1974 450SL

Hello Everyone,

I have a 1974 450SL that's developed a weird running issue and I'm at a loss as to what it could be.

I apologize in advance for only writing in English. Also, please forgive the long post.

The car had been running great around town so I went on a trip with it. I did expressway speeds (75mph) for about 140 miles. When I got off the expressway it stumbled just a little at idle but it was also showing low on fuel. I filled up with fuel. After fueling, I went about 15 miles but it kept stumbling at slow idle. I stopped at a car parts store bc I suspect vacuum hose issues. I do some vacuum hose checking (no leaks) and now the engine is cool. It starts but is stumbling and I am having to pump the accelerator to keep it running. I can get it up to speed if I'm pumping the accelerator and it sounds good at speed but terrible at idle.

I decide to cut my trip short and park the car. It sits parked for three days while I arrange a rescue mission.

I go back with a truck to tow it and try to start it. It runs for 10 seconds and dies (and pumping the accelerator doesn't coax it to keep going like it had previously) so, I tow it the 150 miles home.

Once I'm home, I spend hours pouring over the forums to see if anyone has had similar symptoms. I find lots of information on the D'Jetronic but no one has reported the same symptoms.

I invest in a Pertronix ignition module, new plugs, wires, distributor cap. (Since all the trouble shooting guides say make sure ignition is good first. The old spark plugs are an even caramel color all around.) Unfortunately, I can't check the timing because I can't get it to run long enough to check it.

I pull the distributor out and check the trigger points (they are clean).

I definitely have spark!

I pull the fuel pump, filter and in-tank screen just in case they're fouled (all are clean)

I direct run the fuel pump with 12v (it runs good)

I check the fuses and relays (all are good)

I listen for injector clicks (20 clicks just the way it's supposed to do)

I listen for fuel pump buzz at key on (yes, it's there)

I definitely have power!

I pull the fuel line at the fuel damper and test fuel pressure. I have fuel running through the system at proper amount both with key on and in trying to start it and during the brief running. It also maintains fuel pressure as it should.

No change in symptoms - runs for 10 seconds (unless I pour gas down the gullet and then it runs longer) and dies and pumping accelerator doesn't help keep it running although revs do vary.

It will keep running if I pour a teaspoon of fuel directly into the throttle body but then dies once that's exhausted.

I've gone through the vacuum lines and can't find any leaking.

I've pulled the fuel injectors and cleaned and tested them and they all work. I still need to put the fuel rails back in and test them in place but have been delayed due to waiting on new pintle caps, screens, and rubbers.

I think it's something relatively simple but am feeling like I'm missing something.

So, the basic issue is it starts, runs for 10 seconds and then dies.

I have spark, I have fuel, I have electric power - what am I missing?

Thanks for any insight.

Raven

Hi Raven,

I have just glanced through your description and only one thing jumps to my mind: When you start the engine, it gets fuel from cold start valve. That might make it run for a few seconds. There are 2 things that I'd ask you to check, assuming that your ignition works as you tested:

1. Check fuel pressure (2.0 Bar or 29 psi) by inserting your manometer instead of cold start valve in fuel loop line. Damper is a bad location, as this is already behind pressure regulator in return line to tank.
2. Have someone try to start your car while you feel whether every injector vibrates. Yes, you can feel whether they operate.

Some more hints on ignition:

1. Pertronix does not really make sense, you already have a transistor operated ignition.
2. In Mercedes-Benz workshop manual you will also see an ignition timing value for starting revolutions. This is just to verify that you have nearly correct timing.

Best regards, Volker

Hi Raven,
I fully agree with Volker.

Suggest your injection-valves only work by acceleration-pulses (You tried this).
Just hat a trigger-points-set without function you won't believe when you see the photo in non-working condition: Foto

In addition, one more idea came to my mind: You tested most components, but not the wiring harness.

I recently translated my model-specific measurement-instruction into English language, you find the global instruction here: Introduction / Instruction
Here the instruction for your car (direct download): M117_en

One annotation concerning the measurements: I use a LED-circuit-tester for some measurements because multimeters react too slow.

I forgot one thing in the plan that could be important, too: The seperate mass (ground) contact for the injection valves. Fortunately, Volker just some hours ago posted a photo it can be seen very fine on (middle-left): Foto

Did not get any feedback to the English-translated-plans. Please give me feedback, if anything is wrong or unintelligible.

Regards and success
Norbert

Hi Raven,

in addition I can also recommend Bosch's workshop manual for M116/M117 . I am sure you know Startekinfo for R107 and there Engine Manual - 07.4 Electronically controlled gasoline injection system with regulation.

So now you have a lot to read...

Well,

one more thing which is mandatory on a new car: Please use Appendix A to verify that you have correct components installed. I wouldn't be surpirsed at all if you find yellow or green injectors, wrong MAP sensor or ECU ...