1995 XJR. Is anyone able to tell me how the car determines how much fuel is being consumed, for display on the dash?

I'll be driving along a flat road at a constant speed with no change to throttle input, and the display will show 8 L/100km, then jump up to 17 for ten seconds or more, then back down to 8. It does feel like there's misfiring going on then, perhaps flooding.

Thank you,

 

The engine ECU "knows" the flow rate of the fuel injectors is 5.125grammes/second. The ECU calculates how long to turn on the fuel injector each time it opens. The software can therefore calculate how much fuel is being injected. The ECU sends a pulse to the dashboard for every fixed volume of fuel injected. The dashboard also gets a vehicle signal from the ABS ECU. The dashboard can calculate the instantaneous fuel consumption from these 2 signals. Even if the engine speed, throttle position and vehicle speed are completely constant, the instantaneous fuel consumption will still vary because the closed loop fueling control will continuously cycle the fueling rich and lean, at the frequency that you have noted, in order for the catalyst to work. This is perfectly normal. In practice, it is quite difficult to keep all of these 3 variables perfectly constant. Intantaneous fuel consumption is probably the least useful piece of information that can be output from the dashboard, and that's coming from the person who was responsible for calibrating the function. The average fuel consumption is much more useful, and is as accurate as I could make it. It was calibrated based running the tank from full to empty many times, and measuring the fuel volume added and distance covered.

 

Thak you Andy. That's the info I was looking for, with one bit of clarification needed re: the quoted line above. Is the ECU able to do this because it is measuring the duration that the injector is open?

My real question is, if an injector is sticking open, will the ECU record the time it is actually open, or the time it was told to be open? I'm trying to diagnose a fuelling issue. I just drove 900+ miles in two days with the traction control and ABS lights on most of the time. Probably unrelated. Driving on the flat was mostly ok, with the fuel consumption jumping way up then down from time to time. But every time I went up a hill (the slightest increase in throttle) the L/100km would go up to 48 or 73 etc and the engine would bog down. I'd have to let off the throttle to keep it running. Once early in the trip I had the cruise control on and when it opened the throttle aggressively to climb a hill, the cruise cut off as the revs dropped to nearly zero. Most times this condition happened, I was unable to maintain highway speed and had to pull to the shoulder.

A new fuel pump was just installed. Other than that, I don't know the car. Odo now a genuine 71,000 km.

 

The engine ECU won't know if the injector is stuck open. it only know the time that it should be turned on.

There are lots of possible causes of incorrect fueling:-

failing oxygen sensors
failed coolant temp sensor
failing throttle potentiometer
failing mass air flow sensor (though I've seldom come across this failure mode)

 

I wouldn't have said that! I bought a 1995 XJR a few years ago as a quasi runner. I say that because most of the time the car would run ok, and then randomly flood and stall. The conditions actually sound very similar to the OP's, but mine were more extreme in that the car would stall, and could only be restarted by holding my foot to the floor to shut off the injectors and much cranking.

The previous owner had the car into a well respected Jaguar mechanic in town and I talked to the mechanic after I bought the car. Turned out the previous owner had connected the leads backwards when attempting to jump start the car with a flat battery, and it never ran right after that. Being a good mechanic, he had another XJR in the shop and tried substituting parts one at a time and could never solve the problem before the PO ran out of money.

I was able to solve the problem by watching the instant fuel consumption. I was on a test drive through my neighbourhood at a steady 50 km/h and the instant fuel readout was between 30-40 mpg. Then all of a sudden it went to 2 mpg and the car stalled. I went through what controls fuelling and the main things was the MAF and the ECU. I had my OBD reader attached and I could see that the TPS and coolant temp reported didn't change, so I figured it had to be MAF and ECU. I changed both parts and the problem went away completely.

I have a suspicion that both the ECU and MAF were damaged by the reverse polarity, and were operating in some sort of reduced capacity mode, as the mechanic had tried changing both components one at a time with known good ones and couldn't eliminate the random flooding and stalls. I suspect them with both compromised some particular condition lined up and commanded erroneous full fuelling. As an experiment when I had a clear road, when I noticed the instant fuel readout showing increased fuel consumption I floored the throttle and the car bogged for a moment and then did accelerate at the normal full throttle rate.

I know XJR MAF's are not easy to find, but the OP might try replacing his and see what happens, along with verifying the TPS is reading correctly.

 

I would just add to possible causes of incorrect fueling, the "Fuel pressure regulator" which due to failure may cause "overfueling" at low speeds with subsequent rough running and possible stalling.

John Herbert
(1996 XJR)

 

Thank you to all for your guidance here. So if fuel consumption is shown to be high, it's because the ecu is telling it to be...therefore the ecu's inputs are suspect.

I need to start with some basics like checking hoses, spark plugs, etc to see what I'm dealing with. It's very grimy underhood, having been treated for winter use. I'll give it a good clean and visual inspection before I begin my journey through all the systems. It needs cosmetic fixes as well as the mechanical side, but I liked the price and the car has a good story.



P.S. Robert Laughton If you're here...I'm thinking of visiting in the spring for some parts.

 

Yes, but it might also be the ECU itself. That was the case for my car. The ECU for 1995 is unique, the 1996 and 97 cars have EGR, the 95 doesn't, so if you do replace the ECU you need to find a 95. Or get an working core and I think XJRengineer can reprogram it for you.

 

Also check the condition of the electrical plugs to the engine ECU itself. Water ingress to these connect is a know problem on X300, and can cause all manner of electrical problems.

 

It is clean and dry. However, that scribble on the red connector makes me think someone has been here before.

 

Might be worth unplugging those connectors and having a look. The way the wiring harness is routed it is possible for water to run down the wires and into the connector on the ECU.

 

My shortcut wishful-thinking visual inspection was not enough, thank you Jagboi for politely pointing that out, lol...here's a photo of the plug and you can see something wet. It is thin like water but the colour of used engine oil. I haven't seen that before--would it be dirty water? The male and female brass (?) connectors are not corroded at all.

And in the second photo, the red and blue wires butted to red and brown wires...does it look like factory or later add on? I do know this car was wired for some sort of telephone when new, that's all.

 

The red and blue are definitely aftermarket wires, the factory wouldn't use crimps like that.

 

A small and tangential point, but you mentioned cleaning the engine bay before starting work. I would be tempted not to until you have bottomed out your current issue.. There may be some clues in the grime, and the process of cleaning could itself cause issues, as many components under the bonnet will not enjoy water or cleaning fluid of any sort.

 

Good point! I'll stick to rags where I must for now.