Mercedes 450SL: d-jetronic issues : Enige does not run on all 8 cilinders

Hello all,

first I will introduce myself, I am Hans Kloosterman, I live in the Netherlands and I have a Mercedes 450 SL '73 USA. This car is a so called now a days a bi-fuel car or hybrid car. That means my car runs on gasoline and natural gas (LPG) I have bought the car in april 2008.

My original problem was that the motor runs well on gas but not on gasoline.
When I drove 100 km / h, I could switch to gasoline and the car could barely hold the speed. Accellerate was hardly possible..

in 2015 i found it time to fix this is issue for once and for all

My first step was to add some injector cleaner in the gasoline. But this made no difference.
Then I replaced the fuel filter. Also no positive change.

The distributorcap has been replaced and wires of the spark plugs.
The valves were far ingrained that adjustment was no longer possible.
So both cylinder heads are overhauled.
On LPG the car runs well.
The cams of the gebiss were worn and bough the adjustment tool and cams. mention in this forum..
The new cams are mounted and adjusted to 0.5mm. (no an easy job but we did it.)
However, the engine will still not running on all eight cylinders
The injectors are well controlled, but we suspect that they spray too long.

When I start the engine on gasoline, I have a thick black plume of smoke and the engine is not running regularly.

Can anyone give me advise on how I can best continue this quest?

All tips are welcome.

kind regards
Hans kloosterman

Hi Hans,

welcome on jetronic.org.

Before I will try to analyze with you what is wrong in D-Jetronic, I would like to understand how your gas injection interferes with D-Jetronic. I guess the gas is injected as a single point injection below throttle flap. Or is it a multipoint injection system. And most interesting how does it inhibit D-Jetronic while running on gas. Does it stop fuel pump or does it inhibit injection via removing +12V from pin 24 of ECU? Somehow it must inhibit ECU. Could also be main injection relay. You must first make sure that the gas system does not interfere with D-Jetronic while running on fuel.

You say it does not run on all cylinders on fuel, Do you know which cylinders are not working? If you do not know, use a good rev counter and remove one spark plug connector with gloves one after the other and look how much revs drop. Should be the same on each cylinder. On cylinders not running it will not drop or drop differently.

Hi Hans,
you wrote (and told me via email) the engine is running rich when starting.
Starting the cold engine or the engine in warm condition?

Bad temperature sensors <or wires towards them>, especially the motor-temp-sensor, can richen the mixture so much the the motor dies.
Maybe the reason is situated somewhere else - we need the 'background information' Volker asked for.
We are not so familiar with LPG on classic cars here in Germany. But I know some and own one (not a classic car...).

Is the engine normally being started on LPG?

Regards
Norbert

Hi Norbert and Volker,
Thanks for you're swift reply.

The engine starts, whitout any problems, direct on LPG.
On gasoline warm or cold, that does not change anything. the engine runs irregulary. Irregulary is an understatement. if i let go of the gaspedal its stopped
The gas is single point injected just below the airfilter. There is a simple swich on the dasboard G/B. I don't think there is any interference between the two fuel systems.

"And most interesting how does it inhibit D-Jetronic while running on gas. Does it stop fuel pump or does it inhibit injection via removing +12V from pin 24 of ECU? Somehow it must inhibit ECU."
you suggest that it is running on both fuels? I must check the wiring on the car.

I will try the spark plug test to determine which cilinders do not work.
I will have the answers in a couple of days.

I was thinking:
The main axle in the distributer should open and close the triggerpoints. right? If that axle has too much clearence. could that be the problem?

thanks for tips sofar. As a famous actor would say "i'll be back"

Hi Hans,

the car will either run on gas or on fuel. So the switch has to inhibit one system and activate the other. You have to find out how that is done and wheter it acivates one and deactivates the other reliably. D-Jetronic must not run while gas is active and vice versa. If gas and D-Jetronic run at the same time, your mixture will be far too rich.

The axle and cams for the trigger points has a lot of tolerance. It should have a lot of play before it prevents an injection group to work. When you tell us which cylinders work and which don't, we will be able to see whether it is linked to an injection group.

Hi Volker,
That makes sense. I will try to get the answers.
The car is located on the other site of my town in a garagebox. So I do not have the car besides me.
I hope in the weekend to make some "Quality time" with my car and do some testing and report back to this forum.

Thanks for you're help sofar.

update:

The last week of August I have made an appointment with my garage.
They will check that there is no interference between gas and petrol.
So that answer has to wait.

During the revision of the cylinder heads the wiring was measured for breaks. Everything was OK.
But in the mean time there is so much tinkering with the 40 year old wiring that we will do so again.

Oh yeah I completely forgot to mention. Because of fuel leakage at the pump some time ago
we replaced all fuel lines and we measured the fuel pressure at 2.1 bar. So that's good too.

The mechanic who did the work on my car could not say which injectors did not pulse.
But he told me that all injectors received pulse and that the problem did not occur always on the same injectors.
So that did not help to pin-point the problem.

I suspect that the garage does not know how to proceed anymore and just saying replace this or replace that,
and that costs rise without a solution.

I'm going to print and follow the D-Jetronic checklist.

Hi Hans,

sounds like a plan