What caused my random spike in MPG today? from ~45mpg to 52-55mpg at 45mph??

[ps2fixer]

Ok... so here is the long version, skip below to the * marked short version if you wish.

I was thinking about unhooking 2 injectors (cyl 2 and 3) to see what the MPG impact would be. I started it up, ran as I expected, engine vibrated quite a lot, but ran kind of like a 2 cyl engine under a load. When I reved it a bit to see if it would smooth out, the check engine light started flashing, and when it idled back down it went solid. Engine codes E0300 and 2 other 3xx codes were set. I cleared them, hooked up the fuel injectors and drove home like I normally do.

Driving while cold got me my normal ~30mpg at 40mph, once 130F hit, it shifted and I sped up to 45 and got lockup.

Couple turns later (about 10 miles) I was shocked my instant MPG gauge was saying 55mpg and I was traveling 45mph. I normally get around 42-45mpg at this speed. I put it off as a down hill slop since this is only my 3rd drive home with the scan gauge. Well 3-4 miles down the road it was still 52-55mpg... I was able to keep it over 50mpg until my next stop including a couple small up hills. After the stop I got to speed and it resumed the 50+mpg.

In the last ~4 miles of the trip, I was looking at the different sensors to see what one(s) were out of the normal. Only one I could find is the IGN, normaly it rides around 37-39, but it was running solid at 24. When I sped up slightly it would jump to 37 or 39 instantly with very light gas and lifting off it a little it went back to 24.

*** Short version ***

-I unhooked 2 injectors and was going to drive home like that to test the MPG difference, it set engine codes so I hooked them back up and cleared the codes
-Cold MPG was normal
- around 10 miles into the trip I was getting 55mpg at 45mph, normal is around 42-45 mpg
-IGN reading was 24 instead of the normal 37-39

*** Trip Stats ***

44.7MPG average
36 Miles
204F max WT (192 thermostat installed a few weeks ago, old one stuck open)
49MPH max
38MPH Avg


****

So to me it sounds like my timing is too fast or the gas I have isn't running very well at the more advanced timing (low octane?). I 95% of the time fill up at the same station and same pump because their pumps always gave me the best gas mileage when driving my Camry (before a scan gauge).

Comment what you think is causing this, because what ever it is, I am loving the better MPG. My last best was the same trip, same speed, tiny bit colder day, and it was 41.7mpg, I didn't have 50+ mpg figures at 45mph except going down hills.

Just to note, I have been getting right around 38-39mpg before I dropped my speed to 45mph instead of 50mph. I have yet to have a tank 40mpg+. Comparing to other users, most get 32-38mpg on this site, one I saw at 42mpg. Only mods on my car is a upper grill block, cardboard in radiator (still there when thermostat was stuck open), and the scangauge. The last 3 tanks have had no changes, and the outside temps are around the same +- 5 degrees.

If I can figure this out, I might be able to hit 50mpg average + with such little mods with a car rated for 26mpg average (where the hell they get that low IDK..)

[SentraSE-R]

My guess is your Scangauge initialized itself to the factory default settings (3 liter engine, etc.). Check your vehicle setup and calibration settings.

[ps2fixer]

Scangauge appears to be as it was, 1.8L engine, etc. Driving to work today, I got my normal MPG for at night. I was able to get my car to do the same actions I saw yesterday, but couldn't keep the speed. For at night my TPS reads 18 and sometimes dips to 17. If I let off just a tad bit, say where 17.5 should be, the IGN reading goes from 37/39 down to 24 and mpg instantly jumps to 52-55! So what I'm thinking is that the ECU thinks I'm coasting and it leans the AF mixure and retards the timing. Theory of mine is that there was a slight wind pushing me (north) and since I was running the gas peddel low enough, it was kind of doing a lean burn setup even though this car does not have that function (same engine in other cars can). When I perform more aero mods on the car, I should see this "lean burn" more often if my theory is correct.

While in the "lean burn" thing yesterday, I did notice the engine was a lot smoother than normal and sounded like it was under less load. When my car is fully warmed up, it idles around 675-700rpm and is very smooth running, just before peak temp, it runs 850-900 rpm and vibrates quite a lot. When it is idled to 700 and I give it gas to get the rpm to around 900, it vibrates as well. I think this is just the nature of this engine, but maybe it could be timing or fuel quality (E10+)? My engine spects say 9.5:1 compression, so maybe I should run 89 or 91 gas instead of 87?

[SentraSE-R]

Sounds like a phenomenon called Super Atkinson Highway Mode. I've searched in vain for SAHM in my cars, but can't hold it very long. I think it's found/held easiest by holding the TPS at a specific value over idle position (i.e., 8 over 12 TPS idle, or 20 TPS), or keeping IGN advance high (>40). Google SAHM Atkinson.

[ps2fixer]

Very interesting read on the SAHM Atkinson mode.

Quick stats for when I'm in the high mpg mode:

RPM around 1550-1650 depending for uphill vs down hill etc
LOD 42-46, at about 47 it kicks out of the mode
IGN 24, today I saw it dip to 23 with a lower load (around 35) down a hill
Speed 40-50mph (target of mine is 43-45)
MPG 50-55, mainly around 52-53 on normal flat ground

Now when I'm not in the mode my specs are

RPM around 1550-1650 (same as above, it is in lockup lol)
LOD 50-54
IGN 36-39, spikes to 40 here and there
Speed 40-50mph (target of mine is 43-45)
MPG 40-44


It is very hard to keep it in this mode, 2 days ago I drove home a large % of my trip home in the mode, somewhere around 60-70%. I didn't even know how I stayed in the mode, just knew to keep the foot lightly on the gas. I think there was a small wind pushing me since this is out of the norm. I hope further aero mods will keep me in this mode. On the way home at around 50-55mph I could get it in the mode and at 46 LOD (still slowing down though) it read around 48-51mpg, much better than 37-40...

The only really weird thing is... SAHM mode appears to be based on high ign advanced which my car does very easy when warm. It is when the ign is not advanced as much when I get the better MPG. I really wonder if I ran better gas if it would get better MPG due to the very high ign advanced? I think my next fillup will either be mid grade or prem to test. $3.69 for reg, $389 for prem, 20% better mpg would be a ~$0.54 savings per gal!

[ps2fixer]

New find....

File:Timingmap.jpg - Wikipedia, the free encyclopedia

According to my readings and that map, my 45mph at ~46-50 load should be around 22 for spark advance.

When I deaccerate, IGN reads 12... according to that map, it should be higher not lower? When I give it a tiny bit more gas (around 60% load) IGN reads around 33 while going 45mph (1600rpm)

I have yet to understand this... been half thinking about unhooking my battery for a while to reset the ECU settings and let it rebuild, or keep them and run a higher grade gas.

[Olympiadis]

It sounds to me like it is your torque converter locking that is giving the increased MPG.
To test this you might try tracing your TCC circuit and then wiring in an over-ride switch.

Most newer vehicles use a PWM type of TCC activation strategy to make this function seem smoother to the driver. Even my S10 had this.
An over-ride will force a signal for 100% lock-up of the TCC at will, and will instantly show you the gain on your S.G.2.

Some (perhaps most) TCC circuits supply 12 volts to the pressure solenoid, and pulse the ground signal based on calculated duty cycle. In this case you would over-ride the ground signal using a SPDT relay.
Single Pole Double Throw (SPDT), Single Pole Single Throw (SPST) Automotive Relays

[Ladogaboy]

Advancing the timing too much in the idle/low-load range can lead to an erratic, rough idle. That's why the timing is retarded so much.

[ps2fixer]

I think the car is going into lockup fine since it shifts 4 times (4 speed) and has another spot that feels like another gear (lockup). It is doing about the same RPM in both good mpg vs not so good and going faster, say 50mph or 55 does not show better mpg but worse.


I need to check the idle IGN readings, I think it is around 8-12.

I talked to an engeneer today about this since he had pc issues (i'm a computer tech), and he thinks it is the knock sensor.

[Olympiadis]

Originally Posted by ps2fixer

I think the car is going into lockup fine since it shifts 4 times (4 speed) and has another spot that feels like another gear (lockup). It is doing about the same RPM in both good mpg vs not so good and going faster, say 50mph or 55 does not show better mpg but worse.


I need to check the idle IGN readings, I think it is around 8-12.

I talked to an engeneer today about this since he had pc issues (i'm a computer tech), and he thinks it is the knock sensor.

Yes, I realized that it would feel like that, - that the converter had already locked. The purpose of the PWM strategy (which is based on engine loading by the way) is to make the TCC function less perceptible to the driver.
Also, due to the function of the PWM, you will not see such a clear change in engine RPM at certain loads. This is exactly why the PWM strategy makes the function smoother in terms of driver perception. That's why I suggested a test to rule this out, or confirm.
Most scan/datalogging software will show you the TCC function flag (on or off) and some will show the duty cycle as well.

Speaking of knock sensors, that same scan/datalogging software will show you knock sensor output in a term called "knock counts", or "knock retard", - the retard being the result of the adaptive spark logic in the PCM.

One more thing to check is your EGR function if your car has a PCM controlled EGR valve. Again there are two strategies for EGR function: either on/off, or a PWM or stepper-motor controlled solenoid that delivers based on a calculated duty cycle. When EGR function is active, the spark advance is increased. When you fall below the thresholds for EGR operation (different for different PCM/calibrations), then the spark advance will drop, - usually in the neighborhood of 10*.
The EGR delivery decreases combustion efficiency, hence the need to start the spark sooner, so depending on engine loading & conditions it will definitely affect your fuel mileage.
The SG2 will often report EGR duty cycle on many models once you add it via x-gauge.

All of these (TCC, KR, and EGR) are based on variations in engine loading, so their effects will be noticeable as you change loading by backing off of the throttle while driving.

Given your curiosity about this sort of thing, it sounds like you would be a lot happier if you invested in one of those scan/datalogging smart-phone apps so that you could monitor what exactly is going on in your car. All you would need extra is a Y-cable (one male end plug, and two female plugs) for the ALDL port.

There are other more sophisticated datalogging software that will display the reported data in optional units, and ranges of resolution. For example instead of seeing your throttle position as 15%, you could have it display as 15.34%. This sort of thing comes in handy when working towards higher MPG.

Using the same method (some software will show instant MPG or fuel consumption), you could have your instant fuel economy reported to several extra decimal places in order to monitor very small changes in a narrow range of test conditions.