Something is wrong not sure what

[WVengr]

My 1997 Nissan D21 4cyl manual is getting pretty awful mileage, despite my efforts to coax a few more MPG out of it, I cant seem to beat the EPA estimates by more than 1 or 2 MPG unless I resort to things like following an 18 wheeler at 50 MPH for 150 miles (the two spikes in my mileage record here).

I am coasting (with engine on until i get around to adding a electric booster for power brakes) down hills and to places i know i have to stop, shutting the engine off at stop lights, shifting so that the RPM never goes above 2000, and driving slow on the highway. Using these techniques I have been able to get a lot better MPG in my previous cars, but cant get anything here. (Or maybe I am and would be getting 13mpg without this.)

This is really frustrating me. I changed the Air filter, fuel filter, plugs and wires, changed oil to lowest viscosity recommended by manufacturer, new MAP sensor, New Catalytic converter. The exhaust leaks at the muffler but I don't think that would be causing this (going to fix it soon anyway). Anyone know what I should check? I am getting only a little better mileage than I get in my 1958 Edsel with a 2 speed auto and a 5.9L V8 (and I drive it with alot more spirit than i do the truck) in this and it is a 4 cyl with a 5 speed!

[Daox]

What kind of mileage are you getting? Is the engine light on?

I might take a look at replacing the O2 sensor.

[WVengr]

I am getting about 21 combined right now, where EPA combined is 20MPG. I can get a little more on the highway, but not above EPA unless I am drafting for long periods. The CEL is off, was on a 2 weeks ago but it was a MAP sensor code and a downstream O2 code. new MAP and Cat fixed that though. Also makes a bit of a popping sound from the exhaust when I am decelerating in gear, and at Idle, not sure what that is though.

[cleanspeed1]

Popping in the exhaust system, the leak possibly, and as far as the bad mileage my thought is getting the air/fuel ratio checked along with the timing. The next thing is to do a compression check and leakdown test on the motor to see if something is amiss. Also, what condition are the injectors in? Have you done the cap and rotor? Valve adjustment?

How many miles do you have on that thing?

[WVengr]

Ah the mileage might be helpful. It is at 157k on what I assume is the original engine. It is the correct KA24E, but knowing the fact that this is WV, people swap out engines for such trivial things as head gaskets around here.

[Daox]

Popping in the exhaust sounds like backfiring which means you're running rich which explains the bad mileage. I'd pull your spark plugs out and take a look at them. If you don't know how to read plugs, take a picture of them and post them here.

[WVengr]

Well may have found the problem. I was reading the user manual to see if there was a thing in maintenance that I had forgot. Instead I saw "Fuel: Recommended 91 RON" Since I had a 150 mile trip to make I topped off the tank with 93 octane just to see if the recommended made a difference. I filled up again when I got home 159 miles later and shattered my old mileage record

EDIT: I thought I got 32 MPG, since that's where the gas pump shut off. However this morning I noticed that the gas gauge was down a bit even though I had just driven a few miles back to my house after filling, so i filled up again this morning, this time at the same station I had used when i left, and got a little over a gallon more in, so it is looking more like 26 MPG, but still a marked improvement.

It wasn't knocking so I hadn't thought about octane, but for some reason it made a difference. Not sure why though. Further research is needed to see what octane gives me the lowest $ per mile. Not exactly sure how to do scientific testing on this without draining the whole tank though.

[user removed]

I would tend to agree with Daox that it might be your #1 O2 sensor, which is the one that provides feedback to the ECU for fuel trim. They tend to get lazy with age but will not generally trip a CEL until they are really bad. The second sensor is part of OBD2 and checks the exhaust post CAT.

The popping in the exhaust on deceleration would indicate there is unburned fuel, which is especially significant since it should be in DFCO with your foot off the gas pedal. You can rev it up and listen to the injectors with a auto stethoscope to confirm DFCO.

While premium may be helpful, it should not be necessary even if it is recommended. The wife drove Muranos for 60k with regular when premium was recommended without issues and good mileage, occasional spark knock when dead cold and backing out of our fairly steep driveway.

regards
Mech

[jakobnev]

shifting so that the RPM never goes above 2000

I'd go ahead and experiment with a bit more revs while accelerating.

Also: Tire pressure, u didn't mention tire pressure.

[WVengr]

Well it threw a code on me today. P0446 supposedly a failed vent solenoid in the evap emissions. could this be my problem, and how do i make it go away?