280SEL D-Jet running very rough

[tyrrellnick]

G'day. Hoping there's someone here who can tell me if I'm heading in the right direction, if I've missed something, and potentially throw me a bone on some common causes of my old girl's symptoms...

Here's the run-down:

Bought the car from a neighbour a few months ago. When test driving, checking a few basics, everything was fine. Running smooth. The previous owner had owned it for about 20 years, and had regularly serviced it (just basics of course), and had only really pottered around town on short trips in recent years.

I took the car on a 100km round trip to Canberra to stretch her legs, and by the end she was running very rough at idle and take-off, missing on one or two cylinders.

Went through the process of checking ignition - leads, sparks, firing sequence, timing etc all seem fine. Tick.

Compression tested with local Benz specialist, including adjusting valve clearances, and all cylinders within 5psi of each other, all above 100psi. Tick.

So it has to be fuel, right? Benz specialist said as he was under the hood he could see cylinders 2 & 4 seemed to be leaving fuel residue on the spark plug, which indicated unburnt fuel. 2 & 4 aren't in the firing sequence together, so something to do with injectors the likely culprit.

I put a bottle of Nulon injector cleaner in the tank with 95RON fuel, and after about 200km over a few days, I drove the 50km to work at about 100km/h the whole way, and by the time I pulled up at the first set of traffic lights, she was purring like a kitten! I thought PROBLEM SOLVED! Must've been dirty injectors, how easy was that?

WRONG.

Next day, back to the old rough idle, missfiring on take-off and cruise, so very obviously not firing on at least one cylinder much of the time. Can smell petrol faintly in the cabin too, but no leaks on the ground overnight, so presume some of the unburnt fuel is hanging around in the engine bay and wafting into the cabin? The car hasn't backfired once, which is a surprise if it's not burning fuel on a couple of cylinders.

So what do you think my first port of call should be? Here are some options I've considered, but would love some feedback:

1. Fuel filter/dirty fuel - but doesn't make sense because you'd think this would affect all cylinders equally.
2. Sensors etc - again, would affect all cylinders/injectors equally.
3. Trigger contacts - maybe one or two injectors aren't operating at all?
4. Injector seat seals - if there's air seeping into the chamber it could result in lean mix which would mean it doesn't fire?

The only thing with all this that bothers me is why would it run fine one day, and then terrible again?

Hopefully someone has some ideas... I can take photos of anything if you need any visuals.

Thanks in advance everyone. I'm no mechanic but I love solving problems, and I really am getting quite fond of the old girl! She's a gorgeous car, with all new suspension and otherwise in great nick, no rust etc.

Cheers

[Bartman4800]

Most likely there is a lot of sediment inside your fuel tank. It could also be gum that now starts to dissolve.

Dissolved gum will pass through your fuel filter but will clog up your injectors.

Drop your tank and inspect it thoroughly. When in doubt, bring it to a radiator place to have it chemically cleaned.


Hope this is your problem.



Regards, Bart

[tyrrellnick]

Most likely there is a lot of sediment inside your fuel tank. It could also be gum that now starts to dissolve.

Dissolved gum will pass through your fuel filter but will clog up your injectors.

Drop your tank and inspect it thoroughly. When in doubt, bring it to a radiator place to have it chemically cleaned.


Hope this is your problem.



Regards, Bart

Thanks Bart - do you know if my model has a pressurised fuel line from the tank? Sorry if it's a stupid question.

[Bartman4800]

Your pump is hanging in front of the tank, together with the filter. No pressure between tank and pump.

If the tank has gum, most likely the fuel lines can be gummed up as well. Good idea to clean.
Vinegar is actually a gum dissolver. It does not remove it but softens it so it can be flushed out with carburettor cleaner or the like.

Bart

[tyrrellnick]

Your pump is hanging in front of the tank, together with the filter. No pressure between tank and pump.

If the tank has gum, most likely the fuel lines can be gummed up as well. Good idea to clean.
Vinegar is actually a gum dissolver. It does not remove it but softens it so it can be flushed out with carburettor cleaner or the like.

Bart

Thanks Bart, much appreciated. Will have a look at this over the weekend. The only other question I've got here is how would this be the cause of cylinders 2 & 4 misfiring? Would have thought dirty fuel would affect all cylinders more evenly?

[Bartman4800]

Some injectors might be gummed up more than others.

Change your fuel filter, cut it open and see what horrors live inside....

Tank is inside the boot, filter and pump live under the car.

https://forum.w116.org/mechanicals/fuel ... p-needed!/


Bart

[Lance]

Fuel filter - replace
Check mesh filter in tank (unscrew bung in base of tank)
Clean out tank?
Compression is very low, should be about 130 psi minimum.
Use more injector cleaner, might be a bit of rubbish in the system.
Check electrical connections to those injectors.

[cuisses]

On a D-Jet I would blame the trigger points (your point 3) or the ECU (rare). Sometimes the trigger points give a "bounce" signal which produces two injections per pulse i.e. too much fuel. The cylinders are paired, so there are only 3 different pulses for a 6-cylinder.

Your symptoms, i.e. runs OK most of the time but then turns rough sound like what I had in my D-Jet. The trigger points can be sensitive to all kinds of factors when they are on their way out.

You can check this easily when the engine is running rough. You just hook up an oscilloscope across the injector. The difference between a good pulse and a bad pulse is pretty clear.

[Bartman4800]

On a D-Jet I would blame the trigger points (your point 3) or the ECU (rare). Sometimes the trigger points give a "bounce" signal which produces two injections per pulse i.e. too much fuel. The cylinders are paired, so there are only 3 different pulses for a 6-cylinder.

Your symptoms, i.e. runs OK most of the time but then turns rough sound like what I had in my D-Jet. The trigger points can be sensitive to all kinds of factors when they are on their way out.

You can check this easily when the engine is running rough. You just hook up an oscilloscope across the injector. The difference between a good pulse and a bad pulse is pretty clear.

Not all of us have an oscilloscope sitting on the workbench

[cuisses]

Not all of us have an oscilloscope sitting on the workbench

I have always found a CRO to be much more useful than a spanner - particularly on a D-Jet car.
You can probably buy one for less than the cost of a set of trigger points. Particularly if you get the kind that plug into you PC:

http://www.ebay.com.au/sch/Oscilloscope ... 934/i.html

[tyrrellnick]

On a D-Jet I would blame the trigger points (your point 3) or the ECU (rare). Sometimes the trigger points give a "bounce" signal which produces two injections per pulse i.e. too much fuel. The cylinders are paired, so there are only 3 different pulses for a 6-cylinder.

Your symptoms, i.e. runs OK most of the time but then turns rough sound like what I had in my D-Jet. The trigger points can be sensitive to all kinds of factors when they are on their way out.

You can check this easily when the engine is running rough. You just hook up an oscilloscope across the injector. The difference between a good pulse and a bad pulse is pretty clear.

Thanks cuisses.

I don't have an oscilloscope, but I will check the electrical connections to them. It just seems odd that she would run really smooth for a trip or two, then really rough (it's easily missing on one or two cylinders) for a week, then good for a day. Makes me think it could be something loose and/or electrical.

Definitely runs rough more than it runs smooth. Also, I've noticed a strong petrol odour from the boot when I open it (intermittently can smell inside the car too). I know the tank's there, but maybe there's something else going on? Air getting in the fuel line or something? I suppose fuel pressure could lead to rough running & hesitation. No leaks which is weird, nothing on the garage floor when left for a day or two.

I'm not much of a mechanic, as I said earlier, but the more I can narrow it down, the less I'll have to pay someone else to do...

[Chai]

Check the rubber hose from the manifold to the manifold pressure sensor. Replace if original as it may have an intermittent air leak from a small internal crack. Incorrect vacuum will make the engine run rich (computer thinks you're accelerating). Heat from warming engine may cause air leak in hose to close up and engine then seems to runs normally.

The smell of petrol in the boot may be from a perished fuel hose or the fuel tank cap washer. Same for smell in engine bay. Replacement with 8mm internal diameter fuel hose may be prudent, especially if you can see cracking rubber at the cut ends of the hoses. Buy MB hose if you want to retain the original MB hose clips - do NOT use the worm drive clips as they do not clip around evenly.

Another place for fuel to seep out are the hoses to the injectors.

You can spray electronic cleaner at the trigger contacts. There's low voltage so there's no arcing - therefore expect long life (is good as is very expensive). However, function can be affected by oil vapour mist dirtying the contacts.

[tyrrellnick]

Hi all,

I'm back here after a fair while of fault-finding on weekends when I could find time, as well as an amazing local antique car club which has plenty of helpful blokes who know their way around cars of this era.

In any case, the problem is solved! Turns out the 3-pin clip attached to the plate at the bottom of the distributor (for the D-Jet injector points) had sheared when someone at some time had tried to take the plug out. One of the wires attached to the plate inside (the centre one) had broken its solder and detached, although it was obviously able to make contact sometimes as the engine didn't ALWAYS run rough.

IMG_20170423_213549715.jpg

I wasn't able to isolate this issue until I got the distributor off, and that was by far the biggest task. It had seized on there so tight there, it wasn't able to rotate let along come out. Cue a couple of weekends underneath the car with a foot-long rod belting the underneath of the distributor upwards. Not the most elegant solution, but we got there (thanks to about 2 cans of WD-40 and some elbow grease).

IMG_20170423_161227601.jpg

Finally came out and I could work on the points. Covered in gunk so cleaned it all up and she slides in and out like new now.

A bit of soldering, some plastic welding to repair the clip, and some advance adjustment done - she runs beautifully! Drove her to work this morning (about 50km) and realised the heater isn't working, so there's my next project... engine runs a bit cool, I think the thermostat may be stuck open, hopefully that'll fix the heater. Fingers crossed...

[Lance]

Check the rubber hose from the manifold to the manifold pressure sensor. Replace if original as it may have an intermittent air leak from a small internal crack.

Very good idea, I am going out to do that to mine straight away.

Also, I found my distributor had also seized and it took me three evenings laying under the car tapping away in the same manner until I got it out. Was finally able to correctly set the timing and did that make a difference.
Also my heater stopped working and I think I read somewhere that it is usually the heater valve that needs to be cleaned out with new seals fitted.

[Lance]

engine runs a bit cool, I think the thermostat may be stuck open,

Mine takes ages to warm up, I only realised how bad it was after I bought my W123 which is up to temperature after 5 minutes of driving. So, I would say the thermostat has been removed.

[tyrrellnick]

engine runs a bit cool, I think the thermostat may be stuck open,

Mine takes ages to warm up, I only realised how bad it was after I bought my W123 which is up to temperature after 5 minutes of driving. So, I would say the thermostat has been removed.

I thought the same, but when I removed the housing, the thermostat was definitely in there, seized to the top so it made it very difficult to get out given the compressor for the self levelling suspension sits about 8mm above it...

In any case, I replaced the thermostat anyway, and now I've identified that the problem with the heater is the control switch/valve up on the firewall. I think the diaphragm in there may have sprung a leak, so I'm going to have to remove it this weekend and have a look. It never ends, but I'm really glad the problem with the rough running wasn't something more more serious. Initial thoughts were it could've been compression, but if that were the case I would've just scrapped her I think.