300TE Fuel injection units keep failing

[penguin]

I have had my 1989 w124 300TE auto for around 9 months. Ran pretty well at first then started to use more and more fuel. Took it to a reputable local garage with extensive tuning experience they advised that the entire Bosch Fuel injection unit was U/S and needed replacing as it was "impossible to adjust the mixture". I procured a replacement used unit from a specialist breaker. Once fitted the car ran normally for around 3 months when it became thirsty again around 10mpg and lots of smoke.

I took it back to the garage and was dismayed at their news that the "new" fuel injection unit had exactly the same fault as the first. The breaker is supplying another unit as I write. The unit I mean is the whole gubbins under the air cleaner.

My questions are these.

Is there anything which could be "killing" these units or have I just been unlucky?

Does anyone know of a specific weakness with these units?

Any help, advice, suggestion or opinion welcome.

Thanks in advance

 

[Sp!ke]

I'm wondering if it could be something else too, as I've never heard of them needing replacing.

The fuel pump relays are prone to do strange things at weird times and not too exbenzive to replace - might be worth 'borrowing' another just to test.

 

[Ian B Walker]

A Fuel Pump relay is going to set you back about £80 from the stealers. I have a spare is you want it.

 

[Dieselman]

In what way are the metering heads going faulty?

I'm not familiar with the unit on your car but if it's the mechanical jetronic then other than wear or dirt in the distibutor what can go wrong?

I suspect someting else to be the fault, either the fuel pressure regulator or a vacuum leak. A vacuum leak will be especially apparent at idle condition, which is when the mixture is set.

 

[penguin]

It is the mechanical system, and the garage say it is "worn out" and cannot be adjusted to run at a sensible emissions level. Apparently it sooted up their machine when they tried. What I find so hard to understand is how two of them can "wear out" so quickly. Is there anything that could cause them to wear prematurely? Or are the garage just not getting to the real problem of why it keeps getting richer and richer.

Vacuum leaks seem more likely than worn out PI units to me, but would fitting a different unit make the car run OK without fixing the leaks, which is what happened the first time; then, over a period of a couple of months, the car just ran richer and richer.

The garage say they have checked the fuel pressure and it is OK.

Many thanks

Penguin

 

[Dieselman]

Did the rich mixture come about without any adjustment being performed, if so then I would suspect either the cold start injector is stuck on or one of the breather pipes leading to the throttle body is blocked.

The garage say the pressure is ok so no need to worry about any other part of the system.

The mixture is adjusted by altering the position of the air flap against the spring ballancing and thus the pressure on the distributor needle. The more the needle is pressed the more fuel flows. Unless the needle or barrel are worn then there is nothing to go wrong.


Is the mixture too rich throughout the rev range or just at idle, or indeed is it too lean at idle and then too rich whilst running. If the latter definately suspect an air leak.

 

[V12]

These fuel dividers (the top piece with all the pipes_ are very prone to go! sell everyone i get for the 103 engine.
It is generally the electronic black pack on the side that goes wrong.

With secnd hand parts you can never guarentee how long the parts will last.
some will last years to come, others do become faulty quickly, but most breakers will sort you out a new one no quibbles if within warranty, if not they will 9 times out of 10 sort you one a lot cheaper than normal.

Does sound like cold start injector or fuel pressure regulator, could be another couple of possibilities.

The fuel pump relays have so many differences, not a lot are interchangeable like other relays and ecu's...

Ricky

 

[penguin]

The mixture just got richer without adjustment. Adjustment then had "no effect" according to the garage. I am hoping to get it back today with the second used part fitted - so I should have more idea of how the mixture is behaving. I intend to investigate further myself to try and get to the bottom of this as I really like the car and want to keep her, I'd just prefer it if she drank a little less.

V12 mentioned a black box on the side of the unit. Which one do you mean as there are two black units with terminals on mine?
Do you mean the removable one held on with 2 hex bolts? What is it and can it be replaced with new?

 

[NormanB]

Penguin

I have just read this thread again. I do not wish to level anything against the experienced mechanic with the extensive tuning experience but I would get this to a Bosch specialist or even a good MB dealer - either way you need to pay for accurate diagnosis not endlessly paying out for unnecessary and expensive parts.

Just my 50p worth.

Good Luck

NormanB