Hi guys .I have a xj8 4.0 v8.2001 on lpg. It was due a service so I decided to change the plugs along with oil and filter. Plus air filter. Now because it's on lpg I did a quick search to see what was the best plug to fit. I was told not to bother with all these irrium plugs or triple spark ect but to fit the original ngk ones .pfr54-11. Which are the original ones so I went for these but lazer platinum. Now b4 I fitted them I was getting 16mpg on petrol since replacing them only just about making 14.7 won't do no more. Now the plugs I took out were ngk irium .do you think this is a plug issue as I'm quite lost with it because correct me if I'm wron but in theory mog should of improved after a service and I only brought the best qaulity parts.

 

Might change the title to include LPG as this might get someones attention that has this version .

 

Have you kept the original plugs? Jaguar actually recommend the IR plugs nowadays as a better part. Stick them back in and see if the miles go up...
The other thing to carry out after a service is 'hard reset' - do a search for the term and it's function.

 

If that doesn't get you anything I would suggest a " Hard Reset "

1. With the key in the on position , not running

2. Remove both battery cables

3. Touch both cables in the air together ( the longer the better as in overnight with a clamp or jumper cables )

4. Don't let the cables touch the battery post as you will have to start the sequence over . Put a towel over the post .

5. Turn key to off and remove key

6. Rehook the battery cables to battery post

This will in no way harm anything but it resets or reboots the ECU's back to zero state without effecting the ECU's operating software or firmware .

The engine ECU will have to relearn the Long Term Fuel Trim so it will be off for awhile until it relearns it .
__________________

 

Hi thanks for the reply and I found. Yes I will try putting them back in. And do a hard reset which I didn't do .thanks