A little history. 99 Accord I4 automatic
rotor, cap, plugs and wires, and air filter were replaced last week, and timing checked. Changed oil and switched to Mobil one oil, and Puralator Synthetic filter.

Honda dealership replaced timing belt, h2o pump and seals, also adjusted valves last week.

Friday morning started the car up it idled at 300rpms, and kinda rough idle. hooked up the scan gauge II to it and no codes.

Headed out of town and the Scan Gauge showed the highway mileage was 22mpg at 75mph. Way off from the normal 28 mpg.

This morning started the car to come home, and idle was normal and smooth.
On the highway the mileage was 27. Stopped to get a splash of gas and the mileage dropped to 24.5. When I stopped to fill up 75 miles later, and got back on the road the mileage dropped to 22.5.

When I got home a bit ago I checked all the vacuum lines, and they are all fine.

Any ideas on the rough idle just going away, and the gas mileage going down with each start of the car?

I have to make the same trip this Friday, and would like to try and solve this problem.

TIA

Tortis

 

Have you actually recorded and calculated the mileage by resetting your trip computer and dividing the actual driven mileage by the gallons you put in?

My experience with those digital readers is they are all over the place. The only true method is going to be using the actual gallon and actual mileage. It sounds like you're basing this 100% off the scan gauge?

 

No I haven't. I was using the average, and current mileage the gauge shows.

I will do that on the trip this Friday.

T

 

Give it a whirl, completely fill up the gas but just to the click. No topping off, then run it to at least 1/4 tank. Fill all the way back up to the click and use that method to get the gallons used.

Definitely be curious to se what you're getting after that. What tires are you using too? They can make a significant difference in mileage too. You pretty much have done all the other maintenance items you should look at to keep up the MPG.

 

The tires are brand new Firestone Affinity.

I cant wait to see how the scan gauge compares to the real gas mileage.

T

 

Does your scanner say whether or not the car goes into closed loop mode? ECT showing engine up to temp properly? Assuming you have 180F thermostat, so I assume ~180F indication?

 

yes it goes into closed loop, and engine temp runs according to scanner 195-202 degrees on the highway

 

Assuming your o2 sensor is working right, you should be ok. I too would use the trip odometer to double check the numbers.