Sudden Gas Mileage Drop / Poor Fuel Consumption, HELP!

[Gascan]

Hi all! Need some help from the 80 series brain trust. In the last month or so my city-driving fuel consumption went from approximately 10.5 MPG to 7.5 MPG. I haven't noticed any difference in performance nor do I have any check engine light; I live in Florida so the weather is still as warm as ever. I've since replaced the distributor cap, rotor and spark plugs. This has not helped, still at 7.5 MPG's

Some information about the truck:

Year: 1997

Mileage: 299,3XX

Possibly original O2 sensors

One knuckle is seeping grease (waiting for cooler weather to rebuild it)

Engine has been eating a ton of Rotella 15W40 for as long as I've owned it (several years).


Please advise what could possibly need fixing to get the truck back to double digit MPG.

 

[ppc]

What exactly is the amount of oil consumption? "a ton" means nothing without the actual amount over how many miles. With high oil consumption the O2 sensors most certainly would have their performance effected. The catalytic converter could also be negatively impacted or failed because of other issues causing a restriction of flow or even a melt down. More and better info is needed.

 

[Comet]

I replaced O2 sensors years ago and that drastically improved my mileage. I'm probably due again soon. The valve seal guides were the culprit for my oil loss. Hoist a large sail and pray.....

 

[thabruiser]

Shot in the dark. Is it possible one of the factors you are using to calculate mpg is incorrect? Like, say your fuel gauge is not reading correctly. So you think that it's nearing empty, and it's actually got another 1/3 tank to go. Or your odometer is sticking, and so you don't think you're getting as many miles out of a tank as you used to? Just a thought.

 

[Red Merle]

What exactly is the amount of oil consumption? "a ton" means nothing without the actual amount over how many miles. With high oil consumption the O2 sensors most certainly would have their performance effected. The catalytic converter could also be negatively impacted or failed because of other issues causing a restriction of flow or even a melt down. More and better info is needed.

Rotella especially has been accused of causing these issues. I stopped using it after an all new exhaust system. I get 14 MPG as an average.

 

[Gascan]

Over a quart of oil in 1,000 miles. Been using Rotella for over 4 years without issue. Mileage is getting tracked accurately. The fuel consumption increase was pretty abrupt not gradual.

 

[Formulaonehd]

With these beasts the economy seems to be affected by a multitude of things. If it happened suddenly then its likely to be a failure in the spark, exhaust, 02 sensor (if you have em) or was the case for me when the HG was ever so slightly blown. Im not the first to report a sudden decrease in mileage right before the gasket blew. I hope its not yours but that economy is pretty awful even for an FZ

 

[Tedward]

You could do the old reset the ECU and hope it relearns more better fuel trim.

As for the oil use, mine used a quart every 600-750 miles for the longest. Got about 13 mpg, maybe 14.

 

[morganism]

Check the EGR valve if you have one.

Sometimes you can pull the O2 sensor, and cook em off with a propane handtorch. That will clean off the carbonization, and see if it helps.

Have you pulled the batt connector lately? You may have to reset the computer with the "Readiness Monitor
If you do try and reset comp, you need to turn key to ON position while batt disconnected a couple times to clear capacitor charge.

"Has the outdoor temp swung wildly? A 45 degree temp swing between block coolant temp and outside temp can cause the comp to throw a fit and need to reset the computer. It is a firmware bug, and has caused me to have to reset the family Corolla couple times a year."

http://justsmogs.com/wp-content/uploads/2016/04/READINESS_MONITOR_DRIVE_PATTERNS_T-EG02-003.pdf

 

[Beej]

Rotella especially has been accused of causing these issues. I stopped using it after an all new exhaust system. I get 14 MPG as an average.

Now wait one cotton pickin minute. Every oil post ive seen on here has always read "80s are easy on oil" and "use whatever you want and change it often". Where is this Rotella is causing issues talk coming from? I liked Mobile 1 Turbo Deisel Truck but its like 5-6 bucks more. And i swear if someone says anything about cost of ownership, ill put sand in your tuna salad.

 

[Pascoscout]

Shot in the dark. Is it possible one of the factors you are using to calculate mpg is incorrect? Like, say your fuel gauge is not reading correctly. So you think that it's nearing empty, and it's actually got another 1/3 tank to go. Or your odometer is sticking, and so you don't think you're getting as many miles out of a tank as you used to? Just a thought.

I hope people are not using the gauge to estimate fuel usage. Best way is to read the fuel put in off the pump meter and divide the mileage since last fuel up by that number.

 

[cybrstar]

7.5mpg......

Your engine finally broke in.

 

[Gascan]

Thanks guys! Anyway to diagnose the O2 sensors without pulling them? Would an OBD2 reader show how they’re performing?

Wouldn’t any failure in the EGR system cause a check engine light/OBD2 code?

Just filled up the tank, 7.6 MPG ... will disconnect the battery tonight to see if that old-dog ECU can learn new tricks.

No significant change in outside air temperature; it’s Florida .

7.5mpg......

Your engine finally broke in.

I have to admit, that gave me a chuckle...

 

[Red Merle]

Now wait one cotton pickin minute. Every oil post ive seen on here has always read "80s are easy on oil" and "use whatever you want and change it often". Where is this Rotella is causing issues talk coming from? I liked Mobile 1 Turbo Deisel Truck but its like 5-6 bucks more. And i swear if someone says anything about cost of ownership, ill put sand in your tuna salad.

That is a topic that has caused a lot of back and forth but from what I have read, the higher levels of zinc in Rotella could cause damage to cats and O2 sensors. Apparently, Shell doesn't recommend using Rotella in motors with catalytic converters.

I am not saying this is correct, it was just enough for me to stop using it and switch back to regular 10W30 dino oil in my rig.

 

[White Stripe]

I would look at live data and check the o2 sensor voltage wave form. I would also monitor maf voltage graph to see if anything looks out of the ordinary.

 

[Gascan]

UPDATE: Swung by an auto parts store after work and had the OBD2 codes read, here is what I've learned:

- My Check Engine Light has burned out (I know it worked about 6 years ago when I bought the cruiser). Like a dumbass, I didn't notice it hasn't lit up on start-up. Anyone have the part number for this bulb handy?

- P0125 Coolant Temp

- P0130 O2 Sensor 1, Bank 1

- P0133 O2 Sensor 1, Bank 1 (so I've been told)

- P0141 O2 Sensor 2, Bank 1

- P0401 EGR Flow Insufficient

After doing a bit of searching and reading on Mud, I've concluded the following:

Both of my O2 Sensors are shot (Codes P0130, P0133, P0141). This sets off the Coolant Temp, P0125 (according to the Mud hive mind). Finally, according to some posts, P0401 (EGR Flow Insufficient) is often tripped when the coolant temp hits 182º-185ºF.
My plan of action is to replace both O2 sensors with Denso OEM fit counterparts and get an OBD2 bluetooth/wifi reader to track coolant temp. And, replaced that damn check engine bulb!

Any thoughts/ideas/suggestions?

 

[thabruiser]

Sounds like you've got it pretty well narrowed down. A lying O2 stuck lean will definitely make your gas mileage drop because it's loaded up with fuel the whole time. I think you've got 'er whooped. Make sure to update what you find after replacement parts are put on.

 

[jonheld]

Have you pulled the batt connector lately? You may have to reset the computer with the "Readiness Monitor
If you do try and reset comp, you need to turn key to ON position while batt disconnected a couple times to clear capacitor charge.

OK, I have seen you post this several times and I have to call bull****. When you remove one of the battery terminals, there is no longer any electron flow. When you turn the key to the ON position with one of the battery terminals disconnected, it does NOTHING except exercise the ignition key cylinder. Please don't try to alter the laws of physics.

The easiest way to clear out the stored codes and reset the ECU is to simply pull the 15 amp EFI fuse. Nothing else you do, including swinging a live chicken over your head, is going to make any difference.

 

[rolliges]

Don't ignore coolant temp warnings or reason away the cause.

Drain, flush, flush again until clear, and refill with fresh coolant with new thermostat and rad cap.

 

[thabruiser]

I hope people are not using the gauge to estimate fuel usage. Best way is to read the fuel put in off the pump meter and divide the mileage since last fuel up by that number.

I agree with your advice. I guess a sticking fuel gauge would make you think you were getting better gas mileage, and you'd end up running out of gas. (i.e. WOW I've gone 250 miles, and am still on a half tank! *80 sputters and dies, running out of fuel*....) But a sticking odometer could cause an incorrect calculation of mpg on the "excessive fuel consumption" side.