Let's talk fuel economy

Dave, My 96 has 122k on it... I was consistently getting in the low 20's (mpg) for the past year or so - which was 15% less than what I had been getting when I first acquired the car (with 40k on the odometer). I checked for codes (no CEL), and pulled a knock sensor (0304) code.

As part of the 120k maintenance I have done the following:

1. fuel filter
2. belts (accessory and power steering)
3. air filter
4. oil change
5. radiator drain/fill
6. transmission drain/fill
7. knock sensor
8. pulled plenum and cleaned out EGR port and EGR guide tube
9. cleaned out IACV and throttle body

Since the above items were done, my mpg (same driving profile, 70% highway, 30% city), has gone up to 26-28mpg. The KS made a huge difference to the mpg, as well as to the overall throttle response of the vehicle. I have yet to do the plugs - and hope that helps improve the mpg even further. The current mpg I am getting with the Maxima, is better than I have ever achieved (consistently).

role in fuel economy (I replaced both front O2 sensors at 90k. I am not sure if the ECTS affects fuel economy - since the ECU has feedback from the O2 sensors to adjust the air/fuel ratio... FYI, I have also replaced the ECTS on my car (@ 80k) - in an attempt to resolve an extended cranking problem.

A clogged EGR system would increase No2 emissions (and have you run hotter), but not sure if it directly affects fuel economy.

Anything that causes more drag (perhaps a bad wheel bearing), or more back pressure like clogged exhaust system components would reduce fuel economy.

If you have never replaced the O2 sensors on your car, that maybe something to consider doing.

Not to state the obvious, but the other commonly overlooked factor, is driving style. I have learned to be a little more judicious in the use of the accelerator :), and by keeping a steady constant speed, rather than constantly speeding up and slowing down, have gotten much better mileage (cruise control on the highway helps too).

Just my verbose 2 cents.

Cheers, Nirav

96 Max GLE, 122k

Haven't done these, but planning to. TB hasn't been cleaned in a while.

I did the knock sensor last year. No noticeable change.

Changing my driving style to shift before 3k and makes an apparent 2mpg improvement. But my mileage varies as much as 4 mpg from one fill-up to another.

Never been done, since I haven't had any codes. I'd kinda like to find a whole stock y-pipe assy with low mileage and put new O2 sensors in it.

Now that I've known all along. Gently putting down the throttle also helps rather than opening it right up when you do need to accelerate fast.

Dave

Dave -

there are always stock y-pipes available on maxima.org. I saw one as recently as this week, and from the picture, it looked like it was in great shape. I am contemplating picking one up too (as a just-in-case move)... the flex wrap on mine has been torn for years (at one flange).. the pipe is still intact. I'm nervous however of stripping/shearing the manifold-end bolts...

What sort of mpg are you getting with your Maxima... you have a 5-spd correct?

Nirav

That can do it. If it's "within spec" but not reading the true temp it can cause problems yet never set a code. This applies to the sensor and it's connector.

Yep, same as the above.

That's what I suspected. I had replaced it with an Autozone version, and ever since my temp gauge seemed to read a little higher. I might swap back in the stock one since I don't think there was anything wrong with it in the first place. I changed it back when I was having trouble starting, and that ended up being a failing CmPS.

Would the universals be a good choice, since I'm capable of splicing the wires and doing a decent job?

Thanks for the input.

Dave

Precisely what I was thinking. I saw the corrosion on my studs and got nervous. But last year a mechanic took off the y-pipe to access the steering rack, and didn't seem to have any issues with it. The flex wrap is looking a little rusty and brittle.

I'm getting 24mpg average for about 75/25 highway/city driving. On all-highway trips b/t 70-80mph, I get no more than 26, ever. And that's generally babying it.

Dave

Steve,

I thought the ECTS may affect fuel economy, but wasn't sure if the ECU would compensate for the ECTS input via the O2 sensor readings. It is definitely worthwhile to replace the ECTS. It is relatively cheap at $25, and takes 10 mins to swap out.

Cheers, Nirav

Steve T wrote:

Dave,

I would recommend Bosch OEM equivalents from

They sensors run $70 or so a piece (mention maxima.org for a discount). I cringe at the thought of a splice - with all that snow, salt, and heat around the exhaust area, the last thing I want is to save 15 bucks a sensor and end up with an intermittent problem down the road that becomes a nightmare to diagnose. Thats just my two cents Functionally, I would assume they would work perfectly fine.

Cheers, Nirav

Since your manifold studs have been removed recently, I would expect that you or any exhaust shop should be able to get them out again without any problems. The other choice you have is to just buy some replacement bolts and have a shop swap out your old ones. That way, when you do have to replace the y-pipe (pre-cat failure, etc.) you'll have new hardware to work with.

Your mpg isn't "horrible"... but I think you may see some improvements after swapping the O2 sensors... I assume all your other maintenance is up to date.

Nirav

Get a dealer one and replace the connector as well.

Should be OK, just make SURE to use heat shrink tubing, solder the wires etc and get a quality seonsor like an NTK (NGK's brand of O2 sensor).

You have to remeber there are times that the computer kicks out of closed loop and the O2 sensors aren't working like when accellerating, start up etc. People assume the engine/computer is ALWAYS reading the O2 sensor when they aren't.

There is no separate harness/plug for the ECTS (the nearby Knock Sensor does, though).

Dave

Not sure about the Maxima, but on my wife's 98 Altima, it was getting horrible gas mileage. After some lenghtly investigation I replaced one part: the coolant sensor. Take note, the Altima has 2 coolant sensors and I suspect that Maxima has them as well. One sesnor is just for the temp gage on the dash. The other sensor reports directly to the ECU. That's the one I replaced.

After a few tankfuls the car's mileage, and power, improved. It went from 22 mpg back to a reasonable 25 mpg.

The defective coolant sensor must have fooled the ECU into thinking that the car was running a little cool, and hence the ECU ran a richer mixture, causing power loss and bad gas mileage. Once the sensor was replaced, the ECU re-adjusted the feul curve and I gather that took some time.

CD

Oh, and the best part was that the sensor only costs $6. I was contemplating the O2 sensors, but at $60 each, and there were 2 O2 sensors to replace, I went with the cheap fix 1st:)

CD

Steve,

I knew the system ran in open loop upon startup and until the O2 sensors were warm, but didn't realize that there were other driving conditions that resulted in open-loop operation. Thanks for the info.

Nirav

You have to slpice in a new one..

what in the hell is the big deal with 3 mpg ??? htat coud be the idifferent for waiting for a train and hitting all red lights or not.

10% .. u need to average over probably 10 gas tanks to see if it really is

The comparsion was over several tankfuls, and when it was bad, even at my best I could not crack 22 mpg. It was definitely a problem. This is a combined city and highwy reading, by the way.

CD