Le "français" 350 SE no start !

[Denis]

Hello gentlemen (yes you are that and much more  )

Since we need a game now and then, here is the challenge ! What is wrong with Denis's 350SE ?

She looks sad enough, she just sits there looking good, her starter whirs strongly and her sparks at a sparkplug wire are strong and steady. But she does not awake 

This is a D-jet antique 

The fuel pump whirs and stops its whir after about two seconds when the key is turned on. Then I press the switch into cranking position. The engine cranks strongly without even a hint of igniting a fuel charge.

When I fitted a fuel pressure gauge just behind the cold start injector, I got...almost no pressure build  >:(but this does not keep the cold start injector from using whatever fuel feed  it got : the spray is good as the engine is cranked . Should this not fire at all ?

When I squeezed the fuel return line line, I got 2 bars in 2 seconds. When I stopped cranking, the 2 bars (return line still clamped) stayed strong for another
minute when it dropped to 1,6 bars.

What wisdom comes forth from the group ?
I think that the fuel pressure regulator is questionable

As for fuel flow, I did not test that as I could not get teh pump going by shorting terminals 30 and 87 at the relays. besides, which bloody relay does what in the W116 box : they are identical without any function marking 

What else ?

I tell you, these cars are unreliable, 33 yeras and teh blooming FI has problems - they just dont make them like they used to...

Denis

Paris, France - mild day, a relief in a hard winter that often reaches -5C...

[John Hubertz]

I'm not sure how to diagnose it denis, but I'd be concerned that your injectors (electric pulse-type on a d-jet) aren't firing.  Have you tried misting gasoline directly into the intake as you crank?  (I use a window cleaner pump sprayer to do this).


I am sure you can read the pulses on a digital volt-ohm meter - if I recall it reads some kind of "average" voltage due to the rapid on-off-on-off


The early "brain boxes" are somewhat notorious for failing...  So much so that one 3.5 owner I know (Charles Hart - has a 3.5 SL and a 6.9) actually bought a parts car for the brain and the euro headlamps.


I imagine in Europe they will be easier to find at a dedicated MB junkyard since so few are still on the road. 

[John Hubertz]

Denis,

A friend of mine has one or two 3.5 models and I asked his advice on diagnosis - here is his reply to me:


Are the plugs getting wet, as if there is too much fuel?  Does the
engine
catch at all?  First, he should prove to himself that there really is
good
enough spark. Do that with a set of clean plugs and a shot of ether
with the
brainbox disconnected to ensure that there is no gas getting in.

Second, listen for injector clatter to make sure that the electrical
circuit
is doing what it's supposed to do.  You can check the injectors by
holding a
screwdriver to it and then your ear to the end of it.  You'll feel and
hear
it clicking.  There are four circuits to test.  Not that I recall which
ones
are which off the top of my head.  I'd do cylinders 1 & 2 before moving
on.

The next step is to follow the procedure outlined in the service
manual.  If
the results show up being too wierd, it's easily a few broken wires or
at
least high impedance in the wire harness.

Remember that the 3.5 & 4.5 systems have different wire harnesses and
brainboxes.  The sensors (except the throttle) are interchangable.

Thanks go to Charles Hart of the M-100 board.

[Denis]

Yes, this is pretty strange. If I turn the ignition on and move the throttle with one hand, I can feel the injectors firing by touching them this does not mean that they are firing when the engine is being cranked 

I did try fuel directly into the engine and the cold start injector does spray a good mist. I don't even get a single reaction from the engine -nada-rien !

I will try to confirm that injectors are firing during cranking. If they are not, I will suspect the trigger points or the ECU. I have a spare ECU but believe me, these are now quite rare here. Rust and fuel costs have sent most W116 to the scrap heap or made them collectible if they have a V-8 in them.

Have a good day

Denis

Paris, France

[oscar]

Hey Denis,

What you need is three accurate pressure gauges : 2.5, 4 and 16 bars.    . There's one in every crowd isn't there.

But seriously, your and my car are virtually the same, especially the motor.  Reading about your descriptions of what you've tested and found is really eliminating all the easy remedies unfortunately.  But it is reminiscent and here's another possible cause.  Is it possible that water has come into contact with the top of the motor, by way of condensation or liberally dousing the engine in water? If not I doubt this will hold true.   I raise this question only because of a silly thing I did with a pressure hose in an attempt to clean the motor over a year ago.

With the air filter still attached, I remember squirting the hose underneath the air filter and thinking "that was stupid" because of all the electronic components sitting on top of the motor.  The car barely got me home and then refused to start.  This lasted for a week.  All you describe is exactly what happened to me.  Although I presumed fuel and injectors were mechanically in good order, it was obvious  the water caused an electrical short but no idea where.  I pulled all sorts of electrical connections apart with no visible water detected.  Discharged the battery a number of times trying to start it, pushed the car in and out of the shed to let the motor dry out in the sun.  Eventually after a week, a cough, then nothing, a splutter, nuthin, finally a couple of cylinders fired and rocked the car as though it had never been timed, but the motor kept ticking over. The car ran for some time before each cylinder awoke.

If that doesn't help, I've needlessly let out a dark secret regarding my own incompetence. Bugger!

My only other suggestion is to agree with you regarding the slide in trigger points, which would mean you may be hearing valves instead of injectors whilst cranking.  I only say this because I can't tell the difference, I would have to be told what's what.

[Denis]

Well MB350 the only thing I did prior to this problem is chase/tease a Lotus Seven down a highway. I think teh 350SE might have the dreaded Colin Chapman curse on it now 

While I am at it, I might pull the distributor and clean it out. I might as well do some motor maintenance so that it will run flawlessly when it does 

I tested the pressure sensor and it is within specs, eventually the last thing I will do is ECU measurements, it is so uncomfortable to do in a car like the W116, not like a Volvo 1800 with the ECU under the seat.

I tell you, that W123 diesel sure is a great standby.


Denis

Paris, France

[John Hubertz]

Denis!

I think you've got wet plugs!!  My old 450SL (same injections system) wouldn't even THINK of firing if the plugs were wet. 

No other explanation why you have NO hint of starting that I can think of.  Fresh plugs and a shot of starter fluid as Charles suggested would be a great first spot to try....  I mean, if you can't get it to fire with EXTERNAL fuel it can't be your fuel system either - and you've got spark and air!

Replace those recalcitrant midgets and let us know what happens!

[Denis]

Unfortunately  you are wrong...how can the plugs be wet after I let the car sit for two days ? I tell you, it just cranks over swiftly as if it had no FI or Ignition system 

I am thinking now : no or not enough fuel due to injectors not doing their jobs (no signal to inject available fuel or no trigger to the ECU to say "fire" to the injectors).

Denis

Paris,France

[Denis]

The injectors are firing during cranking - I had a finger in their bodies and could sense the solenoid clic !

This may well be the dreaded Colin Chapman curse
I really did upset that Lotus...

A few more test but it looks like the ECU is working...I suspect fuel supply and ring pressure.

Shouldn't I get 2 steady bars in the ring upon switching the ignition on (fuel pump whirs/stops) ?
I did not get this yesterday. This could be the pressure regulator or the fuel pump check ball. I am wondering about fuel pressure regulator now. I don't see the fuel pressure damper doing anything that would prevent starting.

Yes, all conjecture my dear Watson ...

Denis
Paris, France

[John Hubertz]

Denis,

I'd say the quick diag for this would be adding fuel by hand - have you done this yet?  Splash a bit into the intake and if it fires you've got a definite fueling problem.....  the only reason why I thought "wet" plugs was that at least on my d-jet, once they became fouled the car wouldn't start - even after sitting for a week

I'm not trying to pester you - just wish I could be more help

[Denis]

One of the first things I did is put some fuel over the throttle plate : not a single explosion !
Check the ignition : opening the points with the ignition on creates a huge spark at the coil. Connecting a spare plug/lead to a distributor cylinder lead connector creates an equally steady spark !

This is getting to be interesting hey 

Denis

Paris, France

[John Hubertz]

So either the car has "jumped time" (camshafts not turning in correct sequence) or perhaps Charles Hart has something re replacing the plugs.... or perhaps something as simple as the distributor becoming loose and rotating out of adjustment - that happened on a car I owned once and it took me WEEKS to figure it out.

My 107 with the same system simply WOULDN'T fire without a fresh set of plugs - weird I know, but apparently these engines are spark sensitive.

[Nutz]

Confirm that there is a good ground from ECU terminal # 11 to ground.

[Denis]

Well John, the timing chain was checked about 8000km and was far from worn. the engien a only fed a diet of Mobil 1 oil. And the bloody spark plugs are NEW (1000 km)...

As for Nutz's idea, it had not escaped me : if the ECU is acting strangely, the car wont start.

Just wait till the weekend...


Au revoir

Denis

Paris, France

[Denis]

OK, a silly question...
In the fuse box, which relay does what ? in order, starting from the front.

Thanks

Denis

Paris, France