1972 Merc 280SE 3.5 V8 running probs

[Stoive]

This V8 engine type was also fitted into later 350's .
The problem I've got with it is rough running that started suddenly a couple of months ago . If you slowly depress the throttle it will accelerate smoothly to full revs but if you floor the throttle quickly it thumps and bangs and decelerates to idle speed ( but doesn't cut out ).
I've checked the timing , replaced the points and condenser , and checked the fuel flow to the fuel rail - it's getting loads .
I've also cleaned the 4 breaker points ( that sit under the base plate in the distributor ) that control the electronic fuel injection - 1 set of points runs 2 cylinders each . They were slightly oiled up but it hasn't cleared the acceleration problem .
Has anyone got any ideas please because I'm all out !

[benjj]

Does that engine have some kind of throttle position sensor? If so that can cause the kind of running issues you're encountering.

[Stoive]

Thanks Benjj , I'll check it

[Huntsman]

Is it a Bosch system?

If so, you need to buy a copy of the book by Charles Probst.

[jimxms]

I would have said some form of exhaust blockage. But on a classic I doubt there is anything that could block the exhaust (aside form a small animal or a banana)

[Stoive]

It is I think , thanks a lot I'll get it

[jith]

Stoive said:

This V8 engine type was also fitted into later 350's .
The problem I've got with it is rough running that started suddenly a couple of months ago . If you slowly depress the throttle it will accelerate smoothly to full revs but if you floor the throttle quickly it thumps and bangs and decelerates to idle speed ( but doesn't cut out ).
I've checked the timing , replaced the points and condenser , and checked the fuel flow to the fuel rail - it's getting loads .
I've also cleaned the 4 breaker points ( that sit under the base plate in the distributor ) that control the electronic fuel injection - 1 set of points runs 2 cylinders each . They were slightly oiled up but it hasn't cleared the acceleration problem .
Has anyone got any ideas please because I'm all out !

The 280SE 3.5 is an utterly stonking car that ran 2 different forms of injection, both Bosch. The early cars ran D Jetronic which is the crudest form of Bosch electronic injection. The later cars ran K Jet, which is absolutely superb.

It sounds like you have the early system which is highly sensitive to temperature input changes. This means you have to make sure the cooling system is perfect; no leaks, proper coolant mix, very importantly, the thermostat is spot on and ensure the coolant temperature sensor is working perfectly.

This system also has a vacuum sensor fed by a hose from the inlet manifold. You have to ensure there are no splits or leaks in the hose and the sensor is responding to vacuum changes instantly.

If you have K Jet the most common problem on the V8 is vacuum leaks resulting in a flat spot.

Incidentally, you are not supposed to touch the lower contact set in the distributor, they are very finely setup from new and if giving trouble should be replaced as a unit, although they seldom do.

If you are in Scotland I can set the system up for you.

Best of luck,

J

[Stoive]

Thanks J ,I'll have a closer look at the vacuum tube, unfortunately I'm at the other end of the kingdom otherwise I'd pop by