Root cause of poor gas mileage

I wouldn't expect the change in oil viscosity to make much measurable difference in fuel mileage. What was your fuel mileage prior to the change?

 

There are many other factors that can contribute to poor mpg:

• Tire pressure (what type of pressure do you run)
• Gas choice (top tier, bs what's around at the time you need to fill up)
• Driving style (spirited or mindful)
• Type of driving (mostly city, mostly highway)

If oil consumption issues:

• How many miles since noticed?
• Did a light on the dash tell you about the oil use or did you find it by checking the dipstick?
• Have you done any cleaning of the egr system (intake manifold, egr pipe, egr valve, egr cooler)?
• Have you ever tried cleaning the oil with an engine flush?
• Ever cleaned the MAF or changed the air filter out?
• Have you owned the car since new?

Keep us posted .

 

No, you have jumped to an invalid conclusion.
An almost 50% drop in fuel mileage can NOT be in any way blamed on that slight change in oil viscosity.
You have some other MAJOR problem looming.

 

I actually switched our 2010 Prius II with 162 k miles on it to 5w-30 at 120 k miles and haven't seen a drop in mpg (I average 51.3 mpg with mostly highway driving). I did this to help slow oil consumption (a quart every 4500 miles found by checking the dipstick every weekend).

Things I have done:

• 5w-30 full synthetic (currently running Redline and at 1700 miles on this oil change have seen no movement on the dipstick )
• Add BG Products MOA to the new oil to help keep it basic as long as possible (gas leaks by and gets into the oil causing it to become contaminated and degrades seals)
• Add BG Products EPR to the oil prior to draining to remove build up every oil change (it comes out jet black every time). At our last Bay Area meet up, there were 3 oil changes done so I was able to compare the differences, and there was a difference .
• Instead of running 10-15 mph above the posted highway speed limits, I run at 0-5 mph above the speed limit
• I have cleaned the egr system front to back which helps ensure the air flow is correct. If the ports are plugged with deposits, in correct air to fuel mix can result
• Run top tier fuel (it has higher detergent contents which delays the onset of deposits)
• Cleaned the Mass Air Flow (MAF) sensor along with throttle body

Some things to consider for your situation .

Keep us posted .

 

Did you change the engine oil yourself or one of those jiffy lube places? Check to see if they put more than the specified amount into the engine, I've heard horror stories about those fast oil change places. Check to see if the dipstick is at the right level first, if you did take it to those fast oil change place, it may have more than 4.2.

 

Did they refill too much? If budget ain't a problem, get a fluid extractor (like below) and extract any excess through the dipstick. I moved into an apartment 2 years ago and been extracting engine oil through the dipstick nozzle so no need to lift raise the car to change engine oil. On my salvage camry, I can tap the straw right on the oil pan, straw is somewhat bendable also. If you extract too much, you can put it back as needed.

 

Oil viscosity will only account for 1-2 percent change in efficiency -- maybe less in a hybrid since oil viscosity doesn't effect efficiency of the MG.

 

For your vehicle to drop from 45-25 mpg is huge. Any chance someone hooked up a trailer behind your prius? Outside of joking did you have your rear brakes changed recently? Brakes dragging can be an issue or is your engine on constantly when you come to a stop sign or stoplight once it is warmed up? Could be that your vehicle is constantly charging the HV battery? Just some ideas.

 

Are you manually checking your mileage by writing it down and doing the math over several tanks.
What kind of driver are you and where do you drive?

 

Changing the oil is one thing but the filter coming off without a possible oil mess is another.

Posted via the PriusChat mobile app.

 

What’s seafoam??????