'74 450SEL - D JET - Won't run, but will start under certain cond.

[chinny4290]

Hey guys.

I'm buying a friend's family inherited 1974 DJET 450SEL and I need help diagnosing the reasons as to why it won't run. I've spent the last few weeks trying to search Google and the countless Merc forums for my specific issue but I can't seem to find a similar thread.

The car sat for a year and before that year, it ran and drove. My friend noted that power was down but it still drove and ran.

Fast forward to today, and the car will turn over but die. With new fuel/filter and a flushed system, she'll start, run, then die.

The fuel pump only runs under these conditions: when I remove the fuel pump relay and jump the relay port accordingly, the fuel pump goes. Though, put a working relay in, and the fuel pump stops running.

Even with the fuel pump relay jumped and constantly running, the car won't run for any more than five seconds and will die. During the five seconds, we tried pressing the pedal, and it seemed it wanted to die quicker.

According to the various troubleshooting guides, the next step to ruling out problems if the fuel pump is found to be working is to replace the ECU. The problem is, ECUs are hard to find and not cheap if found. What should I do to diagnose the problem?

I tried slowly removing one of the fuel rail lines AFTER the fuel pressure regulator with a towel and even with the fuel pump NOT running, gas started squirting out under pressure. Another sign that tells me that the fuel pump is fine.

We put a small amount of fuel in the throttle body and when that happened, she would start up immediately and would run beautifully until all the fuel was burned. So that tells me it's still fuel delivery related...

What do you guys think?

Your help = awesome!

Thanks!


PS if I need a new ECU, has anyone tried these?

http://www.autopartswarehouse.com/sku/Mercedes_Benz/450SEL/Programa/Engine_Management_System/1974/Base/8_Cyl_4-dot-5L/W0133-1597140.html?tlc=Engine+%26+Drivetrain

[hi ho]

I would try to set up your fuel pump in the engine bay directly connected to the fuel intake. Below is a diag of what I'm suggesting.
I did this with a non-starting kjet 450 engine (should still work with your djet). This and a bit of Aerostart got the engine going. If this works your fuel system from intake back to the tank is suspect. (fuel system blockages can be an issue when a car has been sitting for some time)

WARNING: Take PRECAUTIONS - if not done carefully this can create a nasty fire. It is important if you decide to do this to make sure all connections are secure, and do it in an open area

[gerry l]

Hi, I am confused - I have a 77 450 on my engine and  the fuel supply is connected to the 17mm connection at the 5pm position. On the fuel distributor in the picture, you have the fuel pump connected to the 17mm connection at 3pm, I thought that this was the fuel return? I am trying to get the car running, I bought it as a non runner that had been sitting for a few years. Thanks Gerry.

[djetronicista]

Try ebay for a used ecu much better priced.  I have a few used ecu's most erratic running djet cars have either a bad ecu or map sensor.

[arman]

It sounds either injector related or signal to injectors related. Ignition is obviously fine and fuel delivery from pump as well and the start injector seems to work, while cranking. However I have no idea how to trouble shoot this.
Good luck!

[ters]

I agree with Arman. Found some useful links to help explain the D-Jet system: http://w107.pbworks.com/f/DJetronic.pdf  also http://www.vclassics.com/archive/efi.htm

Please update any improvement/diagnostics?!

Ters.

[gavin116]

Good observation Gerry! 


The supply line is the 5 o'clock one, the return line is at the 3 o'clock position.  The screen on the inlet line should probably be replaced if you haven't already.  The part number is 000 074 60 86 for the filter and the copper washer is 007603 012110.  Ensure you use two spanners then removing and inserting the filter so that you do not weld the closest connector into the FD but over tightening the fitting.


Hope this helps.


Gavin


See EPC diagram on page 3 of my 'blog'

[rob280sel]

Have you checked the contacts that are on the lower side of the distributor? behind a plate with electrical connector on it.
I have a 280 D-jet and it has contacts in the dissy which activate the fuel pump and injection pulse. (3 wire connector). i.e. if the engine stops for some reason the fuel pump stops too.
The 450 will have 2 sets of trigger points (contacts)

sounds like the fuel system pressurizes with the key, the engine starts on this pressurized system then runs out of fuel.

Every now and then I get a miss-fire, clean these contacts and all good again.

Have a look at this, it's for a 6cyll, but could give you an idea of what I was on about.
http://www.peachparts.com/shopforum/vintage-mercedes/264678-testing-trigger-points.html

[Guillaume C]

Hello Guys.

First, I beg your pardon for introducing me in this conversation without any presentation and for bad English (I am French).

About the 450SEL, I want to tell you that on the D-Jet system, there is a main relay for the ECU.
On W116 models, this relay is located under the dashboard, under the glove compartment.

If this relay is bad, there is no "12V" to the ECU and there is no "12V" to commute the pump relay. That's why the pump can't run, even with a good pump relay.

When trigger contacts are bad, the pump run one second when turning the key in the ignition switch.

In Chinny's car case, pump don't work at all with the relay. So I think that it's the ECU relay or the ECU which is bad.

I saw this main relay trouble two years ago, on a 1974 450SEL.

[hi ho]

Hi, I am confused - I have a 77 450 on my engine and  the fuel supply is connected to the 17mm connection at the 5pm position. On the fuel distributor in the picture, you have the fuel pump connected to the 17mm connection at 3pm, I thought that this was the fuel return? I am trying to get the car running, I bought it as a non runner that had been sitting for a few years. Thanks Gerry.

Just got back to this. My apologies Gerry, that pic was taken before I realised the fuel line was in the wrong spot! The intake is the one at 5 oclock. Sorry for the confusion

[arman]

Hi chinny4290

Any news on your starting problems?

Regards
Arman

[chinny4290]

Hi Guys.

I thought I'd revive this once more...sorry it's been a while since I replied...I haven't had time to visit the car again (it's 3 hours away in Pittsburgh, PA).

But I will be returning to visit the car again to see if I can get it starting this up and coming weekend.

So I read some more and I also learned that my friend's caretaker replaced the trigger points on the 450 and measured them out by sight...I also read in regards to the trigger points, they need to be spot on, otherwise the car won't run well or at all.

So the plan is to pickup a Feeler gauge at the store and to measure the exact trigger point gap...can anyone link a guide as to how to do this and to check the trigger points?

I reread my OP and I've been able to deduct through hindsight that it sounds like the injectors aren't pulsing either...Would fixing the trigger points heal this? Could this also cure my possible issue with the fuel pump running only when jumped (I read into the trigger points some more, I read that the fuel pump and injectors won't run/pulse if the trigger points aren't perfect because it tells the ECU the location of the cylinders for timing and stuff).

And there was mention of checking the injector grounds, can someone supply a picture as to where they are?

Also, any other advice for me as to what to prepare myself with before I see the car this weekend, would be greatly appreciated so I can order the parts or what not from my local shop.

TIA!

[carl888]

You can't measure the trigger point gap sadly because when the points are installed there is no way into the distributor.  There is a very small amount of lateral movement in the assembly itself which I set up to make the pulses as symmetrical as possible with a continuity meter.

I think it's pretty obvious your car is not getting fuel, but starting on the cold start injector.

I would measure the loom to see if you are getting a pulse to the injectors.  If not, try the trigger points first.  The fact that this has been played with before suggests there may be an issue in this area.  Also, whilst you may have fuel, make sure the pump provides 28PSI when primed.