Excess Fuel Consumption - Volvo 960

Many thanks for such a comprehensive answer.

It may be the Oxygen Sensor. I bought the car 6 months ago.

Checked underneath and found 2 wires going to the exaust had been disconnected.

I connected them and heaps of black smoke poured out of the exaust. (could not see cars behind me) so promptly disconnected them. I presume the 2 devices on the exaust that the wires go to are oxygen sensors.

Can you advise me if I can take the sensors out and clean them or need to find replacements.

Interestingly with the wires disconnected there is no black smoke coming out of the exaust. I started the engine then put mt hand over the exaust and some black fleks come out and also my hand was a bit wet and smelt of petrol

Just a comment on the Engine type: The Volvo 960 does not sport the V6 PVR engine, but rather an all Aluminum 24Valve Inline 6, which came in two sizes: 3.0ltr (204 HP) or 2.5ltr (170 HP). Best Regards Tommy Zielinski

You need to start at the beginning and check cylinder compression. It should be a minimum of 120 PSI, if not there is a problem, tired motor. If not then check and see if the injector is leaking and also check for vacuum leaks in the intake, injector seals and throttle body. A black plug is sure sign of too much fuel. The engine needs to have 195F coolant temp for the management system to switch into closed loop and then operate efficiently another thing you need to check. O2 sensors could be worn out and cause the problem. They should produce 0.5 volts at temp with the engine running at 2000 RPM. Check those things and then tell me what you have found and we can go further into the problem! 35 year experience Jaguar and Rolls Royce tech expert! johnd7618@cox.net

Hello

The car has only done 100,000 Km, and is generally in very good condition. Problems so far has allways been caused by corroded electric wire connections. I don,t see any leaks but I am living in malaysia with minimun tempertures around 30 degrees centrigrade and leaking petrol quickly evaporates.

Hello

Because I use this car in Malaysia at 30 - 40 degrees temp it stikes me that many features on the car like heated windows and heated seats are redundant.

With the thermostat in the car runs hot. Without the thermostat it runs at about normal temperture. Stikes me the thermostat is not needed and leads to overheating when going up hills or heavily loaded.

If I disconnect theTemperture Sensors (so no signal) will the computer think that the engine is hot, or will it think the engine is cold? (and enrich the mixture)

Are these sensors either on or off. ie signal or no signal ?

So if there is no signal from the oxygen sensor then the computer assumes there is no oxygen in the exaust

Many thanks for your interest. Actually my desired object is to customise the controls to suit Malaysian conditions and avoid expensive sensor replacement.

I agree with others that you need the thermostat. I also agree with many of the comments already made....

Also:-

You could have a blocked radiator, both externally or internally.....

Ignition timing may be wrong.....this can cause overheating.....

Wrong type of petrol (Agriculture petrol - low Octane - rather than say Super!) will also cause overheating.....

If you have no vacum leaks then look to the manifold pressure sensor (MAP). Like the first responder said, this engine is a candidate for boat achorage. I might even look to the valve train for anything irregular. Somehow the engine is being told to over fuel the system.

The 'story' has lots of 'mystery'. The previous owner undoubtedly (high probability) experienced the 'super rich, cloud of black smoke out the tailpipe' condition, and found out the Oxygen sensor and/or ECU circuitry was the cause of the super rich condition. So he permanently disconnected the Oxygen sensor. (Good for a $10,000 fine in the U.S.) So you purchased a car that had been bootlegged rather than fixed.

The ECU reverts to a back-up 'open-loop' operating mode when key sensor inputs are not available. The ECU uses a 'default value' in the calculations. Those values are selected to provide reliable, but not economical operation until the fault is corrected. The engine power is reduced, and fuel economy is usually very poor, but it DOES start and run. And the 'check engine' or Malfunction Indicator Light (MIL) is switched ON when there are faults of this nature.

Then, something else happens (slowly most likely) to cause the 2 center fuel injectors to feed more fuel (or the other 4 to feed less). Partially plugged fuel injectors, sticking open/leaking injectors, leaking O-rings where the center injectors seal to the fuel rail-allowing gasoline to leak directly into the intake, OR a leak in the fuel header pipe. Volvo made a lot of aluminum tube fuel headers with an aluminum disc 'plug' in the ends. (and replacement parts are made exactly same way) The disk is held in and sealed with a simple rolled joint---and it is VERY, repeat VERY common for them to start leaking. The leak can be tiny, or significant--4 L/100 KM at highway speed is not uncommon. The fix for the leaky fuel rail is replacement-which will also eventually start leaking, or welding up the end plug (or using gas tank sealing epoxy all over the end cap joint.)

Summary: You have multiple problems/failures to address. 1) Disconnected Oxygen sensor system MUST be repaired and returned to factory specified operating condition by replacing the sensor and fixing the reason it was disconnected in the first place

2) figure out why the 2 center fuel injectors are not balancing with the other 4 injectors--probably leaking O-rings AND dirty fuel injectors

3) Inspect carefully to ensure the fuel rail is not leaking at the end plugs (or anywhere else.)

Good Luck--this IS NOT going to be low cost. OEM Volvo Oxygen Sensors are VERY costly and substitutes often are not satisfactory for some reason. If the ECU is bad--find a good (working) used one. A new one will cost more than the vehicle is worth. Pull all the fuel injectors with the fuel rail and have them professionally cleaned and the O-rings replaced.