Engine RPM Oscillating when warming up

[xJohnnyOx]

My handle on the Explorer Web Site is Fixing Daily. The Expedition is not so bad, but I encountered a new issue over the last couple of weeks now that the daily low temperature is in the low teens.

2000 Expedition, E.B. with 5.4 Triton.

Starting on cold morning, runs fine in stop and go traffic until the temperature gauge is reading about normal (at this point I am sure some things in the motor must still be coming up to temp). At that point, at stop lights, it will idle smoothly for about 10-15 seconds and then the motor rpm will start oscillating quickly between 200 and 1200 RPM. If I give it a little throttle, it smooth’s out. The Service Engine light is not coming on (SES light is operational as it works correctly when turning the key from Off to K-on, E-off). This will go on for about 10 minutes of additional warm up time when I idle at each stop light. After that, the engine and engine compartment are probably all at normal operating temp and the problem goes away. At this point the engine idles smoothly.

I have not been able to scan when this condition comes up, but will keep the scanner in the truck in case it does it again.

When warm and running, the scan results I am getting now are:

Pending Code P0174 – Bank 2 Too Lean

Throttle Position (when at idle with no pressure on the pedal): 18% (this seems weird)
Idle speed, constant 780 RPM.
Running in Closed Loop (CL) Mode.

My first too thoughts are: 1 – Dirty Fuel Filter , and 2) Intermittently faulty IAC valve when cold.

Any other opinions are welcome and gratefully accepted.

 

[bedrck46]

sounds like a vacuum leak. maybe a small leak in a hose and when it heats up the hose swells enough that you don't notice it as much. Take a can of Carb Cleaner and spray around the rear of the intake manifold and the hose that goes to the PCV valve if your rpm's increase you found the problem if not check all the vacuum hoses

 

[tonydiv]

I wouldn't rule out the throttle position sensor either.

 

[xJohnnyOx]

Both good ideas. Thank you. Has anyone scanned their Expedition to see what the T.P.S. is reading at idle? If so, let me know so that I can compare. (From my previous Suburban experience, it would read 0.5% at idle).

Thank you,

John O.

 

[Copper93]

Try cleaning the IAC. It's quick and easy. Every time I have seen that type of issue, it has been the IAC.

 

[IMINYOURCHAIR]

Mine is reading at about %19 at idle. my past car, a 2004 volvo xc70 did the same, but the car before that, a 2000 buick lesabre read at about 0.2 to 1.2%. But the volvo had an electronic throttle that had no idle air valve, it actually opened the throttle to idle. i'll glance at it tomorrow to see what the actual value was.. but a +1 on checking for a vac leak, if it says too lean that means that air that wasn't counted by the maf sensor is getting in to the mix. you can also use water if you don't have carb cleaner, just get a squirt bottle and listen for it to be sucked in and also for an idle change.

 

[1997SCEBFEX]

All good suggestions above. How many miles on her?
When was the last time you cleaned/replaced the IAC? It's pretty easy to remove and can be cleaned w/a reputable cleaner, then reinstalled, or a new one. I'd also take it upstream all the way to your air cleaner element/MAF too and ensure everything is good-to-go.

TPS usually jumps around at certain speeds, mine was horrible around 34 and 45 mph...took over a year for the CEL to come on....originally thought trans, but the codes are pretty good at diagnosing the problem.

PanHandler

 

[xJohnnyOx]

I just bought this Expedition about 3 months ago. I have had several other threads working various issues. I am really liking this truck. AWD and 4x4 were fully tested yesterday in our first significant snowstorm of the season. Both worked great! This current issue is just a minor annoyance and given the responses, all options can be checked easily. One other item I want to change is the PCV valve grommet. I think the current one may be brittle and not providing a good seal. I will report back if any one item seems to be the culprit. Oh, as to mileage, 180K. I do not have any service records so i am going through all the normal stuff as I get to it. I am a DIYer for most stuff, but i am letting my trusted mechanic change out the spark plugs next week. I just read to many horror stories about simple things going wrong on that job.