Why I think my ECU needs a replacement? As I sit in idle my car MPG goes up rapidly and while driving it decreases considerably. On my normal drive to work, I usually get around 17 & 18. Right now it is at 14.3 and continues decrease as I drive further.

What I think caused this? Well whenever I had someone jump my car earlier this week while I was hooking up the jumper cables to my battery this person touched the positive with the negative cables causing it to spark. I didn't think of anything at that time but now realize it may have fried some components of my ECU. Everything else runs fine (as far as I know) except my MPG and Fuel used. The reason it is important is because my fuel sensor is a dud. I will use primitive methods to estimate my range until I replace the ECU.

I wanted to make sure before I buy one if that could the problem.

Thank you

Pedro

 

If you change the ECU keep in mind there are many different part numbers in many different parts of the world as well as 3.2L / 4.0 L and S/C / normally aspirated versions .

Have you tried a hard reset to get the ECU straightened out ? The engine will need to relearn some things for a while so it will be off until it relearns them .

I hooked my battery up backwards and fried the voltage regulator .

 

Yup I will make sure to keep that in mind when purchasing the new ECU (if I will need to).

I did hard reset the computer yesterday and still is behaving erratically. I have hard reset the computer before but I do not recall the jumping up and down of the MPG. Before driving to work today the computer indicated I had 0.5 gal of used fuel. When I arrived to work the computer indicated 2.1 gal of used fuel. My trip from home to work is only 6.1 miles. So mathematically it does not make sense.

LOL are you trying to imply that I was the one who did this??

 

No , I set my battery in backwards and hooked up the post . Easy to do .That will never happen again , I promise .

By reset do you mean removing and putting both cables together ?

 

I almost did that when I purchased a battery for Jacklyn lol I wasn't too confident and I decided to call a tech guy at wally world. There was no indication of positive or negative on the battery except on the side wall which I didn't see when I placed the battery in the trunk.

But I did not, I disconnected the negative cable for about an hour or so. I will do what you recommend above and see if that works.

EDIT

To confirm, disconnect both positive and negative cables and momentarily touch them?

 

There is a key on procedure with the hard reset I'll get after I eat my Fruit Loops

 

Hi Pedro,

Have you had the car scanned for any diagnostic trouble codes (DTCs)? Any flagged codes may help narrow down your diagnosis and possibly save you the cost of an engine control module (ECM).

Bear in mind that the onboard computer estimates fuel economy readings based on inputs from sensors that communicate with the ECM. A problem with any critical sensor such as the fuel level sender can affect the computer's calculations. And my own scientific fuel economy studies have shown that Jag's onboard fuel economy estimates are frequently off by at least 2 mpg, so you should view them as a general trend, but not truly accurate.

Cheers,

Don

 

The main difference is the PROMs (chipset) in the ECM.
I performed this task DOZENS of times at the dealer when the recall was issued!!!!!

Easy enough to check part numbers.

bob

 

Hi Parker,

I was able to find the thread on it, disconnect negative cable (ignition in off position), turn on ignition, touch negative cable while positive is on post, turn ignition off and connect battery. However using this method did not work.

Hi Don,

the only codes are the fuel sensor, which does not work. I do not have any other codes at this time. I know the calculated MPG is off, hence if I add 10 gal of petrol, I assume I have 8. Using this method has not left me stranded since I have owned the car.
I drove another 4 miles after the hard reset mentioned by Parker and my computer is reading that I used 1.5 gallons.

 

The calculation of the fuel economy comes from information other then a direct fuel flow rate meter , but does come from a direct wheel rotation count ( but that is subject to error with non - standard tire size and tire pressure ) . Can't remember which wheel sensor it uses . Or the wheel rotation count can come from the transmission output speed sensor at pins CC48 - 36 and 51 at the transmission ECU as it gets converted to a digital signal inputting The engine ECU with the orange data bus wire at pin Papa Indy 105 - 10 . You would have to read the orange wire data bus with a reader that is set to that protocol . The ECU sees the fuel injector pulse time and frequency and assumed fuel pressure as this varies with the intake manifold vacuum pressure dependent on the RPM and butterfly position . The X300 doesn't have a MAP sensor so the ECU must derive this from EPROM tables ( or data array in computer science terms )( is you FPR functioning properly ? ) . This is how it calculates the fuel flow rate . So the final computation is the amount / distance . The easiest thing would be the the cleaning of the wheel sensor as the signal may be good enough to not get a ABS light but not good enough for the ECU mileage calculations as that does not have a CEL code . Are your fuel injectors clean as this would effect the true flowrate calculation .

I wish I had more detailed information on the ECU so what is above is a guess .

I could be wrong I'm under the influence of a bowl of Fruit Loops

 

More info for you.

bob

 

Your oxygen sensor on your catalytic converter needs replacing asap! Try that and see if your milage improves. The oxy sensor or "lambada sensor" tells the injection system how much fuel to utilize and if the sensors damaged or nonfunctional it has a tendency to make the fuel ratio too rich leading to reduced MPG and clogs up the catalytic converter with carbonized fuel deposits which can eventually lead to an engine fire. Not to mention how expensive it can be to replace the catalytic converter. So, spending $20-$60.00 on a new sensor is more then well worth the while for a your Jaguar! Do it ASAP!

 

Hi Chris,

Note that the thread to which you have replied has had no activity for more than two years, so presumably Pedro has resolved his issue by now.

I am curious to know how you diagnosed failure of the oxygen sensor without Pedro reporting that he had scanned any Diagnostic Trouble Codes (DTCs) related to an O2S, fuel mixture or catalytic converter problem? Unlike your XJ40, an X300 has four oxygen sensors. How would you know which one to test and, if necessary, replace?

We appreciate your sharing your knowledge, but in this forum we try to encourage the careful diagnosis of a problem before spending money and time replacing parts. With Jaguars, there is always something to spend money on, and throwing parts at a problem gets very expensive very quickly.

Cheers!

Don