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Do you have or can you borrow a vacuum gauge? If so, check the engine vacuum. It can tell you a lot and will definitely let you know if your timing is off.
I finally had some time to dig into it. The spark plugs I put in it not realizing heat ranges were a thing were Bosch 7's (FR7LPP33), I now know the OEM ones are Denso 16's (12582002). So I have Oreillys trying to special order the ones the owner's manual directly calls for now. Not sure if that's drastic enough to cause an almost 90 mile per tank difference but it's worth a go. Also for the Timing belt Water pump and thermostat, I'm just going to replace all of them this weekend for preventative maint. so hopefully the issue will be fixed by one of them. If they can't get into the OEM spark plugs that Saturn calls for does anyone know what the next best one would be?
Bosch spark plugs are notorious pieces of crap. I wouldn't run one of those in my lawn mower. It's ALWAYS best to stick with the OEM brand and heat range plugs for optimal performance.
As someone else who lives in Texas, I do believe we run a higher ethanol content down here. Something I've considered starting to check and sample as of lately. I'm sure my 16 Cherokee can account for it. My 06 grand caravan runs off turpentine or premium. But I drive upwards of 1000 miles a week. Always weighed down (mobile mechanic) so I'd like to try to get my mileage from mid teens to upper teens if possible.
Anyway.
You could also try cleaning the throttle body. Doing a top end induction service. Making sure the air filter is clean.
Premium oil such as Amsoil can help a tad bit also.
I run Amsoil in my Cherokee, along with rotating between the performance improver and upper cylinder lubricant (both fuel additives) and I saw almost a 2mpg gain out of it.
Doing a level kit and all terrains kind of hurt the mileage 🤣
Work van will get the same treatment once I have time to tear the engine down and reseal top to bottom.
I am an Amsoil dealer if interested in learning more about that. Not trying to give the gimmick salesman speech or anything. Just sharing a real life gain from using it.
For spark plugs, Honda uses NGK and Denso iridiums in their J35 V6 engine lines that our Vue's use also.
NGK: IZFR5FK-11
Denso: SKJ16DR-M11
Either of these options will work well for you.
Also, be careful where you buy spark plugs from because there is (surprisingly) quite a problem with fake spark plugs online (yes you heard that right!). Fakes are no longer just a problem for Hermès hand-bags; apparently there is a large enough market for scammers to make and sell cheap copies of spark plugs. I'd avoid anything on eBay or even Amazon for that matter.
As to your severe mileage reduction, the only other thing I can think of to check, in addition to what's been mentioned already, is a front alignment and the front brake hoses. An iffy alignment will rob some fuel economy if it's severe enough, and the front brake lines on the Vue have been problematic (at least for me), I have had to change them twice now over the past 8 years. They can develop an internal restriction that prevents the brake calipers from disengaging fully, even when the brake hose looks perfect on the outside, so you are essentially driving around with your brakes half engaged all day. Have them inspected carefully.
I finally bit the bullet today and just bought a ****ty 100$ code reader, It shows EVAP as red but won't give me an actual code other than some weird ones. I'm just surprised it never threw a check engine light. So now it's time to dig into this.
But that image shows 9 (in Hg), which is 17 to 19 inches of vacuum, depending on the elevation of the location. I've been tripped up on this too, because different tools give readings in different units.
