal canister under the nearside[ rhd] wing. This is to evacuate the charcoal canister which forms part of the fuel tank evaporation system In later models it was replaced by a solenoid valve controlled by the ECU. The only way to set the mixture accurately is to use an exhaust gas analyser. Having watched numerous MB techs use this technique the screw needs very little movement to alter the mixture strength. You may also have to disable the EHA while you do this not sure.

ps vacuum leaks often occur at the small rubber pipes that join parts of the vacuum system together--- they go brittle and crack and this not obvious to the casual inspection - worth checking those out.
 
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ahhhh hang on a sec. I've just realized that the loose transparent vacuum pipe which i discovered a couple of days ago was supposed to connect to this 'thermostatic valve' as it ends up on the throttle body, right?
I actually connected this to the fuel pressure regulator. So, i i put it onto the thermostatic valve THEN i should get some hose from a hardware shop and connect the fuel pressure regulator and the tiny tube which comes off the rocker cover. Would that help?
thanks for your patience
 

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