Engine rebuild woes

[Canned Benz]

Hi guys,

I've just finnished a year and a half long engine rebuild of my 1975 280ce (W114). Engine is the M110.981 with Bosch D-jetronic. When I turn the key the engine fires up then dies about 3 seconds later. With the throttle wide open it'll run for about 5 to 8 seconds. If you immediately try to start it again it'll do the same but on the third try it'll just crank and has to be left for about 1 minute before it'll start again. I managed to keep it going for about twenty seconds by squirting carb cleaner into the throttle body but then it backfired and took the hairs off my hand, won't be trying that again!
I can't get it running long enough to properly check the timing with my light but I've adjusted it to the point where it seems happiest. Adjusting it in either direction now, makes it even worse.
Any advise would be much appreciated, I have a wedding on the 29th and I don't want to let the bride and groom down!

Thanks
Simon

 

[PobodY]

First things first. - Are you getting (enough) fuel to the injectors? The reason I ask is that behaviour is like a carburettor with a float bowl that fills too slowly; it only runs until the available fuel is used.
What does the fuel filter look like? That's a pretty simple (and cheap) reason for low fuel flow.

Google tells me that the D-jetronic uses vacuum from the inlet manifold; any vacuum leaks? - That might explain why it runs slightly longer with the throttle open (more venturi effect).

 

[Canned Benz]

I stupidly tested the fuel lines and rail for leaks with the 1 and a half year old fuel. I've drained the tank and put super in since then but there must have been at least some stale fuel in the lines. I spent 2 hours last night repeatedly starting the car to try to get to the good fuel. It seemed to gradually run for longer, but the engine was also getting warmer from the attempts which may explain that.
Don't think I have any vacuum leaks. Engine timing is spot on and compressions are good.
I'll buy a fuel filter and a in-line fuel pressure tester. Any other advice in the meantime?

 

[PobodY]

If you've got fuel, spark, and timing, there isn't really a good reason why it doesn't keep running.

What about "idle air control" and related cold-start components? - On my Nissan they can get gummed-up over time, but the symptom is usually high idle rather than not running.
A good spray with throttle body cleaner seems to remedy the problem; I normally take the idle adjustment screw out and spray behind it.

Throttle stops? Could they be set too low, so it's just not doing enough RPM to idle?

I'm just hoping that bouncing some ideas about might help you find the problem.

 

[Canned Benz]

I think you're on to something with the fuel system. The fact that I could help it run a bit longer with carb cleaner makes me think it's fuel starvation. I'm happy with the rest of the engine because of the rebuild, I've also replaced every hose on the vehicle with silicon or braided steel and stainless steel jubilee clips so I'm happy there is no leaks of any sort. All the ignition system is also new. Don't think air flow is the problem because it doesn't run with the throttle in any position.
The fuel pump looks ancient, maybe it's pumping enough to fire but then can't supply the engine when it's running. Also the D-jets have a 7th cold start injector that squirts into the manifold at start up. Maybe this is working but the injectors aren't for some reason. However they have all been cleaned and flow tested by a specialist so should be as good as new.
I think I'll order a fuel pump, filter and pressure tester and test the injectors and harness in the meantime.
Thanks for helping me. You're right about bouncing ideas around, it's easy to go off on a tangent when you're on your own.

 

[ernie]

Many years ago, I had a similar problem on a friend's BMW 7 series petrol which would have been roughly of the same era as yours. Long story short, engine only started if the ignition switch was held in the start position.

As the car was under warranty, I didn't see it sorted as it was trailered to BMW, but I was told it turned out to be a faulty relay in the fuel injection system. Could it be a fuel cut-off valve or something similar?

Just a suggestion!

Ernie

 

[PobodY]

It could be as simple as a poor earth; the fuel pump is running, but not at full speed because it can't draw enough current.

 

[Canned Benz]

Update.

I tested injector number one. The injector has continuity and clicks when probed. It has a two pin connector. One pin is a brown wire and has a good earth, the other pin is a green wire and is not getting voltage when cranking.

I also tested the cold start injector. This injector also clicks and you can see the puff of fuel through the throttle body. This also has a two pin connector and briefly gets voltage when cranking. Interestingly it briefly gets voltage on the first two attempts at cranking and not on subsequent attempts unless left for a minute. This explains why the car fires up briefly and only on the first two attempts, then has to be left for a minute. It's only running on the fuel from the injector.

So now I've got to work out why I'm not getting voltage to the injectors. Not easy as D-jet is a bit complicated, despite it's simplicity!

 

[Canned Benz]

Update 2.

Think I lead myself up the garden path. Spent all day checking each D-Jet component, sensor and wire to and from the ECU. All ok. Then tested the injectors by feeling them instead of using a multi-meter, and while it cranks they are all activating.

With everything plugged in except the cold start injector it coughs and splutters while cranking but never quite fires up. So the injectors must be injecting some fuel at least. I tried advancing and retarding the timing again but if adjusted in either direction it doesn't even try to fire. Throttle position makes no difference at all.

Back to thinking it's fuel starvation. I shall buy a fuel filter, fuel pump and fuel pressure tester. Seven days until the wedding!

 

[PobodY]

Crikey! It's getting a bit close, but it sounds like you're making progress.

 

[jaymanek]

I'd be looking at fuel pressure also. Sounds like you may have crud in the tank, lines and pump restricting fuel flow.

 

[Canned Benz]

Well it wasn't the pump or filter. Replaced both. New pump sounds a lot stronger so not a waste of money at least. I'm still waiting on the pressure tester to arrive. I've read online that some classic EFI cars have two fuel pump relays, one for starting and one for when the engine is running. No idea if the W114 is one of them but I might try giving the fuel pump a constant 12 volts to rule it out.

 

[Canned Benz]

Gave the fuel pump a constant feed and the engine still dies a couple of seconds after starting so it's not that the fuel pump is cutting out.

 

[PobodY]

What do the spark plugs look like? - Do they tell you anything about the fuelling; maybe too much rather than too little if everything seems in order otherwise?