Closed Loop Mode
 

Ok, I just got my scangauge installed and have a question about the Open/Closed Loop mode. While I know what they signify, I don't know why my Jeep goes into Closed mode so quickly. Example, on a 45 degree morning, startup, wait about 30 seconds while I get my seatbelt on, adjust the wipers and such, and pull out of the parking lot, and it goes into closed loop as I pull out. The Temp needle hasn't pulled off of the 110 degree mark (sorry, didn't get the water temp off of the Scangauge this morning).

On the upside - my scangauge says that I got an average 21.7 MPG for my last 90 mile trip to my relatives house in moderate highway traffic, heavy rain and no aeromods as of yet.

I'm not seeing the problem here, closed loop is good. Open loop is really only needed for starting, extra power, and drivability under certain circumstances.

My brain is obviously not working right this morning, so I'm gonna have to ask...what exactly is "open loop" and "closed loop"? I'm almost positive I've seen it explained elsewhere, but could somebody give me a refresher, please?

Very simply, open loop is when you are not running at stoichiometric air/fuel ratios. Your vehicle is using the maf or map sensor in combination with a few other sensors to determine how much fuel to inject. In this mode you are almost always injecting more fuel than necessary (except in a fuel cutoff situation, in gear deceleration for instance). During closed loop the fuel injection is based mainly off of the oxygen sensor in the exhaust. It holds the air/fuel ratio to stoichiometric ratios and is in general more efficient than running in open loop.

Definately not a problem, I just wanted to know if it was unusual for an unwarmed vehicle to go into closed loop so quickly.

It would not be unusual at all for a car with a wideband O2 sensor to enter closed loop mode quickly. With the wideband the closed loop can control warmup at a richer AFR and then transition to a leaner AFR when warmed up. With a wideband there is almost no reason to stay open loop except start up and maybe after start enrichment because AFR can be controlled across a wide range. With a narrowband O2 sensor it wouldn't make a lot of sense for the car to enter closed loop until the engine is warm because you will only be correcting to 14.7 AFR. My 2 cents

JeepNmpg2 -

Can you find out how many wires are attached to your exhaust manifold 02 sensor(s)? You could have a wideband sensor as mobythevan says, or you could have a heated narrowband 02 sensor. I think wideband are also always heated(?). In both cases it would go into closed-loop faster.

CarloSW2

O2 Sensor
 

Sure, when I get off work, I'll go and look. Do you know which sensor I should look at, I think mine has two or three O2 sensors (1 or 2 on the manifold and 1 after the cat).

-Andrew

JeepNmpg2 -

Thanks, the one(s) on the manifold. Those are the ones that are most important for closed-loop AFR (Air/Fuel Ratio).

CarloSW2

It's probably worth noting that stoichiometric ratio (lambda 1.0) is the peak thermal reaction, but it is not the best place to run an engine for either fuel economy or power.

Best economy is leaner (around 1.05 lambda), best power is richer (around .86 lambda, but with a lot of variation depending on engine design).

The only reason that vehicles target stoich is emissions. Stoich is the highest EGT, and that gas temp is needed for the CAT (which has a very narrow efficiency band). Also, it is a good spot on the HC, CO2, NOx emissions curve.

As a practical matter, closed loop IS usually more efficient than open loop because the cars always error rich open loop.

Generally speaking, vehicles with heated narrowband sensors tend to go closed loop a little faster than cars with wideband sensors (which are always heated as well). There was actually a good discussion about this at the recent Consolidated Research Council conference.

Much of the time, the limiting factor in keeping a vehicle from going closed loop right after startup is not rather or not the engine is ready to run at stoich, but waiting on the sensors for reliable measurements.

In an unheated UEGO sensor, the system can't go closed loop until exhaust gas heats the zirconia ceramic to the point that a reaction can occur.

A heated sensor is faster (and generally heated so that the vehicle can get closed loop for emissions purposes faster), but there are still some undesired delays. You can't pre-heat the sensor for startup, because unburnt fuel or condensation would shock cool the ceramic and crack it.

A wideband takes longer to bring up because of the way UEGO sensors work. Stoich is a reversal in chemistry, and effectively correct as long as a reaction is occuring. But the curve off lambda 1.0 with a wideband sensor is complicated, and only 'calibrated' at a specific temp. So instead of just heating above the reaction threshold, a wideband is generally heated more, up to about 750 degrees C, and held there before readings are reliable.

You ALWAYs want to measure lambda pre-CAT. The sensors don't really measure AFR, but equivilency ratio (which is inverted to give lambda). Basically O2 is added or removed to the test chamber until stoich is reached. Since CAT is supposed to be removing hydrocarbons, the post cat sensors are there to see rather or not there is a proper shift from the real (pre-cat) equivelency ratio measurements.

When you get a 'cat efficiency' DTC it generally means that one of the sensors has gone bad, or the shift between pre and post CAT is below expected.

-jjf