I have mentioned this before - many times! I have 3 V8 4.0L Jags. One, a 2002 XJ8 SE is sluggish on throttle response. Another, a yr 2000 XK8 is exactly the same, and the other, a yr 2000 XJ8 Sovereign goes like a rocket when using the throttle! The problem can best be described as flat spot when the throttle is used. So if I'm in traffic at 60mph and pull out to overtake, two of the cars have no initial response until the throttle is pushed down a fair way (not to kickdown). The Sovereign however just tears off and I have to lift off!

It's not the gearbox because none of the cars change down. It's not throttle cable slack because with your foot on the pedal the slack (if any) is taken up. I have adjusted absolutely any trace of slackness out anyway.

I have swapped the MAF from the 'rocket' to the 'snail' with no difference. The connectors to the throttle body have been looked at, cleaned, and refitted. Nothing there (perfect condition). So what's left? The throttle bodies - and the ECU!

We are threatened with storms tomorrow, but if none come I might just open both bonnets, find the ECU's, and swap them over! (I'm assuming they are under the bonnet).

Fingers crossed !!

 

are you putting in the correct gasoline grade? I know in the US the car says PREMIUM only.

 

Could it be the gas you're using? Two cars with the same symptoms. Hmmmm.

Injectors dirty maybe.

 

We only have one gas grade in the UK (we're broke you know!). Both cars - same gas. The 2002 XJ8 SE has now done 240,000 miles over 10 years and has always been like it. So I wasn't too surprised when I got the XK (and it was the same). The surprise came when I bought the Sovereign (to replace the SE) and found it goes like a rocket.

It's so ruddy annoying !!

 

Sounds like my "she's lazy" post. I've got an interesting thought. Is there a difference in rear end ratios?

 

Wouldn't make any difference Lox. The car doesn't accelerate, your suggestion would mean that the rate of acceleration will be less.

 

I'd put the torque app on and measure your fuel trims and post them.

 

I've been trying to find a way to install 3.73 gears in my MY03 because I know the acceleration would be much improved. My thought was that maybe your "rocket" has something other than the pathetic 3.06s.

 

All the cars are bog standard 'as built' units. To be honest I wouldn't mind if it did accelerate a bit less fast - but it doesn't even try! When the engine does finally respond it speeds off just fine, it's just that annoying delay that bugs me.

As it happens it's now tipping with rain - job delayed for a couple of days. (My son's Mini is in the garage, the shiny new Carport is in bits awaiting assembly, and I'm not going to open up the electrics in the rain!)

First job is to locate both ECU's and take a look to see if they are the same. The two I'm swapping are on 4.0L V8's that are within 3 months of each other so I'm hoping they will be identical.

 

Check the VCATS list in the bottom of the boot where the spare is. If the letters and numbers match exactly it might work.

 

Just realized you plan on swapping the Sovereign ECU into the XK8. Not likely to work.

 

Even if they have the same part number that does not mean they have the same software. You should compare the the ECU entries on the 'VCATs' labels in the trunk (usually near the edge of the spare tire well in the boot) for part number and software (VCATS) version. The software version is usually not on the module itself.

Also check the driver door jamb for any service action stickers that might point to a service action that included a software update.

 

I think that as one is an XK, and the other an XJ, the engine mapping is bound to be different - but what I'm looking for is the elimination of that wretched throttle delay. If the delay goes (or moves the XJ), I will replace the XK ECU.

Worst case scenario - a bang to rival the one in China and a big hole where Hampshire once was!

 

Jim - Let me know what you find out... I too have some issues when I hammer the pedal and the car seems to momentarily "die" before it kicks in and gets going. Occasionally I was getting some code (can't remember which) when I would go for a spirited drive and when searching that code it referenced cleaning the MAF sensor and/or replacing the fuel filter. I am hoping that the fuel filter is the cause for my issue (fingers crossed) and will let you know what I find out (even though my car only has 31k miles, the fuel filter is probably almost 15 years old).