20mpg city on a GA16DE automatic only makes sense if you're always accelerating full throttle from stops.
b13boy mentioned the temp sensor. What does your temp gauge read? Does it reach normal operating temp? If the temp sensor is bad or has a bad connection, or the thermostat is stuck open, the ECU will stay in closed loop and run rich.
Your car only needs 87 octane - 89 is wasting money - UNLESS you have lots of deposits on the pistons, valves, etc. causing higher compression or hot spots, resulting in pre-ignition.
Bosch vs. NGK plugs will not cause a 10mpg difference, gotta be something else. NGK are better, last longer.
I'm sure new Duralast wires are O.K., but OEM plug wires are the best. I still have original wires (147,xxx miles) on my '93 se-r with bolt-on mods & S4 cams - runs great - 35.77 mpg over the last ~3000 miles, about 60%city/40%hwy.
You still might have a bad or leaking injector.
If you're using that much oil in 350 miles, you might want to determine the cause. You said there are no puddles, a compression & leak-down test might be in order.
Below is from my post on a different forum about bad mpg, listing more possibilities. That list is SE-R specific, some things will be different for a GA16DE automatic.
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Have you checked the oxygen sensor to see if it's working properly?
Put the ECU in diagnostic mode (see procedure below).
It would be easier using a Consult or Techtom MDM-100.
The check engine light on the dash will flash indicating a rich/lean
condition for the oxygen sensor. With the engine at normal operating
temperature, at about 2000 rpm, it should blink 5 times within 10
seconds. If it doesn't, you have a bad oxygen sensor, resulting in
bad mpg.
Another thing to check is your MAF ground wire.
How to re-ground your MAF:
http://www.se-r.net/engine/maf_ground.html
Look throught this a post I saved back in 2002, about bad mpg and things to try:
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On Thu, 31 Jan 2002 23:07:11 -0600 BlackSE-R <nissanser92@yahoo.com> wrote:
>I think I'm beginning to lose my head. I have tried everything people have
>suggested to me. I checked the famous archives. I talked to mechanics. I
>keep on getting low miles per gallon. Last time I tanked: 137 miles; 8.402
>gallons. That's roughly about 16.3mpg. I drive a 92 se-r, no mods, 115k
>miles on it, I bought it from this one guy who had to sell it, because he
>was forced to leave the country. Other se-rs get more than 20mpg. Guys, if
>this happens to me, I'm popping the Champaign bottles and having a huge
>party. Any suggestions on how I can fix this problem? I'm hopeless now...
>Thank you up front, for any suggestions...
Yep, your mileage sucks. Something is wrong. It's not hopeless. Don't
lose your head, use your head. You can fix this.
For good mpg, your car needs sufficient well-mixed, accurately metered
air/fuel, good compression and a properly-timed good spark in all 4
cylinders, efficient transfer of power to the wheels, and an minimally
restrictive exhaust system. That's all.
-ECU show any codes? If your ECU never gets out of open-loop mode (or is
stuck in fail-safe) you will waste a LOT of fuel...16mpg sounds about
right. All critical sensors (O2, TPS, crank angle, MAF, coolant temp,
knock sens.) must operate in approved range to reach closed-loop (feedback).
-Spark plugs show any clues? Lean, rich, oil? Are they the right plugs,
properly gapped? Don't mess with anything but spec NGK plats...you can use
5's if 6's foul.
Have you:
-checked the O2 sensor function via the ECU diagnostic?
-verified idle speed 850/ignition timing at 15-17 deg. BTDC (and not, say,
10 deg)?
-regrounded the MAF and checked for proper function?
-looked for vacuum or intake air leak(s)? (idle manifold pressure should be
steady 18-19 inHg @sea-level) Check the AAC/FICD, (cold-)air regulator,
EGR, PAIR, and PCV valves for proper function.
-thoroughly cleaned the TB/intake manifold? Lots of gunk builds up here.
-replaced/cleaned the air filter?
-checked battery, alternator, coil, corroded connections etc. as potential
cause of weak spark under load?
-checked clutch for gross slippage (you'd probably have noticed
this...popping clutch in 5th @idle should stall engine)
-checked wheels for proper tire pressure and alignment?
-checked brakes (incl parking brake) to be sure they aren't dragging?
-checked fuel system for leaks, proper fuel pressure, dirty filters? (there
was a recall on pre-93(?) fuel pumps...check w/ dealer to see if yours was
done.)
-checked your cat. converter and muffler for fouling/blockage?
-looked for a dead cylinder? Check for healthy spark and compression
(misfire: fouled plug, bad HT wires, bad distributor cap/rotor; OR fuel:
dirty/plugged fuel injector; OR mechanical: worn/bad piston
rings/valves)? A dead cylinder would waste about 25% of your fuel, so
again 16mpg sounds about right.
Good luck!
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ECU diagnostic mode:
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Self-diagnostic system will operate in 2 modes which are
manually selected using a screwdriver through access port on ECM. With
ignition switch in ON position and engine not running, use screwdriver
to turn switch fully clockwise. Wait at least 2 seconds. Turn
screwdriver fully counterclockwise. Inspection light will begin to
flash.
NOTE: Modes cannot be switched while engine is running. When
ignition switch is turned to OFF position while in Mode II,
ECM switches to Mode I.
Mode I (Bulb Check)
Engine is not running in this mode. Turn ignition switch to
ON position. Red LED on ECM and CHECK ENGINE light (if equipped)
should be lit. If lights do not illuminate, check and replace bulbs as
necessary.
Mode I (Malfunction Warning)
This is normal vehicle operating mode and engine must be
running. On California vehicles, if a malfunction occurs, Red LED and
CHECK ENGINE light (if equipped) will illuminate, indicating an engine
control system malfunction has occurred and a code has been stored. On
Federal vehicles, codes are stored and only Red LED will illuminate
when ECM’s Central Processing Unit (CPU) malfunctions.
Mode II (Self-Diagnostics)
When Mode II is accessed (engine not running), codes stored
in ECM memory will be flashed by CHECK ENGINE light (if equipped) and
Red LED on side of ECM.
Long flashes (.6 second) indicate first digit of code. Short
flashes (.3 second) indicates second digit of code. For example: one
long flash followed by 2 short flashes indicate a Code 12.
DTC 33, OXYGEN SENSOR
1) Check System Function
During self-diagnostic test Mode II, Red LED on ECM and MIL
on instrument panel should blink 5 times within 10 seconds when engine
speed is about 2000 RPM.
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Linear Mode
