I took the Ls on a 40 mile highway trip and lost half a tank of gas, that was the first time I have drove the car that far after the tuning it up with new plugs, wires, filters, fluid. but 5MPG is crazy do you think a bad aftermarket coolant temp sensor could cause that? Or does my problem sound worse?

 

How was your gas mileage before the tuneup?

 

Model year?

Is this the mileage being reported on the instrument panel or is this calculated? Don't discount a faulty fuel gauge or sender.

 

It'd be nice to know what year your car is so we don't have to search to find out. (1990) But anyway, I highly doubt you're getting 5 MPG. The only way to know how many MPG ya get is to fill the tank, go on your trip, and then fill your tank when you're back, then do the math for how many gallons you used for the miles ya went. Just going by the gauge, or even the computer in the newer models, isn't accurate at all. I'd say there's something wrong with your fuel gauge or the caps in the cluster. Very common problem on these cars.
Or you have a very bad leak in the fuel system just dumping it out, but you would smell that.

A little tidbit on mileage; When I bought my 93, I filled it and drove home over 300 miles all highway and filled it when I got home. I got 28 MPG. (In a 1993) Now most of the time I only drive about 2 miles a day. After tune-up, ECU, and everything else I did, I get about 14 MPG and believe it or not I'm happy with that. Most of the gas used goes to warming the car up or sitting at 2 lights and 2 stop signs. Ya gotta add that wasted gas into your mileage also. I laugh when people brag that they're getting 35-40 MPG. Ha. Hell, if ya go that way then my daughters Civic I get 70 MPG. (when I'm going down hill not touching the gas pedal)

 

thank you guys for the advice I appreciate it, I do have a 90 and I never calculated my mileage before because it was never a big issue to me but I do have problems that cause high idle and over revving and I have a rich gas smell from exhaust. But I did use the instrument panel to calculate my mileage because I started with exactly half a tank on the meter and when I got home it was on E. With these gas prices I just parked it in the driveway and haven’t drove it anymore haha but that would be awesome if I didn’t actually burn all that gas and it’s just a faulty gauge, that hurt watching my money be burned like that

 

what commonly causes that is a bad 02 sensor, maybe see if you can borrow a code scanner from a parts store or whatever, see what comes up

 

These fuel gauges, like most cars, when full to half tank it looks like it hardly burns any gas. Then from half to empty looks like it just pours out. Ya can't use the gauge to figure your mileage. Like said before, ya need to divide gallons into miles to get correct MPG.

 

If it smells too rich then that is probably related to the problem directly. Hopefully your cats are not bright red after a long drive.

Use trip meter B on the cluster of your 1990 to calculate gas mileage with the number of gallons you fill up with at the station. It'll be the most accurate

Bad O2 sensors could be your problem, but it takes proper diagnosis to find out. Better to not fire the parts cannon yet

I would suggest testing them first. This is a good guide -


Unfortunately, the O2 sensors are not in a good spot on our cars.. They're accessible but beneath the car behind the front wheels. If you're not an expert with electronics or maintenance, I would suggest to let a shop diagnose this. They should be able to test it safely

 

On my way to work I filled my tank and drove about 21 miles(lots of stops) to another gas station, put 1.5 gallons back in so 13 or 14mpg?

But last week I for sure had $35 in my tank which is half a tank, and it was all burned in the same distance I drove today so that’s weird…

The only thing I changed was I wiped my really dirty throttle body out a bit and rechecked my coolant sensor but could something that simple make that big of a difference?

 

No, not 8 MPG difference by just wiping the throttle body. When the gauge is on the E mark there's still more fuel in the tank. How much??,,who knows?? I know there's 3 gallons left when the idiot light comes on. If you burn the full tank then fill it and divide it up you'll get a much better average MPG highway, city, stops and gos. Doing that also helps in the future if the gauge ever breaks so you know how many miles you can go so you don't run out. But getting 13-14 in a 1990 is pretty good nowadays. I think it only got 18 when new. It's tired, no matter how much maintenance has, or has not, been done to it. After all, it is a 32 year old V-8. You could throw a grand or 2 in parts to tune this up to try and get more mileage, but you'll be lucky to pick up 1 - 2 MPG. But hey, that's not we buy a V-8, right?
At least now ya don't have that stress of thinking you're only getting 5MPG. (110 miles a tank)
On a side note; You say you put $35 for a half tank? Wow ya got some cheap gas in Ok. You are using premium fuel, right? I just filled mine (21 gallons) for $115. Eh, it is what it is. That's Life.

 

I wonder if your fuel level sender might be worn or faulty. How many gallons did you fill up for 1/2 tank? 11.25 gallons would be half.

13-14 mpg sounds more reasonable for stop and go traffic conditions. Use the trip meter and the number of gallons you fill up with to calculate your mpg for the next few trips and see if that seems more accurate.

If the car still smells like it's running rich all the time, that still should be checked out soon. It's not good for your catalytic converters.

It feels better after I finish fueling up and start driving away Can't get myself to sell despite the prices.

I've been hypermiling now and can manage 16-18 city, 23 mixed. I sometimes think about having a vacuum gauge

 

Concern #1 is that you don't have a gas leak somewhere, or your car will burn down. If you've ever had a car fire.. that's obviously No-Bueno. So don't drive it if it's possible fuel is leaking somewhere, don't park it near anything, or inside anywhere.

 

Definitely took stress off my shoulders thank you guys, that fuel gauge making me think I’m crazy but I get unleaded gas for 3.50/gal so about $75 to fill up.
I’ve never drove it long distances like I am now so I never put premium in but does it noticeably change performance? I have pretty weak acceleration anyways so I’m not too worried unless cheap gas can hurt something

 

There's no performance gain by using premium (91 octane + up) but it is "recommended" cuz of the higher compression ratio. I used garbage regular (87 octane) gas in my last LS400 after a while only cuz it was a piece of garbage and I really didn't care about it. But my 93 now I only use premium cuz I want this one to last and only $10 more for a full tank, what's the difference if it's gonna run cleaner?

 

Putting lower octane gas is fine in modern computer controlled engines because if they start pre-detonating, the computer will adjust accordingly to protect the engine. However, you're putting a hell of a lot of trust into the engine knock sensors to advance ignition timing and save your engine from grenading itself with pre-detonations due to lower-than-designed octane fuel.

20+ year old knock sensors and a ECU controlling them... It's gonna be fine. But...

As for performance - it will be slightly limited, the engine will be less "dynamic". I'm unsure if peak power numbers or "area under the curve" numbers will be changed, I've never seen dyno sheets comparing the two fuel types for this engine. This probably depends a lot on air temperature, humidity and a million other factors. Toyota recommends 91 octane because it guarantees the engine will always be able to make it's rated power and not need to pull timing to save the engine because of engine knock. Your engine may be able to make it's rated power from 87 octane fuel. But it may not. "it depends".

"fuel quality" has nothing to do with premium vs. regular and everything to do with who is making it. As far as I understand, fuel stations must sell the same "grade" of fuel across all octane ranges. So the only difference between 87 and 93, at one gas station's pumps, is the octane rating. The detergents or other "quality" metrics of what makes good\bad gasoline should be identical. At that gas station. Shell's mix may be different from Costco who may be different from Loves.

In terms of variations across different brands, fuel from Shell and a brand called "Top Tier" gasoline is known to be the best available, given the options in terms of fuel quality. Number of detergents and other "quality" metrics I don't understand. Top Tier is sold across a huge range of 3rd party stations, just google it to find their logo and then look for that on a pump. There should be a sticker.