Good Engine Start-up Tip

[ProtractedSilence]

I don't know if anyone else has catloged this before, but it should be helpful.

One of my friends is an engineer at Honda and he told me that at least on Honda vehicles you should be able to get better mileage by turning the key to "ACC" prior to engine start-up for 1-2 minutes. The reason being that in this state, there are element heaters on some of the wide and narrow band fuel mixture sensors that are activated. Then, when you do do turn on the engine and start driving these sensors should be able to effectively adjust the fuel/air ratio.

I don't know how far back this tip works for Hondas, or if it will work for other vehicles.

Who will commit to try it and see if it makes a difference?

[Frank Lee]

I get the premise of heating those sensors; what I don't get is turning the key to "acc". Wouldn't it need to be on "run"?

[Christ]

It wouldn't work on pre-OBD2 cars, and I think some of the OBD-2a versions still didn't have heated sensors.

While heating the sensor surely will have some effect on mippigs, it'll be well within the noise of day to day variation, and unable to be tested without serious testing procedures. The O2 feedback sensors have very little overall effect on AFR compared to the other sensors in the control loop.

[cfg83]

ProtractedSilence -

I just enabled this for my previously non-heated narrow-band 02 sensor. Now I can *see* the 02 sensor in the exhaust manifold heat up using my digital AFR gauge :

Using heated 02 sensor in place of non-heated 02 sensor

My post-cat 02 sensor is already heated, so nothing to do there.

However, it doesn't appear that my ECU/PCM is taking advantage of the pre-heated 02 sensor, or at least I can't prove it. My reason for doing this mod was for emissions and *hopefully* quicker switching to closed-loop operation after EOC and engine-off at stoplights.

I think anyone can test your friend's advice if they have a ScanGauge and all of their 02 sensors are wired for self-heating. The driver just has to compare how fast the car goes into closed-loop operation.

I agree it's a small benefit, but I like the idea of it nonetheless.

CarloSW2

[thatguitarguy]

Looking at the videos, the driver turns the key to the on position and not ACC. This makes more sense. On ACC there shouldn't be any juice going to the sensors.

[cfg83]

thatguitarguy -

Originally Posted by thatguitarguy

Looking at the videos, the driver turns the key to the on position and not ACC. This makes more sense. On ACC there shouldn't be any juice going to the sensors.

That's right, I use the RUN position because the AFR gauge doesn't work unless it's in the RUN position.

I just did a test. I set the key to the ACC position and I waited for a bit. Then I set it to the RUN position. If the heating element had been on, the gauge would have shown "L" for lean when I changed to the RUN position. This did not happen, so the heating element (at least on my car) will only heat up in the RUN position.

CarloSW2

[dcb]

IIRC, the heated sensor on saturns (HO2S) is the rear one. It is basically a sanity check of the front (non heated) one so the computer knows when to throw a cat or one of a few o2 sensor codes. Pretty sure most obd cars have at least a rear heated sensor but not necessarily a front one, and the front one is the one that you "drive" on.

[cfg83]

dcb -

Originally Posted by dcb

IIRC, the heated sensor on saturns (HO2S) is the rear one. It is basically a sanity check of the front (non heated) one so the computer knows when to throw a cat or one of a few o2 sensor codes. Pretty sure most obd cars have at least a rear heated sensor but not necessarily a front one, and the front one is the one that you "drive" on.

I agree, but Honda is really good with emissions, so (these days) all of their sensors are heated. For instance, on my wife's car, both her sensors are heated :

Replacement Oxygen Sensor - Front or rear - Fits 2003 Honda Civic EX 4 Cyl 1.7L

Features
Features
Brand: Replacement
Location: Front or rear
Number of Wires: 4
Wiring Type: 15 cm
Product Fit: Universal
Warranty: 1-year, unlimited-mileage warranty
Heated: Yes

Description
UNIVERSAL OXYGEN SENSOR, HEATED, 4-WIRE -- 15 cm long wiring; Includes instructions and butt type connectors; Without plug; A high quality oxygen sensor; Covered by a 1-year unlimited mileage warranty.

You should be able to eyeball your sensor. If you see 1 or 2 wires, it's probably unheated.

I think the second sensor is always heated because you can't count on the exhaust being hot enough to make the sensor work. It's more than a sanity check, it's for keeping the cat healthy.

CarloSW2

[dcb]

From an EPA perspective, the rear sensor is there to double check that the front sensor is being responsive, and so it needs to be heated so it can tell when the front sensor comes "on line". It is also to see that there is actually less O2 downstream of the cat, to make sure that the cat is actually doing something. But the sensor immediately downstream of the engine should be more accurate and thus the primary sensor.

FYI, it may never run the obdII o2 sensor/cat converter readiness monitors if you have a heated front sensor and always heat it up before starting the engine.

[thatguitarguy]

I haven't been able to find a 2nd sensor on my Civic. There aren't any wires at the cat or anywhere on the exhaust except for the O2 sensor sticking out of the front of the exhaust manifold. Pricey, but easy to get to.