I have a 97 5.4 expedition with a KB blower. How long should the engine stay in open loop at cold start? It stays in open loop for about 20 seconds or so which seems kinda short. Does closed loop activate at a certain ect?

 

I did a little research and heres what I found. Open Loop and Closed Loop are two terms used to describe the fueling mode the computer is in.

In closed loop fueling, the computer takes readings from the oxygen sensors which read +/- of 14.64 AFR (Stoich) and makes corrections for it. If your car is idling @ 13.5 AFR, the 02s can see this, and will know to pull out enough fuel to get back to stoich.

In open loop, the fueling is purely based off the MAF transfer function and the open loop fuel table. This table has to be calibrated any time you make MAF changes, and changes that affect the airflow of the MAF.

When a car is started, the computer is in open loop until certain criteria is met. The o2 sensors need to be heated to work properly, so this is the main reason there is a delay.
After the sensors are heated up, the car goes into closed loop and the O2 sensors make their corrections. Just as example my AF Meter's 02 sensor takes 20secs to heat up.

The computer will go back into open loop under a variety of conditions. Throttle Position (TP) is one way. There are parameters in the computer that determine what the TP is for Open Loop, and other things. When that TP is reached, and the car goes open loop, and richens up.

It sounds like your Expe is operating normally. Again its all sensor based.

 

Thanks for the info. I also noticed my ect sensor was reading 80 degrees when it is 30 outside? This was after the truck had been sitting for several hours. How close should the ect be to ambient temp?

 

The ECT is monitoring coolant temperature, so its possible that it is reading correctly, although I would expect the coolant temperature to have reached ambient after several hours. I'd see what it's reading over night. It should be close to ambient on a cold start over night. If 80 degrees is the lowest you see, its bad. It should be capable of monitoring coolant temps as low as -40 degrees. I would think it should stay in open loop on a cold start for longer than 20 seconds too on a cold engine, but if the ECT is telling the PCM the engine is at operating temperatures, if could be that short. I'd continue to monitor your ECT temperatures because it should like you might be onto something.

 

Twenty seconds is about right. After that the pcm uses a cold fuel table to correct / choke for another minute and a half or until operating temps are reached.

 

Btw the ect is not an actual reading of the ect. These engines are equiped with a head temp sensor. Per Troyer the sensor reads cylinder head temp and then other factors such as load, length of time the engine has been running and more are factored in and the ecm produces a number or readout of ect though it is not actual. For example my truck is equiped with a 170 degree stat from Troyer. However my ecm reads 180 as a normal temp and 186 is what I see as a max. When in reality coolant temp is 172ish and reaches a maximum of 178.

 

I let the truck sit overnight and it was a lot closer to ambient. The sensor/open vs. closed loop is not my problem.

 

The Expy may be different, but the 97'-98' trucks have 2 ECT and a CHT. one ECT to the pcm, one to the dash gauge.
I have datalogged the ECT and CHT and after warm up they stay very close to the same during prolonged driving.