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Hello,
I got a 280 CE W123 from 1983 that was flooded. The motor has a K-Jetronic with Aftermarket deactivated catalyst (Euro-version came stock without catalyst)
Currently, the engine starts fine with a bit high revs but after a few minutes when it warms up, it starts to run uncertain and the pipe headers get uneven cold. Especially Cylinder 6 then has about 100 °C less than the others so I think it doesnt run at all.
What was done already:
-Spark distributor is new
-Ignition cables are new
-Interchanged Injectors (Issue stays on the 6)
-Interchanged fuel lines from distributor to Injectors (no change)
-Tested resistance at ignition, all ok
-Warm-up regulator was rusty inside and dirty from flood water. Renewed that with new gaskets/membranes
-System pressure about 5,6 bar, control pressure starts below 1 bar and stays at about 3 bars when warmed up
-Aux air valve tested
-Fuel distributor cleaned with new gaskets
-The air-mix screw was set on very rich so without air flow fuel was pumped. Setting is now done to start injecting when the measurement plate is moved slightly.
Do you have any clue where to look at? Underpressure system maybe?
I really hope to get it back on the road as the condition of the car is really good for an old Mercedes that has been used in germany since 83.
you have done everything right about ignition and fuel delivery.
Inspect cylinder walls
measure compressions
What kind of oil do you run??
Sorry forgot to tell, the compression on all cylinders is between 11,6 and 12 bars cold. No. 6 was also tested with 12 bars warm.
Oil is a 5w-40 with MB spec.
Engine has about 208.000 km and was as far as I know running good before being flooded. Ignition control device is also replaced. Will get new Injectors as well but mixing them made no change yet. Tank is cleaned and gas filter replaced.
Engine has about 208.000 km and was as far as I know running good before being flooded.
Ignition control device is also replaced.
Will get new Injectors as well but mixing them made no change yet.
Tank is cleaned and gas filter replaced.
I see...
What sort of camshaft drive does this engine sport?
>> I'd question your Chain Tensionerbefore dealing with more injectors. Swapping #6 out was perfectly good enough test.
>> Is there a way you can READ FUEL TRIMS?
I know pressure is satisfied but then injection timing gets involved.
LTFT would give us a live status.
>> How is the distributor set...
Do you have a timing-gun to check live ignition timing marks are ok time??
Last edited by CaliBenzDriver; Mar 12, 2025 at 06:37 PM.
When I had the valve cover open to check the valve clearance, the chain looked very good and was tightened. The sound when #6 is out is a bit like the sound my Motorcycle made when the chain was loose but this ticking is only there when the 6 gets out after the motor warmed up a few minutes.
Not sure about fuel trims but I guess there is nothing to read out from the injection as the system is 95% mechanical. Its the K-Jetronic which I think is called Bosch CIS in the US and the injection takes place continuously. I wonder why this works at all as there is no timing between the crankshaft and the injection system so its basically closer to a carburetor than to modern EFI.
When trying to change the injectors today (They definitely got some muddy fuel) I saw that there are O-rings in between that are hard as plastic which can cause an air leakage right in front of the valves. The injector guides have not been interchanged so I think that could cause issues.
When I had the valve cover open to check the valve clearance, the chain looked very good and was tightened. The sound when #6 is out is a bit like the sound my Motorcycle made when the chain was loose but this ticking is only there when the 6 gets out after the motor warmed up a few minutes.
Not sure about fuel trims but I guess there is nothing to read out from the injection as the system is 95% mechanical. Its the K-Jetronic which I think is called Bosch CIS in the US and the injection takes place continuously. I wonder why this works at all as there is no timing between the crankshaft and the injection system so its basically closer to a carburetor than to modern EFI.
When trying to change the injectors today (They definitely got some muddy fuel) I saw that there are O-rings in between that are hard as plastic which can cause an air leakage right in front of the valves. The injector guides have not been interchanged so I think that could cause issues.
When your injectors are back in place, check timing light moving trigger to cyl6 to see if ignition timing moves around
I have everything back together with the new Injectors and in general it runs better but the main issue is still there. Cylinder 3 and 6 are colder at the header. #3 not that much and there is the big combustion gas valve that absorbs heat but the 6 is at least 100 °C below the 5. I checked with a timing strobe at all the cylinders, however there are markings at the timing for 1 and 6 that show ignition at around 6° at idle speed.
With underpressure-adjustment it shall be 4 after 0°, thats what I found in the manual.
Can that explain ticking at low-rpm and #6 not really running?
Yellow value is with underpressure adjust and orange without. Underpressure adjust is connected but not working.
Ignition spark must only before TDC, never after it.
Like 5 to 10° degree in advance.
Your ignition seems under control and now your injection is as well... what gives?
How about the intake air side then ...:
Let's see if the PCV or EGR are making Cyl.#6 lean?
Can you test/plug/disable these temporarily ?
Seems like. The adjustment of fuel/air is only estimated as it was completely on the moon from previous owner (made it pumping liters of gas into the engine, recognized it by smell of the oil) but tahts something same on every cylinder. EGR was blocked totally, for some reason I cleaned that. May it works may not but no difference.
The leakage connection at WUR (70) to intake (12) is located in front of 6, that could may lean it. PCV valve, not sure if the car has it.
That's a great diagram to empower your troubleshooting - This makes CAN Networking look simple, right?
> VACUUM DIAGRAM TESTS:
Do you have a vacuum hand-pump?
Use it to test these circuits.
We know German rubber is amazin'... that what diaphragm are made of.... you'll find something!
First vacuum test all circuits statically without engine running then select interesting ones to test with engine: ON.
The assumption is your Cyl.#6 is getting more air supply that other cylinder ie. "vacuum leak", fresh air supply.
> CRANKCASE VENTING:
I looked through the diagram for PCV... did not see it. Engine is clean enough to have an EGR, so your guaranteed it must have a PCV that needs service attention.
Look for niple on top of valve cover. Follow the line to intake... Us it near No6??
Block the intake side of pipe from sucking air through it for testing only.
Yeah that system is a bit of a mess and later it was even more messed with putting some electronics into it.
Yes, got that handpump and tested this underpressure actuator on the ignition which is supposed to move the ignition to late moves it to early from 6° to about 21° when I put underpressure in. It gets the underpressure from the throttle valve body but should that be higher underpressure when the valve is closed?
At the picture, you see a small hose going from the valve cover into the air filter box, there is no PCV valve in between.
I removed the oil cap and closed it with an rubber glove and there is no over or underpressure, think thats good.
Did another test on the underpressure system by removing a leak inlet pro fuel pressure regulator and putting the hand pump on it. I blocked the pipe that goes to the air filter housing by hand and gave -500 mmHg underpressure. It declined but took around 20 seconds to reach zero. Is that more a tight system or a leakage?
I removed the oil cap and closed it with an rubber glove and there is no over or underpressure.
I think that's good.
Did another test on the underpressure system by removing a leak inlet pro fuel pressure regulator and putting the hand pump on it. I blocked the pipe that goes to the air filter housing by hand and gave -500 mmHg underpressure.
It declined but took around 20 seconds to reach zero. Is that more a tight system or a leakage?
There are a lot of systems operating well... you just need to find the one that's throwing a wrench in.
Test that each vacuum system is :
-1- functional
-2- not bringing in air leak
Your glove on the oil cap is an excellent test!
But your test conclusion is not...
At idle, you need to a vacuum at the oil cap.
So your primary focus may be the PCV: Positive Crankcase Ventilation.
In original German it can be called a "breather hose"... its in charge of suckung oily air fume out of crankcase with a limited intake vacuum.
Last edited by CaliBenzDriver; Mar 17, 2025 at 04:44 PM.
When the engine is running, there is a vacuum at the black hose where it is plugged onto the air filter. The vacuum either comes from the small pipe I close with my finger on the picture or from the engine. Probably from both. If I close it by hand while the engine is running, there is about -200 mmHg generated when I wait a bit
When the engine is running, there is a vacuum at the black hose where it is plugged onto the air filter. The vacuum either comes from the small pipe I close with my finger on the picture or from the engine. Probably from both. If I close it by hand while the engine is running, there is about -200 mmHg generated when I wait a bit
If you test is all right: I take your word.
You are on sight and can see what's happening.
Checked everything approx. a hundred times and at the end...seems like number 6 is back in business.
To check fuel pressures, I had to loosen a pressure regulator that is mounted on the intake manifold. It is fixed there with two screws while one of them has not a blind hole thread, its drilled through. The whole is right in front of #6 intake. Putting back the red marked screw on the picture changed a lot and it does now run a lot better and with 98% chance on 6 cylinders.
Thanks for all the support here, now I can start with getting the Interiour nice and all the electrics working.
Also got a picture of the whole car, still with steel rims but alloys are alredy there.
Checked everything approx. a hundred times and at the end...seems like number 6 is back in business.
To check fuel pressures, I had to loosen a pressure regulator that is mounted on the intake manifold. It is fixed there with two screws while one of them has not a blind hole thread, its drilled through. The whole is right in front of #6 intake. Putting back the red marked screw on the picture changed a lot and it does now run a lot better and with 98% chance on 6 cylinders.
Thanks for all the support here, now I can start with getting the Interiour nice and all the electrics working.
Also got a picture of the whole car, still with steel rims but alloys are alredy there.
She is a gorgeous machine, you're it's master!
How safe are steering & brakes ?
New caliper kits + control arms + stab bars...
How safe are steering & brakes ?
New caliper kits + control arms + stab bars...
Thanks. I can not really say how good the brakes are as I cant drive fast without having it registered but steering works well. Tires are old winter tired but I have new ones on the alloy wheels. Control arms and so on are in good condition, before the flooding the car was in a great shape.
First owner ordered almost nothing except biggest engine but thats good for me as all these missing options are not damaged now.
It has:
-Manual Transmission
-No AC
-No anti-lock braking
-No Airbags
-No rpm gauge (but a nice clock instead)
But electric windows in the front (working)
When everything is ready, I plan to start a trip to Italy.
The next steps are getting Interior Lights and so on nice and working and also do some adjustments to the injection system. The perfect adjustment has to be done in a workshop with CO meters to get it adjusted to correct CO values.
It already has a certificate that it is "Historic" that means it is cheaper in taxes and allowed in restricted areas for cars with older emission classes. Numberplate then will show an H at the end.
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. The whole is right in front of #6 intake. Putting back the red marked screw on the picture changed a lot and it does now run a lot better and with 98% chance on 6 cylinders. 
