W108 250SE fuel injection problem - UPDATED 16/10/21

[julian]

Hi folks, this is a new thread as the issue remains unsolved and the matters raised in my previous one I think are a bit of a distractor.

So... a 1967 250SE with 400,000km of well maintained single ownership.

Starting can be troublesome but has a sound idle and revs well when cold. Soon after this, as it warms the running condition is poor and backfires through the intake and eventually stalls.

So far work includes:
- confirming spark
- new battery
- fuel pressure 1.2 Bar
- electric pump flow rate 600mL per 15 seconds (this is lower than the litre recommended)
- all injection pump elements cleaned ultrasonically
- injector rack moves freely
- all injectors cleaned ultrasonically and pop tested in satisfactory range
- cold start air valve not blocked
- cold start enrichment working

I am being assisted by a very knowledgeable Benz technician in Adelaide who is starting to run out of ideas.

Where to next?

[Bartman4800]

Has the ignition been timed at 8 deg BTDC with vacuum disconnected?

Time to do some reading:

viewtopic.php?f=9&t=18483&p=130427&hilit=adjust#p130427

viewtopic.php?f=9&t=17018&p=136262&hilit=adjust#p136262

there is a set routine to trouble shooting a Mechanical fuel injection.
First is; DO NOT TOUCH THE INJECTION!
actually it's ;
1/ adjust the valves (010 intake and 020 exhaust on dead cold engine).
Check the timing chain.
2/ check the compression .leak down test is best.
3/ Check the fuel flow. 14Psi with 0ne liter per seconds measured at the cold start valve.
4/ Check the spark plugs. Use NGK BP6ES ,or if you want hotter plugs use NGK BP5ES. DO NOT USE resistor plugs .
5/check the plug leads .If there are carbon string leads ,chuck them over the neighbor's fence. They will be useful there .
you should only have solid copper core leads with 3K ohm resistor terminals on the plugs.
6/ check the coils resistance .Getting 12 volts to the resistor and 6-8 volts on the coil side?
7/ Check rotor and cap for cracks. replace the points and condensor.
it should now run if everything has been gone through in order .
Once it's up to running temp. use a C/O meter to check the C/O % which should be 4 -5 at idle.
From this point it's a matter of tuning the injection which can take a full day sometimes.

If this is not enough info, do a search for the member "mercmad" in the W111 folder with the keyword "adjust"

Regards, Bart

[John Green]

Hi folks, this is a new thread as the issue remains unsolved and the matters raised in my previous one I think are a bit of a distractor.

- all injection pump elements cleaned ultrasonically

I am being assisted by a very knowledgeable Benz technician in Adelaide who is starting to run out of ideas.

By this do you mean that the injection pump has been stripped, inspected "elements cleaned" and then reassembled and tested according to the Bosch test sheet?

[julian]

Beg your pardon John, but I meant the small (deliver/backflow?) valves under the six outlets of the pump. I have not performed any dis-assembly or modifications to the pump whatsoever beyond removing these valves, ultrasonically cleaning and ensuring free movement of the 'pintle'. I can see the little pistons moving and rotating freely and collectively under these valves.

The previous owner had the car since new and was fastidious with maintenance. The injection pump was overhauled 20 years ago at great expense and by his reports was running well until being parked 2 years ago.

I am approaching this from an ignition angle now. I can't see how the car can start with difficulty, then sustain a normal idle, then progressively weaken, then finally hunt and stall once warm after 2-3 mins of running, after all the attention being paid to the fuel injection items.

Will replace coil as resistance values a little high, and new cap and rotor, and start from there.

[Bartman4800]

And don't forget the capacitor please! I recall something similar, which turned out the capacitor in the end.

Might as well replace the breaker points too. At least have a look at them and see if these close neatly and don't have any pitting.

Both can still be got from the stealer.

Bart

[John Green]

I am approaching this from an ignition angle now. I can't see how the car can start with difficulty, then sustain a normal idle, then progressively weaken, then finally hunt and stall once warm after 2-3 mins of running, after all the attention being paid to the fuel injection items.

Smart move to do the various ignition parts, these are after all consumable parts. Are you aware that there is a small thermostat on the top of the governor on the pump? This controls the warm up cycle this being the case there is still a chance it is the pump at fault, as the trouble starts once the warm up cycle finishes?

[Bartman4800]

And it's easy to check whether your ignition starts failing. Hook up the strobe light; if you see the flashes start to disappear intermittently, you know you are onto something.

My money is on the capacitor...

Once you know what the problem is, spend another 500 odd dollar on a 123 Ignition system (not W123) and you will never look back.

https://www.123ignition.com.au/product/ ... -6-r-v-ie/



Bart

[CraigB]

Another variation on Bart's suggestion - and I can't remember where I bought them now, but fairly cheaply I bought these things, shaped like the end of a spark plug that you put on the plug and then lead on to it and they light up when fires - so same thing but you can see multiples at once.

[julian]

Hi everyone.
No change with replacement of parts as part of servicing ignition.

My money is on the capacitor...

Bart

Hi Bart, are you referring to the condensor?

Julian

[Christo C]

In electronics Condenser and Capacitor are the same item.

[julian]

So the journey continues.
I have had the injector pump rebuilt along with the injectors.
I have replaced points and gapped and timed as best I can. The car can run quite sweetly.
I have ordered new linkages and brass bushes to remove a bit of slop from this aspect of the system.
We are getting close, now with nearly the whole injection and ignition system renewed.

One part I cannot rationalise is that the car cannot start once it is hot. It runs well at temperature but if stopped will not restart. I figure once I improve the precision of the air/fuel metering with new linkages this might recover, but does anyone want to assist on this particular aspect?

Thanks

[CraigB]

I see you have checked fuel pressure, but is it the original pump or a new one? I know back when my fuel pump went on my pagoda (same system I think), I got a djet one from JG who said that it might also help with the hot start problems they sometimes have - because it has greater fuel flow and that helps to cool the fuel - the hot start problem from vaporisation. Recently I needed to replace a pump on a W108, and djet pumps now not easy to find and JG recommended a VL commodore pump - I got one for that I think was for RB30 turbo but it is a bit lower in pressure than say the M117 pumps. Works great. There will be a thread on here about all that I think in the last year. Cheap and easy to get one and fit it and see what happens.

[CraigB]

And just adding that the early pumps have a fairly big surface area that could maybe drag and slow it down....just my assessment having pulled this 108 one apart - and depends when you did your pressure - just wondering if car not running and pushing the pressure around might read ok, but then on demand and certainly if hot and vapourised that its not pushing enough volume around.... all speculating - but a cheap commodore pump off ebay would at least rule that out.

[julian]

Thanks Craig.
You are the second person to comment on fuel flow. I’ve been reluctant to change the pump but if there is objective experience to the contrary and using a cheap EFI unit is appropriate then there is little to lose. I’ve had a look but can’t see this thread you refer to. Any tips from anyone re finding it?
Julian

[julian]

https://ozbenz.net/viewtopic.php?f=25&t ... mp#p192896

This is the comment I’ve since found. Is this the reference you refer to? You later commented about deleting a T piece. Anyone got some experience here with a photo to assist in changing this?

[Chai]

As it's a D-Jet, suggest you replace the vacuum tube from the intake manifold to the EFI vacuum sensor.
The warming engine can cause incorrect manifold vacuum readings if that hose is porous.
If the rubber tube look old, replace it anyway. Once there's an air leak, the air-fuel will be over rich, preventing proper combustion.

Check the wiring near the solder joints of the vacuum sensor do not have broken strands from being moved around.

[CraigB]

Its pre - djet Chai - with the mechanical pump that looks like it came off a tractor!

That's the thread - its really straight forward when you look at it - one line out of the tank to the pump inlet - one line back without the t piece you will see. Go into ebay and there are any number of choices of vl commodore pumps.

You can still rebuild your old pump etc or look for one, but this cheap simple step I think is your easiest for trouble shooting if you are chasing the right thing.

[julian]

I am completely lost, and my Mercedes mechanic in Adelaide (Olaf at Horst's Auto) is stumped also. Here is what we are facing.
Ignition components all replaced with exception of the body of the distributor. Even leads re-made. Gap, dwell, timing, advance all set and function checked on oscilloscope. It is not ignition.
The fuel pump has been rebuilt by a Bosch agent in Adelaide on correct test rig. The metering has been reset to spec sheets and the fuel rates matched across the cylinders. The warm running device thermostat replaced with a new Mercedes part. The lines and injectors have been cleaned and patterns checked. The rack moves freely. The electromechanical devices are operating- putting the rack into the start position, and also the time switch for starting enrichment injector.
Subsequent to the reinstallation the rod lengths and position have been reset to factory specification.
Rear feed pump checked flow and pressure- within spec, including when hot. Return lines have been rodded and blown with air. The fuel filter is new.

The car starts briskly and idles very well when cold, revving appropriately. Using a Bosch CO meter you can see an enriched running condition during warm up. As the car approaches standard operating temperature the CO reading starts to sag indicating a lean condition. This coincides with hunting, poor running condition and eventually stalling. At this point, disconnecting the mechanical linkage rod to the pump and adding a little more fuel can prevent stalling.

So we have a perfect running car at idle, with perfect ignition, on spec pump metering, but the damned thing dies when warm under a lean condition.

HELP!

Can the plunger under the warm up enrichment device depress the plunger and return the rack to a standard fuel metering but allow for continued air flow through the small filter, thus allowing a lean state to occur?

We are at a loss here and have spent a bit of money getting the car to this point.

[Bartman4800]

Have you checked you got no vacuum leak anywhere? Could be the brake servo, or an automatic gearbox diaphragm (if it is an auto).
You could do a smoke test to find out other than these 2 leaks.

https://www.google.com/search?q=find+va ... P7de14As25

Regards, Bart

[julian]

Thanks, yes we have checked for leaks. Including with gauges.
Again the condition at warm up is excellent. It falls off the cliff rapidly once it reaches operating treatment.

[tsharkey]

I know nothing of these systems but ..... When it warms up, what is supposed to happen, timing (advance etc), fuel wise burn, fuel mix, idle speed.

[julian]

Hi mate.
There is a wax pellet that expands as it is heated by the flow of warming coolant, pushing a small rod outwards. In the cold position this is in a retracted state, allowing simultaneous additional fuel by deflection of the rack/cam in the pump, as well as a small bypass of additional air, ostensibly like a complicated 'choke' setup on less refined systems of the period.
As it warms this little rod/plunger extends, reducing flow of fuel and air until it reaches a neutral state. This is referred to as the WRD for warm running device by Mercedes.
There is a magnet/eletromechanical unit that can be energised to deflect the rack to a start, idle, and off position but is seperate to this WRD system. There are load specific adjusters, idle mixture adjusters, air bypass and of course the throttle affecting mixture. None of these should be touched without first ensuring basics like rod length, injector condition, fuel delivery flow from the tank, and cylinder balancing are sorted to standard, and should be left to someone who knows what they are doing.

[Bartman4800]

Julian,

Ron Bunting (Mercmad) would most definitely be able to help you. He sets up these systems regularly. He is over in Queensland (Rocklea) though.

If you google Benzkraft, you can find his telephone number.


Regards, Bart

[julian]

So ultimately it seems that the 3 dimensional cam used for fuel injection metering was worn in the idle/low running speed/load position. When the pump was rebuilt it seems the spec sheets for the pump are insufficient to accomodate for this wear, and meant that the vehicle ran very lean when the additional fuelling from the warm running device was progressively withdrawn as it rose to normal operating temperature.
The car has over 400,000 miles of well maintained/documented history so it is a fair point to make about the cam wear.
This was compensated by adjusting the load screws on the rear of the rack in a methodical pattern. The car now warms up and drives beautifully for the first 10 minutes at low and high speeds.

Unfortunately there is a new issue emerging. Once the car reaches operating temperature it suddenly loses one or two cylinders and chugs with almost no power, unable to move forward from a stopped position unless buried right foot and even then it would barely move off beyond walking pace. Again almost all components on the ignition are new, and tested with an oscilloscope.

Any ideas where to start on this issue?

[John Green]

Unfortunately there is a new issue emerging. Once the car reaches operating temperature it suddenly loses one or two cylinders and chugs with almost no power, unable to move forward from a stopped position unless buried right foot and even then it would barely move off beyond walking pace. Again almost all components on the ignition are new, and tested with an oscilloscope.
Any ideas where to start on this issue?

Is it he same two cylinders that fail, or does it just feel like two cylinders have failed? I have seen a couple of cases where the pump was blamed for a similar situation where it was close to impossible to move away when stationary. The issue turned out to be that the distributor wouldn't advance the spark when the vacuum was taken away from the adjust diaphragm. Add either jammed advance weights and/or broken springs (and someone has timed it with the broken springs) and the engine can't gain enough revs to get the 3D cam to move out of the idle position.


Fit a 123 ignition distributor to it.

[julian]

John, that is exactly the description. I have been looking for a point where I could justify one of those ignition setups, and I think the threshold has been met.
Order commencing!
I will update.

[pastelgrey300D]

Just thinking about this and wondering if it may be the coil? If you have the wrong coil or wrong resistor it may be that the coil is overheating and this will slowly reduce the performance.

Had similar symptoms on my 230S where the previous owner had bypassed the coil resistor and after 10-20 minutes the coil was getting hot and the car would lose power and run poorly. Check the coil - it will be warm but shouldn't be too hot to touch or hold.

[julian]

So further updates.

John:
I installed a new 123ignition distributor. New plugs. Correct timing.
The issue remains- warm running condition has low power, poor response to pedal input, and stalling when selecting gears.
I haven't changed the fuel tune, since this was setup with CO metering.

David:
Good tip and worth checking. Symptoms occurred prior to changing coil to correct new one. Also changed resistor. I just went out and checked. Coil is no warmer than the engine bay ambient temperature. Winding values in ohms 1.8 primary and ~13000 secondary. I'm wondering if this is a little high?

I am totally over this.

[Bartman4800]

123 ignition actually advised me to remove the ballast resistor altogether. Their electronics can take the extra amperage.
They sold me a new Beru coil.

Bart