I changed my plugs a week ago and i was getting 25+ highway 16-18 city on old plugs. Now under 22 highway 14-16 city . Makes no sense to me. 94k on car. The old plugs were not bad at all.

 

Did you verify that all the plugs had the proper gap? While they should be pre-gapped, and you shouldn't gap them, it's still important to verify that they have the proper gap. And did you use OEM plugs? And did you make sure that all the coils were on properly?

 

I did not gap them. Yes they are OEM plugs. I did make sure every thing was put together the right way. I would think if something was wrong it would not run very smooth.

 

First thing that come to mind is that the LS430 still "remembers" how the old spark plugs performed, and now with new and different plugs, the whole performance dynamic is suffering a mismatch - have you tried disconnecting the battery to reset the ECM and start again from factory default settings?

Otherwise, maybe one of your cylinder is not firing. See attached procedures to troubleshoot ignition problems.

 

I took the battery out to change the plugs.So yes every thing was reset. The next warm day i think i will put the old ones back and see what happens Should be tomorrow.Run a week then will repost results.

 

Well went change plugs back to the originals and found i did not tighen the 2 big clamps on the air breather. Not sure if that has anything to do with gas mileage. We'll see

 

From what I've heard , that unless the plugs being replaced are playing havoc and the car is running poorly, new plugs should not really improve the gas mileage to any great degree on our air craft carriers.

 

That "reset" could very well be the reason for the reduction in performance. Your LS430 took a long time to "learn" your driving habits, and the factory default settings maybe a bit more aggressive than the way you actually drive.

Remember, the LS430 is constantly learning to optimize performance. It may take a bit of time for it to return to the optimized settings for your driving style.

 

A week's worth of driving and you know that your MPGs have dropped from 25 / 16-18 to 22 / 14-16?? I question the validity of that data. I'd suggest giving it more time and measure your MPG carefully over the next 3 or 4 or 10 or 20 tanks.

 

The car ran perfect with the old plugs . I just thought i would replace them at 94k miles. I know if it is not broken don't fix it. I have only had the car for 6 weeks and fixed a few things.I have had 3 outher 430 so i know something about these cars. 2- 2001 2003ul now a 2004 sport. Preffered the non sport. They road smoother less bumpy.

 

Three observations:
1) I left the two tubes off of the air breather and it definitely affect that performance of the car.
2) I've actually bent the end of the spark plug when inserting it in the engine once. I would pull all the new spark plugs and verify gap. Also use a torque wrench, set at 14 ft-lbs, and make sure each spark plug is seated uniformly.
3) Also check each electrical plug to the Coil on plugs to make sure it is seated tightly.

 

I paid a repair shop to replace mine simply because I do not like screwing a metal based plug into an aluminum block and having to use multiple socket extensions to perform the task. Cost me some $235 for OEM plugs but this is one project that I cower from performing.

 

While you are at it clean the MAF sensor and throttle body. The TB will definitely need it if it has never been done.

 

the MAF sensor looked brand new.There was not 1 speck of dirt on it and the wire was still silver and I did clean throttle body did not really need it tho. Even tho the carfax showed no service there has been a good amount of work done because of new parts and how clean every thing is under the hood;

 

So i went back with a torque wrench and tighten then about a full turn or more to 14 ft pounds. Maybe that will help. I travel the same route everyday. Same traffic, same stop signs, same lights,. I reset tank MPG and i am down 2 MPG every trip. Will see if tightening the plugs will make a difference