Starting Issue, 1974 280 SE

Hello Everyone, a quick update. And thank you all for your replies.

Since my last message I have performed the following:

1. During cranking that the fuel injectors are receiving a signal from the ECU, and that they are flowing correctly.

2. During cranking from cold, the cold start injector is firing.

3. During cranking, the ignition system is firing.

4. The spray pattern from the injectors is lovely!

I decided to make a simple switch box to test some minor functionality from the drivers seat. Sometimes a little distraction is good for the soul. I painted it in grey hammertone in order to have a 1960s feel All it does is allow me to close a circuit remotely. I made up some 3m cables to use with it. I tested the following:

1. Switching the fuel pump on and off before cranking.

2. Remotely shorting out the ignition ballast resistors.

3. Remotely being able to crank the engine from the engine bay (Using the red push button) so I can check the fuel pressure during cranking and warm up phase.

I can tell you that after these tests, I still cannot identify the problem. When the engine finally starts, it runs fine!

I think my next item to look at may the following:

1. I will have a look at the distributor during cranking to check that the firing point and dwell for both the ignition system and fuel injection system is within specification. Whilst I have tested the distributor for play in the shaft I have not done so recently. Additionally, I am curious to plot the ignition curve again and I am wondering if during cranking the timing is OK

2. I am concerned that possibly the starter motor is not turning over as fast as it should be. In fact, I am surprised that the starter still operates after the hell I've been giving it! I have taken the larger battery from my V8 W126 to ensure there are no issues with cranking current.





More later......

Lance, you are 100% correct and the car shows all the systems of low cranking fuel pressure.

However, the pressure tests show otherwise, it is bang on 2.1Bar.

lsmoz wrote: Hello Carl888,
I just wonder if the problem is due to the fuel pump losing pressure and then having difficulty self priming. I had starting issues and removed the pump, connected power and dipped the suction hose in a pool of fuel and it took a few goes to get it to suck fuel. Once it did the fuel came gushing out. My fuel pressure drops off readily after the engine is stopped which may suggest that the non return valve isn't working. Also, I once changed the fuel line from the tank to the pump and had a struggle getting the car to start and at the time put that down to the pump struggling to prime itself, as once it was going I had no further issues.. This may be due to wear in the pump.
My car also takes quite a few seconds to start when it has been sitting for a while (longer than an hour), but starts almost instantly if I start it within half an hour of stopping. I may have the same problem, but less severe.
And, rather than set it on fire give it to me!
Regards,
Lance.

Hello All,

This afternoon I made further tests on the W116. I have reconnected the pressure gauge to the system and I have made two short videos of the car cranking and finally starting.

The first one is here.

vid115.photobucket.com/albums/n281/carl3...0020_zps1lwzwb0h.mp4

I have taped the pressure gauge to the windscreen. The slight fluctuation you see on the gauge is the pump priming the system. I note this effect only when using a long extension on the pressure gauge, if it's connected without the extension the gauge does not fluctuate. At about 1:09 the car finally starts however it does not run on all 6 cylinders and takes some seconds to fire on all 6, you can see the revs settling down at just over 1,000 rpm cold idle. You will need to turn the sound up. The "Clicking" noise is the ignition key being turned.



The second video is here:

vid115.photobucket.com/albums/n281/carl3...0021_zpsdxcmmshh.mp4

This is simply the engine being started AFTER it has run for about 30 seconds and switched off, then restarted almost immediately. You see there is no issue with it once it has been running provided it is re-started within 30 minutes, hot or cold.

The pressure gauge in connected to the cold start circuit:

Hi Carl,

that reminds me of a 350SLC I once cured fur a friend. When it was cold it would start badly and run incredibly bad. In winter you would not be able to go faster than 40 km/h before it would warm up. After I had fixed some problems with vacuum hose leakage I finally found the main mistake: It was a hanging rpm governor in ignition timer. When it was cold, it would not change properly. When it was warm, it looked okay. I only found that mistake when I put the car in my workshop and let it stay there over night. Next morning I attached it to my Bosch Compac Tester and saw horrible firings on the ignition oscilloscope. That all had looked okay when the engine was warm.

Maybe you have a spare igintion distributor to test. It is a bit tricky to test ignition timing with just the starter running.

According to your videos you have no problem with fuel pressure. And for your motivation: I like the work you are doing, precise and analytic. Good to see that.

Update as of this evening, a nice 24 degrees here in Melbourne at 8:30pm. I have made a VERY slight adjustment to the fuel pressure, 1Psi increase to bring it up to exactly 2.1Bar. Of course, I expect this to make no difference. Incidentally, at junior school, I was the last year where imperial measurements were taught, so you will notice in my posts I mix imperial and metric

I have left the pressure gauge connected, after two hours, the residual pressure is still very good:



Volker, thanks for your message, the more I work on this car, the more I am coming to the conclusion that the starting issues are probably not related to the fuel injection system

The reason I wish to test the distributor at cranking speed is because I want to make sure there is a reliable well times spark at this low speed. I use this handy Australian made distributor tester, it's very good. But these items, like D-Jetronic testers, are very difficult to find working as they have usually had a very hard life and no one knows how to fix them any more.

I drove the 280 SE again today and this morning the starting procedure was particularly tiresome. None the less I drove for some 20kms and parked the car. After 45 minutes standing I re-started the car. I made a short video of re-starting it. After the engine started I then switched it off and re-started it. It's interesting how the car always re-starts hot or cold immediately after running, but if you leave it for more than 30 minutes, it gives trouble. Ambient temperature is about 23 degrees today, good driving weather.

Unfortunately, following up from yesterday, I cannot fault the ignition distributor nor the timing during cranking.

The engine temperature after 45 minutes standing:




And here is the video:

vid115.photobucket.com/albums/n281/carl3...0031_zps6gxtvcro.mp4