Odd idle problem

[cpumatt]

So, I'll start my explorer and It idles perfectly. Smooth, between 800-1000 RPM. Then, maybe 2 minutes into idling, the car drops to 500 rpm and my truck will start shaking back and forth until it stalls after maybe 5 minutes.

I cleaned out my throttle body, idle speed sensor, and I got a new air filter to no avail. Any tips?

 

[J_C]

Any OBDII trouble codes set? Which engine?

Clean the IAC valve if you haven't in recent thousands of miles, and MAF sensor. Check for oil in the coolant or coolant in the oil. If all else fails do a compression test and note the condition of the spark plugs while they're out, though if you have a cylinder leak that doesn't get bad enough to cause a problem till the engine starts heating up, that could be harder to find.

Is there any chance this started right after a new tank of gas? Beyond idle does it run okay driving?

 

[cpumatt]

I have the 4.0 SOHC. I haven't cleaned out the IAC valve ever, or the MAF. Coolant is fine and the oil change guys never said anything was in the oil. Spark plugs haven't been done in around 60k miles so I need to get around to doing that.

This started happening though since I got the vehicle a year ago. It's just gotten worse so I decided to look into it. Often times when the truck is cold when I first start it it'll start and then instantly stall.

Beyond idle, it runs great!

 

[J_C]

I'd also look for vacuum leaks, thinking maybe it's deliberately running rich when first started but then when the system tries to lean the fuel mix, it gets too lean.

 

[swshawaii]

Excellent advice from J_C above. Speaking of vacuum leaks, check the notoriously failed PCV elbow.

Does NOT always throw a CEL or p0171/174 trouble codes. See thread linked below for location.
P0171 & P0174 4.0L SOHC's Sport/Sport Trac | Ford Explorer and Ranger Forums - Serious Explorations

 

[swshawaii]

Many prefer to just spray the MAF wire filaments with CRC MAF Cleaner without removing the sensor.
If possible, service one part at a time and test. Just curious what the actual culprit(s) is. Good luck.

Mass Air Flow Sensor cleaning 03 Sport Trac | Ford Explorer and Ranger Forums - Serious Explorations

 

[angstrom08]

I discovered a similar issue a few months after I first purchased my '01 almost four years ago. Cleaning the IAC would fix the problem for a few weeks. In the end, I replaced it and haven't had an issue since. I seem to recall the part was cheap on amazon.com and replacing it is really ridiculously easy.

 

[cpumatt]

I discovered a similar issue a few months after I first purchased my '01 almost four years ago. Cleaning the IAC would fix the problem for a few weeks. In the end, I replaced it and haven't had an issue since. I seem to recall the part was cheap on amazon.com and replacing it is really ridiculously easy.

Yup! I cleaned my IAC, things worked really well and then in a few weeks it came back. I will replace it and update the thread.

 

[J_C]

BUT, did you guys put a generous drop of thin oil on the shaft and work it down into the bushing? I suspect that's what the real problem is, shaft seizing in the bushing, as it's not as though the small amount of carbon (in mine at least) was enough to change the airflow rate much.

Then again there could be more than one failure mode, bushing friction vs solenoid coil shorting or something else.

 

[angstrom08]

BUT, did you guys put a generous drop of thin oil on the shaft and work it down into the bushing? I suspect that's what the real problem is, shaft seizing in the bushing, as it's not as though the small amount of carbon (in mine at least) was enough to change the airflow rate much.

Then again there could be more than one failure mode, bushing friction vs solenoid coil shorting or something else.

I agree, I think the solvent used to clean probably loosens/softens buildup on the shaft and/or seal temporarily. That would explain why sometimes cleaning only works for a brief while.

 

[cpumatt]

Update: Deep cleaned the sensor with electrical parts degreaser, and it's working much better. All the other times I cleaned it I focused more on the throttle area. My valve doesn't have the black cap on it though. Maybe aftermarket?