2001 F150, 5.4L gas, 4wd, stock, 217k miles, auto trans. Bought it used about 4 years back.

Currently away from home base. Left Saturday for a 600+ mile trip. All fluid levels are good. Truck has been reliable. No major issues other than an alarm malfunction a while back which got sorted.

About 90 miles into the trip down the highway, the service engine light comes on. Hmm. At the next fuel stop I tanked up. Started and it was a bit rough the first few seconds and then settled. Seemed to have an engine noise I had not heard before.

Pulled over in the station and popped the hood. Oil good. Coolant fine. Power steering fine. Listening to it I could definitely hear an unusual sort of "ghost" like sound. Hard to describe. Sort of like a low "woooooo." It was low, but definitely there and not normal. Could be unrelated to the engine light. Pulling out of the station the sound varied and got louder with slight acceleration. Possibly beginning of an accessory bearing meltdown?

No acceleration or dying issues. Got back on the highway and kept on trucking for 600+ miles. No running issues. No shifting issues. Ghost noise not noticed at highway speeds.

The power steering pump is probably in need of replacement at this point with this mileage. It does drip slightly, but not a lot. It may be the original pump for all I know. Same with the water pump.

The alternator was replaced last year with a Bosch unit.

I have never replaced the O2 sensors. Stock exhaust.

Spark plugs were replaced a while back with Motorcraft platinums.

It is now Sunday night and I went out to listen for the sound, and it is not there anymore, but the service engine light is still on.

Any thoughts on first things to check with a truck that has 217k miles on it?

Was tempted to bring it to the dealer here, but it seems to be running ok in spite of the light coming on. So I may leave it until I get back home and check it more myself then. Can't do much here since I don't have my tools/service manuals etc with me.

 

Go get the codes read. Most auto parts stores can do it for free. Write down the EXACT fault code(s) and come back here and post them. Wait for advice.

 

Went to Auto Zone. Tried NAPA first and they were like no, go to Autozone and spend your money. I was like, okay, will do.

They printed out the code info:

Only code that came up was PO125 Coolant temp is always low. The powertrain control module verifies that the engine is at proper operating temperature.

Probable cause: Coolant level - checked and is fine
Thermostat defective
ECT sensor defective

Neither of those items has been replaced since I have owned the truck.

The coolant temp gauge on the dash is not showing abnormally low temp. Todays temp is 60F right now. So I am guessing it is the ECT.

 

P0125 is a lazy or slow thermostat. It causes the engine to warm up too slowly to keep the computer happy. I don't think its responsible for your symptoms, though, which sound more like a vacuum leak or a bearing.

 

I can swap the thermostat out easily enough.

The strange sound is now gone of course. The vehicle is not living in the easy southern temps anymore like it was since manufacture. It's spending it's first winter in the cold northern climates. So one of the bearings may not be happy with the single digit temps of winter there.

I will just have to keep an eye on it and see if the mystery sound comes back when I head north again next week.

 

you can look for bearing noises with a stethoscope...... I actually use a piece of 1 inch PVC pipe about 18 inches long. Set one end of the pipe on the front bearing of the alternator and lean you ear up against the other end. Listen for a ROUGH whine.. test the other end of alternator, idler pulley, exhaust manifold, etc......... you can narrow down a bearing problem / vacuum leak this way.

 

LOL...Living in a cold climate all my life I can almost tell the temp. on how things sound when I start it...some never go away completely until summer time.