Hey guys. I recently purchased a 04 LS 430 that currently has about 178k. I have two issues that I would like to address on this forum. Whenever I top up on gas, I notice that my tank doesn’t stay full. The weird part about this is the needle goes down as I’m driving for a few minutes, then goes back up again. I know I have a 4.3 liter V8 but, it seems my gas can’t even last me a whole day smh. The second issue I’m having is that my car is burning a lot of motor oil. I took it to a repair shop and was advised by the customer service rep to not put any more money into the car. They saw oil on my spark plugs and told me that it’s and internal issue, what’s scaring me is…..I’m just hoping that it has nothing to do with my pistons, valves or anything that requires me to take the engine out. Any thoughts on this guys ???

What's the consumption on the trip computer? I haven't drive my car much since I got it but I think it averages 16.8mpg in the winter. Meanwhile my sc430 gets like 19mpg in the summer. I guess it's cuz the ls is way heavier.

We need to know what the actual fuel economy is- fill up, drive 100 miles, re-fill, tell us how much fuel you used. The fouled spark plugs and oil consumption looks to me like piston rings might be bad. Unusual for this engine but if it was not maintained for those 178000 miles it is conceivable the engine needs a rebuild or replacement. If the plugs are truly fouled your mileage will be bad no matter what.

thanks for the info. I’m going to get a second opinion somewhere else to see what’s really going on.

Check your PCV system, if it's clogged, it can cause major oil consumption. outside of that, the previous owner may have skipped oil changes and the motor has a lot of wear in the cylinder walls, causing oil to be burned in the cylinders. if that is the case, I would look into buying a new/used engine

Man that’s a scary insight. Like I stated before I would have to get a second opinion. Thanks anyway for the advice!!!

Couldn't these be 2 completely separate issues? The fuel gauge floating from full to lower then back up (while driving) has NOTHING to do with your current fuel economy or perceived poor fuel economy. It's likely a sending unit that is going flakey. To make it super easy on you, what does the dash/computer say your fuel economy is? It can range from 15mpg (terrible-probably stop/go/city/idling behaviors), to 29mpg (great-probably cruise control at 65mph on a level highway). The fuel economy MOSTLY depends on how the car is driven.

Relating to the "oil on my spark plugs"....do you know if the ELECTRODE was fouled or if oil was on the ceramic/insulation/EXTERIOR part of your spark plugs? It's a HUGE difference WHERE the oil is.

Regardless WHERE the oil is, you will THINK that you're "burning" oil, but actually might not be. You could simply have loose bolts on the valve covers or the gaskets are leaking.

If you were burning oil, you might see blue smoke out the tail pipes. Do you? If you were leaking oil, you might see oil stains/drips where you park the car. Do you?

There's a ton of unknowns here.....and what has my hairs on end are shady/dishonest mechanics who will portray they're doing you a favor, and buying this amazingly reliable car from you, as they scare you to sell it at a bargain to them. Just my 2 cents.

Those are good valid points you listed for the “burning oil issues”. Not sure what the Electrode is. As for oil leaking on the ground…..none blue smoke from my tail pipe…..none. I went to a mechanic shop and the service advisor juts told me that the techs found oil on my spark plugs, didn’t specify anything that you mentioned above.


I will re-evaluate what the dash computer says thoroughly for a compete analysis on the mpg. The feedback you gave was greatly appreciated……thanks a lot sir!!!