I have a 03 explorer 4.6 at cold start-up it wants to stall and it idles very bad just for maybe 5-6 seconds and then the idle picks up and it's fine. This only happens on cold start-ups, if I drive it and restart it's fine.

This started when I replaced the two front (up-stream) O2 sensors, I didn't need to change them just have 150,000+ mile and was trying to do something to get better mpg.

Dose anyone know if you should disconnect and reconnect the battery after replacing the O2 sensors on an 03? Reset the computer?

Thanks

 

time to connect a LIVE DATA SCANNER, 5min and we would know.

COLD engine,no start (KOEO),

• AIT (air temp)and ECT(coolant temp) must be with in 2% of ambient temperature
• TPS, 0% at stop and rise smooth and steady as throttle is advanced
• MAF, 0% with no air flow
• Battery, 12.6vdc, fully charged.

Start engine.

• ECT, must start to rise and attain and hold 180* at idle and 2000rpm
• CL (ECU) must maintain at idle and 2000rpm
• MAF, small reading at idle and rise steadily as throttle advances.
• O2, (B1S1,B2S1 precat), must start actively switching with no lean/rich bias...
• LTFT, (long term fuel trim), close to 0%, +/-2 is allowable, if more, time to investigate fuel delivery/illegal air
• Vacuum, at unregulated manifold port, 19-21”hg..with a dead steady needle, investigate jitter or bounce.

 

The primary sensors responsible for starting fuel level, especially coldweather starts, are the AIT(air input Temp) and the ECT(engine coolant temp). If they are malfunctioning the ECU will not fuel the car for cold start (rich).

On cold start the ECU checks the coolant temp and if <180* will initiate rich injection and open ISC (idle control), colder the value the richer the injection(longer pulse).
As the coolant temp increases the ECU commands the ISC and Injection pulse to reduce fuel/air, lowering RPM to +/-800.
On reaching coolant temp >170* the ECU obtains CL (closed loop) and allows the O2 S1 to control the data based on the amount of unburnt oxygen in the exhaust prior to CAT.
In order to achieve the stoic 14.1/1 AFR ratio, the ECU considers the coolant temp + MAF + O2 + TPS.

...Philip

 

Hey thanks Philip thats alot of great info, I really think it has something to do with the O2 sensors maybe I need to change the other two down stream? Something screwed up after I changed them out, I'm thinking the computer is getting the wrong, (too lean) message at cold start-up because after it runs for 10 seconds I can shut it off and it will start up and idle normal.

 

Go back and read post #3.......

It would not the the S2 O2's, they just monitor the CAT efficiency and do not contribute to the AFR (Air/Fuel Ratio).

The O2 does not contribute or affect fuel pulse until the CPU goes into closed Loop (CL) that means the engine must be started and warm with O2 +/-650*. So NO it is not the O2 sensors....

Do not guess/switch parts, that will cost you $$$. Beg, borrow, rent or buy a "LIVE DATA SCANNER" and do it right..... Philip

 

Your right, that's the most cost effective way to find the problem.

Thanks again

 

You have a air valve that sets over the trottle body (silver cylinder) that controls the cold start idle. They can stick or corrode depending on the enviroment. Easy to replace and cost about the same to test. I replaced mine at about 115K miles.