I have a 2000 Explorer W/4.0 SOHC engine. With summer nearly over and fall on the way, I need to figure out what is going on with my engine. This problem has been going on for the past 2 years, and I am about out of ideas. As the outdoor temperatures get cooler, the harder it is to start and keep the engine running. This starts happening around 65° ambient temperature, and gets worse as the temps drop. By 30°, I can barely keep the engine running. Odd thing is, all I have to do if the engine starts is, keep my foot on the pedal to keep the engine around 2000RPM, put it in gear and start driving. After about a mile or two, the engine runs normally, and will do so the rest of the day.
So far I have cleaned the air flow sensor, replaced the IAC, replaced all the injector "O" rings, and cussed a lot. I smoke tested the engine and the only vacuum leak is a very small one at the EGR valve.
Winter is right around the corner, and this is my winter daily driver. Any ideas??

 

I suggest you start by fixing the vacuum leak at the EGR valve. Beyond that, do you have access to a scan tool so you can monitor what the Intake Air Temperature (IAT) sensor is reporting as well as what the Engine Coolant Temperature (ECT) sensor is reporting?

-Rod

 

Thanks! I will order an EGR valve in the near future. I will have to wait for some cold mornings to see if it makes any difference. I do have a scanner, and I will hook it up to monitor coolant and intake air. I will post results when I know more.

 

Update:
The mornings are now cold enough that the problems are present.
The leak from the EGR is so small that I have not replaced it yet. Intake air temp is normal, as is coolant temp. I ran some fuel injector cleaner along with a tank of non-ethanol fuel through it last week. It starts a whole lot easier, but I still have to hold the throttle part way open while cranking until it starts, then keep my foot on the pedal for about 60 seconds. After that, the engine still idles rough, but it will stay running. After idling for about 3 minutes, the engine smooths out and runs fine the rest of the day.
I checked the upstream O2 sensors by warming up the engine and spraying carburetor cleaner into a vacuum connection. Both sensors responded properly by immediately drawing 0.9 volts until the carb cleaner was burned off, then went to their normal fluctuation of 0.1V to 0.9V. So I am now thinking that that the O2 sensors are not the problem. However, this morning, with the outside temp around 20°F, I started the engine and read the live data. Both bank 1 and 2 sensor 1, were only reading 0.03V. I don't know if this is normal or not. Seems to me like this is indicating an extremely lean fuel mix just when an engine would need more fuel to over come the cold. When I looked at the O2 sensors on the milivolt scale, bank 1 sensor 1 was at 30mV and bank 2 sensor1 was at 0.0 mV. As the engine warmed up, the values gradually returned to normal.

 

Have you looked at IAT and ECT values at cold start yet? Both can play a big role in how lean or rich the mixture is during a cold start.

-Rod

 

IAT (intake air temp) and ECT (engine coolant temp) start at ambient, and rise as the engine warms up.