MPG Drop to 35 MPG after New Battery, EGR Overhaul, and Tires (2010 Five/Tech)

Hi Mendel, thanks for following up.

Regarding the MAF cleaner: The 'household dry cleaner' I used was a generic solvent I checked my LTFT (Long Term Fuel Trim) today at a warm idle in Park, and it is sitting at +6.25%. Given that it’s positive, do you think the cleaner left a film that is causing the car to over-fuel slightly?

Regarding the 12V Battery:

• The sticker on the battery is A23, so it is exactly 3.5 years old (Jan 2023).
  
  
• The shop tester showed it hit the rated 325 CCA, but noted the State of Charge was only 66%.
  
  
• My multimeter shows 12.42V at rest, but the dash menu shows it dropping to 10.9V - 11.1Vas soon as the car enters IG-ON mode (load).
  

Tire Update:
I confirmed they are 215/50 R17 Bridgestone Turanza QuietTracks. My previous set was 215/55 R17. I've set them to 38 PSI Front / 36 PSI Rearto stay safe as suggested.

Since I have a 111% hybrid pack and a clean EGR, do you think that +6.25% trim and the 11.1V load drop are enough to explain the drop to 35 MPG, or is it mostly just the rolling resistance of these specific tires?

 

Thanks for the chart, bisco. It’s very helpful for comparison.

To clarify my car's behavior: My engine does shut off at stoplights as long as the hybrid battery has a sufficient charge, so the hybrid stages (S4) seem to be working.

However, my voltage still shows a significant drop based on your chart:

• 12.42V resting (Multimeter) = ~75% Charge (Green Zone).
  
  
• 10.9V - 11.1V under load (Dash) = <10% Charge (Red Zone).
  

I've also checked my LTFT, which is sitting at +6.25% at a warm idle.

Since the engine is cycling off correctly at lights, but I'm still stuck at 34.6 MPG (Trip A reset this morning), it seems my fuel is being lost while the engine is running. I suspect it's the combination of the +6.25% fuel trim and the rolling resistance of these 215/50 R17 QuietTracks. I've set them to 38 PSI to see if that helps.

 

Final Update with Trip Data:

I finished the 36.2-mile round trip from Plano to Irving (mostly highway). Here are the results:

• MPG for the trip: 29.9 MPG(Reset Trip A this morning).
  
  
• 12V Voltage after trip: 12.55V(Multimeter) and 11.4V (Dash load).
  

Getting only 29.9 MPG on a highway run seems to prove that the engine is over-fueling significantly. Given my +6.25% LTFT and the fact that I saw the MAF hit 7.0 g/sec at idle, I suspect the 'dry cleaning' of the MAF sensor has ruined its accuracy.

Does 29.9 MPG on a highway trip with a 111% hybrid pack confirm that the MAF sensor is my main problem? I'm planning to order a Denso 197-6160 tonight. What do you think?

 

Final Update with Trip Data:

I finished the 36.2-mile round trip from Plano to Irving (mostly highway). Here are the results:

• MPG for the trip: 29.9 MPG(Reset Trip A this morning).
  
  
• 12V Voltage after trip: 12.55V(Multimeter) and 11.4V (Dash load).
  

Getting only 29.9 MPG on a highway run seems to prove that the engine is over-fueling significantly. Given my +6.25% LTFT and the fact that I saw the MAF hit 7.0 g/sec at idle, I suspect the 'dry cleaning' of the MAF sensor has ruined its accuracy.

Does 29.9 MPG on a highway trip with a 111% hybrid pack confirm that the MAF sensor is my main problem? I'm planning to order a Denso 197-6160 tonight. What do you think?

 

Final Update with Trip Data:

I finished the 36.2-mile round trip from Plano to Irving (mostly highway). Here are the results:

• MPG for the trip: 29.9 MPG(Reset Trip A this morning).
  
  
• 12V Voltage after trip: 12.55V(Multimeter) and 11.4V (Dash load).
  

Getting only 29.9 MPG on a highway run seems to prove that the engine is over-fueling significantly. Given my +6.25% LTFT and the fact that I saw the MAF hit 7.0 g/sec at idle, I suspect the 'dry cleaning' of the MAF sensor has ruined its accuracy.

Does 29.9 MPG on a highway trip with a 111% hybrid pack confirm that the MAF sensor is my main problem? I'm planning to order a Denso 197-6160 tonight. What do you think?

 

Thanks, ASRDogman and Mendel. I'll take your advice and try a 'redo' cleaning with the proper CRC MAF Sensor Cleaner today. It's much cheaper than a new sensor, so it's worth a shot to see if I can wash off whatever residue the other cleaner left behind.

That 29.9 MPG highway result on my Plano-to-Irving trip really shocked me. I've spent so much on the hybrid pack and EGR that seeing 29 MPG feels like something is seriously wrong with the air-fuel mix.

If the CRC cleaning doesn't bring my highway MPG back up to 40+ and my LTFT back down to 0%, I'll head to the dealership for a guaranteed OEM sensor. I don't want to risk a fake one from Amazon. I'll report back after the CRC cleaning!

 

That’s a great tip, Mendel. I didn’t realize it was good for other electronics too.

I’m heading to the store now to get a can of CRC Mass Air Flow Sensor Cleaner. I’m going to give the original sensor a very thorough spray-cleaning, let it dry completely, and then perform a 12V reset to clear that +6.25% LTFT memory.

I really hope this fixes that 29.9 MPGhighway result. It’s crazy to see a Prius getting worse mileage than a standard Camry after doing all this work! I’ll report back once I’ve cleaned it and done another test drive. Wish me luck!