Could someone explain to me what this is and how it works. Also, what are the indications that my 91 is in either loop.

 

http://www.modularfords.com/forums/s...p-vs-Open-Loop

 

Short answer. Open loop, ECM not reading O2 sensor, closed loop, O2 sensor is warmed up and supplying data to the ECM.

 

"Open loop" operation occurs before the engine has warmed up. In open loop, the ECM ignores real-time voltages from the various sensors, other than the CTS (coolant temp sensor) in favor of default values. When the engine warms up, as determined by the CTS, the ECM goes to "Closed loop" and begins metering fuel, adjusting timing, etc. based on inputs from the sensors.

 

With an ALDL cable and a laptop, you can download software that will provide a flag -- showing you which mode you're in. If the 91 is similar to the 89 system, open loop lasts 5min from a cold start, about 3min from a warm start, and just one minute when hot. The delay is to allow the O2 to get to temp (600ish) where it can accurately measure oxygen content. That's how is adjusts fuel (aka trim).

When cold (open loop), calculations are used to determine fueling requirements. To make sure it never goes too lean, those calculations err on the side of running rich. So, during open loop operation, you're probably burning more fuel than closed loop. Once it hits closed loop, oxygen levels can be monitored to compensate. And, slices of data are taken every few milliseconds.

This trim data is stored in volital (eraseable) computer memory. It's a version of memory that helps better anticipate how the car "needs" to run for best economy/emissions compliance.

Associate open with "open circuit" or closed with "closed circuit" so you can remember which state has the ECM monitoring performance.

One final note: operating parameters can be changed. We call it tuning.

 

Its worth noting that full throttle (or near to full) will also produce an Open Loop state as the required air to fuel ratio is less than the O2 sensor's range.

 

Yeah, I'm in open loop a lot.

 

The reason I posted this thread about the Open & Closed loop systems is that I just had to have all three cats replaced on my 91 and it only has 36K on it. Exhaust guy said the pre-cats where " melted " < his words > and that it was due to it running to rich. I bought a new O2 senors and he installed it prior to me leaving his shop. SailorSteve mentioned the CTS sensor controlling the fuel metering in the Open Loop. Where is that located on the engine and could that be something I need to replace / clean also ?????

 

Actually, what I sais was, the ECM monitors the CTS to determine when the engine is warmed up enough to go over to closed loop. During open loop running, the ECM meters fuel using default parameters.

The CTS is located on the lower front side of the water pump, screws into the pump casting, has an electrical connector on it. Codes 14 and 15 are associated with this sensor.

 

CTS=Cold Temp Sensor

There must be a difference in the early vs late C4s. On mine, the CTS is mounted in the head near the dipstick. There's another one in the front of the intake. One sends info to the dash display. The other is used for the ECM to determine engine temp.

If your CTS was bad, and you were always running in open loop, it should have smelled super rich -- before you melted the cats.

 

So, Greggpenn witch one is used by the ECM for engine temp?

 

It's the one it that screws into the front of the intake manifold (just above the water pump).

FYI: I have seen posts where the sensor wasn't bad, it was the ECM not getting it's signal. That info was posted in this thread....http://forums.corvetteforum.com/c4-t...old-start.html