760 GLE Starting Problem, Help

[niloc]

Sensor is held by the Allen screw only, but can be a nightmare removing from the bell housing. Copious quantities of WD40...

I had to remove the gearbox and whack it from behind as it broke during removal.

[stinky]

Sorry that I didn't pick this up earlier; it sounds exactly like the problem I've been having. If so the left bank (right side of engine bay) spark plugs will be the ones covered in fuel. To see if it is like my problem; get hold of a compression tester (£15 in Halfords, or borrow one) and test each cylinder in turn. Hopefully all your cylinders will show the same results either way please post the results of a compression test on here.

As for the spark plugs; S7YCC should work OK but they are a different heat range and will probably make the car run slightly rougher; stick with the 9's.

Do a compression test ASAP.

[rogerthechorister]

The above quote may be the key.

If you can check that you are still getting a nice regular spark once teh misfiring starts (try 6 little spark sensors, or even a gunson colourtune or a timing light, none of them are dear) then it isn't sparks. If teh sparks get feeble, erratic, and disappear it's sparks. You have done the obvious, and the next steps would be the crank sensor and ignition amplifier - all covered above.

So, what changes after a few minutes? The engine warms up (the thermostat opens) and the fuel injection system goes from being in "cold start" mode to "ordinary running" mode. Mr Haynes is not giving me the info I need to go further.....

[BillB]

If you're ALWAYS and I do mean ALWAYS hearing the fuel pump each time you crank, then it's NOT the RPM sensor - it can't be. The fuel pumps will not run if the RPM (crank) sensor is faulty. But it could be the power stage aka ignition amp. You really need a timing light as I've explained previously, to ascertain REPEATEDLY if you have intermittent spark. Regards.

[TheJoyOfSix]

Some very good advice from previous posters and I'm suprised we haven't got to the bottom of this yet.

Three other things spring to mind;

The electrical plug to the Air Mass Meter, the Fuel Pressure Regulator, and the cat/lambda sensor.

On the passenger side of the engine just behind the airbox (where the air filter lives) is a ally bit of pipe with a plastic box on top which has an electrical cable going to it. This is the Air Mass Meter. the connection between the cable and the unit can be prone to failure as the cable is fixed to the inner wing, but the AMM moves about as the engine rocks when running. Over time this can pull gently on the terminals in the plug and cause all sorts of bad running problems. Try disconnecting the plug (remembering to depress the catch first) and check that all the female connectors are level with the end of the plastic inner housing. Also check for cleanliness.

On the top of the engine near the front, between the two cylinder banks and just behind the waterpump, sits the Fuel Pressure Regulator. This is a brass cylinder about 2" in diameter and the same in length. It has a fuel line frome each fuel rail entering at each side, a fuel return line exiting at the front, and a vacuum line exiting at the rear that is connected to the inlet manifold. This unit maintains a constant 2.5bar (on the B280) of fuel pressure in the fuel rails by means of a vacuum operated diaphragm. If the diaphragm should split then neat fuel is sucked straight into the cylinders resulting in very rich running. The car will still start as the mixture needs to rich, but as soon as it starts to warm up it will swamp the engine most likely causing it to stall. The best way to test this is to pull the vac line off and see if any neat fuel dribbles out. There should be none. A new FPR is about £35-40ish.

Given you car is a 1990 one it will either be a 1990 or 1991 Model Year. thus it is fitted with a Catalytic Converter. If the Cat is blocked (not unknown on a car of that age) then it will initially start, but then stall as the gas pressure builds up in the exhaust. Equally a fault with the Lambda (aka Oxygen) Sensor could cause bad running.

If the Sensor is at fault (a competant garage should be able to test this for you) then it will need to be replaced. If the Cat is at fault then given they were not a legal requirement in the Uk until 1992 it can be replaced with a straight section of pipe, but note the Lambda Sensor will still have to be fitted to the exhaust to allow the Engine Management System to get the correct readings.


EDIT: just thoughtof something else!

It could be the Engine Coolant Temp Sensor. This is located on the passenger side of the waterpump and is prone to having dirty/damp connections. Unplug the sensor and clean up the terminals a la the AMM tip above.

[brainsayers]

Finally problem solved, crushed the heap of S***.

[stinky]

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