Went for a drive this morning and noticed both the oil light and coolant light were on, checked oil and had plenty, checked coolant and while at the bottom of the cold range, it has coolant in there. I then noticed that even though the engine was plenty warmed up, that the temperature gauge was sitting at the bottom entirely. From a brief bit of research, the temperature sensor on the 5.4 is damn near impossible to get at, so while a cheap part, replacing it looks to be a complete headache. I also have read that it could be the thermostat that is failing / failed. So, any suggestions on what to check first? Scanning for codes I see P0118 - Engine Coolant Temperature Circuit High Input, P1289 - Powertrain and P0171 System too Lean. Looks like the first 2 are related, with the P0118 being specific to the ECT sensor.

The coolant is pretty dirty and in need of a flush, is it worth starting with that and seeing if that helps? Or should I prepare myself to have to replace the sensor?

Thanks.

 

Both 118 and 1289 tells you the same thing. Circuit is high or open. Ground the lead and check the dash gauge. If it goes full scale the Sensor is likely at fault.
171 is a vacuum leak. Look at the long term fuel trim tables both banks and you will see the shift that caused the code/s 171/174.
Two different troubles.
Do the trouble shooting first before the guess work.
Good luck.

 

Thanks for the reply, when you say "ground the lead" what exactly does that mean? I'm not sure if it means run a piece of wire from the sensor to a ground location, or use a wire to jump across the terminals.

That also means me to a question I meant to ask previously, where is the sensor? It's a 2000 5.4, I've read it's under the intake manifold, behind the alternator and sometimes both.

Edit: Went outside to take a quick look and see if I could find the sensor, I think I have found the problem.... if I am facing the truck and look just back and to the right (towards the drivers side) of the alternator I can see what I believe is the sensor. If I look a little closer on the pigtail leading to it, I can see just a tiny bit of bare wire, looking a little further down below the wiring I can see a few small bits of chewed wire. I have had mice chew through the wiring on the oil sender in the past (replaced that a few months ago) as well as one of the fuel injectors. I would bet the little f***ers have chewed the wiring leading to the coolant sensor and that is the problem.

My biggest issue looks like it will be can I reach in there without removing the alternator, to unplug the wire in order to repair it and then reconnect it once the repair is complete.

 

You got it.
A rubber hose may have been chewed as well causing a vacuum leak. (171/174 codes)
Gota feed dem mice better food.
Good luck.

 

You might be right on the vacuum hose, they pretty much make a mess of anything. Hateful creatures. Think I'll feed them some peanut butter, on a trap door, above a bucket of water..... Will get the sensor wiring repaired first then start inspecting vacuum hoses.

 

It is much easier to R&R the alternator than the intake manifold.

 

Agreed, just looked up removing the alternator, looks like that will add about 10-15 minutes to the job. Also managed to reach my hand underneath to feel the wiring, yep, chewed right through. I'll sort it out on Saturday, just have to decide early morning or late evening... supposed to be 97F here on Saturday (but feel like 110F with the humidity).

 

you have two coolant codes.. The 1289 I believe is HEAD temp sensor and the 0118 is Coolant temp sensor.. I thought there were two different sensors....... look for ADDITIONAL wires that have been chewed.

 

Just got done repairing the wires, bought a pigtail with new sensor from AutoZone, of course it did not fit despite saying it does. But, I was able to de-pin the connector and rebuild my chewed up one. Alternator back on, started right up, no coolant/oil light and the gauge started moving up immediately.

Moved onto looking for vacuum leaks, think I found it. There's a small red plastic tube that looks like it comes out the intake manifold on the passengers side and routes around to the fuel rail on the drivers side. Anyway, that's broken right before it drops down to the fuel rail. What I'm not sure is what to replace that section with, it's a semi rigid plastic tube. Not sure what it's called or where to find some. Wondering if small diameter fuel line would be good if I could somehow connect it in securely.

 

If the original tube condition is GOOD, just a bad spot or break........... sometimes you can take a small hos with the ID the same as the tube OD... cut out the bad section of the tube and push both ends into the new hose.

 

Other than the broken section it looks decent enough, although I'm sure it's as brittle and almost 20 year old plastic on an engine bay can be. Might take a small piece with me to the parts store and find something that it slides tightly into.

Will be nice if that takes care of my other codes. Then I can get back to the maintenance I planned on this thing.... Oil change, spark plugs, coil packs, fuel injectors and then will add a coolant flush into the mix shortly as well. Fingers crossed it should be running nicely after all that.