owkaye

I had a strange problem yesterday with my 2003 4.6L 2WD Supercrew:

While driving normally at about 40mph the oil pressure light came on. That's when I also noticed that the engine temperature gauge was indicating "full hot".

I have never seen the temperature gauge read hot before. Normally it runs at slightly less than halfway between hot and cold, so this was a substantial change from normal. I had added coolant to the reservoir the day before, but I could not imagine how or why the addition of coolant might cause the truck to overheat all of a sudden. Nevertheless I decided to stop and check the fluids.

But before I could pull over the truck started to run poorly. This freaked me out. My first thought was that some serious engine damage had occurred because of overheating ... but now I believe that this is when the truck went into "limp mode".

I pulled over, shut the engine off, and popped the hood. Everything appeared completely normal. There was no "hot smell" whatsoever. When I carefully opened the coolant filler cap the pressure was released slowly and normally. There was no excess pressure, and there was no steam or boiling water expelled. The coolant was still at the proper fill level, and it wasn't any hotter than normal.

I was puzzled by this, because in every instance of overheating that I have ever seen, there is always steam or super-heated water blasting out of the cooling system somewhere -- either out of a broken gasket or hose, or the radiator when the cap is loosened prematurely. But in this case, nothing.

I checked the engine oil level and it was fine. No indication of water in the oil. I looked around for fluid leaks and there were none. So I got back in the truck, started it up, and "surprise surprise" ... the oil pressure light was out and the temperature gauge was once again reading normal!

Obviously a truck cannot cool down from being overheated to normal in only a couple minutes, yet the truck was once again running normally and all the indicators were normal again -- aside from the check engine light which was now on (it might have been on during this problem, I don't remember for sure). Later when I checked the code it was P1299: Cylinder Head Overtemperature Protection Active. Is this what triggered limp mode?

So now my suspicion is that the engine temperature sensor flaked out and sent the erroneous "overheated" signal to both the dashboard temperature gauge and the PCM. Am I wrong about this? Is there some other sensor that may have misbehaved temporarily and caused this situation?

By the way, does this truck have an Engine Coolant Temperature sensor, or a Cylinder Head Temperature sensor, or both? If one of these sensors is failing intermittently I will probably have to replace it.

And what about that low oil pressure light? Why would that light come on at the same time when there was plenty of oil in th engine and apparently nothing wrong with the oil pressure?

Is there something that might cause of both of these indicators to temporarily fail at the same time?

Fordrocks689

If it's a p1299 code I'd say start looking for a bad ground or electrical issue somewhere (good luck) if you havnt already tried swapping the sensor

owkaye

Thanks, I'll start looking for an electrical malfunction today.

Last night while driving I saw the problem develop as it was happening. I watched the temp gauge move quickly into the hot/red zone. That's when the oil pressure light came on and the truck started "limping". I knew these indicators were in error so I kept driving the truck until we got to the store. A half hour later when leaving the store, the truck started and ran 100% normally -- no indication of any problems whatsoever (except for the check engine light which has been on since this problem first occurred two days ago).

If this is an electrical problem I hope I can track it down. But if I cannot, is there any way to disable "limp mode" so I can drive the truck normally when these false indicators are on?

Also note: I tried to turn off and restart the truck when we got to the store, but it wouldn't idle properly. Instead it kept stalling unless I pressed the accelerator pedal to give it some gas.

rcairbear

Have you done a coolant drain/flush?- just thinking maybe an air bubble at the temp sensor(?) maybe the temp sender is bad-
Another thought, does it happen if youre not driving, just idling?- ( bump or vibration related)

Jbrew

It's your CHT giving way, or the harness. Could be the connector as well. Not THAT common but it does happen. Yes, it's a wrench in the spokes until you R&R. No ECT in that model, - it's all CHT and a PITA to change out. That's because of what it takes to get to the sensor.

owkaye

No, not in a very long time. I'm going to do this later today, not only "because it's time" but also to see if this might resolve the issue.

So if there's an air bubble at the sensor, you think it would it read hot rather than cold? I guess I might have assumed just the opposite.

This is certainly possible. I haven't checked it, but AutoZone printed out some simple multimeter-based instructions to use when testing my Coolant Temperature Sensor:

1- Remove the sensor.
2- Run it under cold water.
3- Check and note its resistance in ohms.
4- Hold the sensor in my hand for 15 seconds to warm it up a bit.
5- Check and note its resistance again.

The difference in the two readings should be more than 100 ohms. If not, the sensor is faulty and must be replaced.

So far it has only happened after driving for a while, usually at least 5 minutes or so, but this delay before it happens is not consistent. This morning it happened again after I was driving for about 5 minutes. I cannot recall a time when it happened sooner than 5 minutes on the road.

One possibly related thing that happened again today is that the truck stopped blowing hot air into the cab and began to blow cold air instead. I have no explanation for this change, but when it's cold out and you're trying to get warm you definitely notice when it switches from hot to cold! Today I drove for long enough that it actually switched back to heating again after about 20 minutes.

Very strange.

owkaye

You seem pretty confident that the CHT sensor or the related wiring is cause all the problems I've suddenly been seeing lately. Is this the only thing you've seen that might create all these issues at the same time?

If it's just a sensor, or an electrical wire or connection between the sensor and the PCM, then at least I will have a clue where to start dealing with this issue. Thanks!

If you can suggest any pictures or videos that might help me to understand what I'm getting into in terms of replacing this sensor please tell me. If not, hopefully I can find a video showing how someone else changed the CHT sensor in his truck with the same engine.

Thanks again for this suggestion.

akdoggie

I'm betting on the CHT also. That is the one common denominator. I'm a bit confused on the oil pressure gauge coming on also. That bothers me a bit. It could very well be a wiring rub or similar disruption, but the CHT is somehow involved.

Here is a Youtube vid on replacing the CHT on an Expy, but the 2003 Expy is the same exact set up, so you are good to follow this video.

owkaye

Thanks akdoggie, I will try this later today. First I want to flush the system and replace the coolant (never got to it yesterday) so I can check the sensor wires at the same time to make sure the problem is not just a bad wire or connector. But the evidence seems to be mounting that the problem is the sensor itself:

That's the oil "warning light" that's coming on, not the gauge. This was bothering me too, for a while, but now I believe that this warning light is not a low oil pressure warning light, but instead it is probably a high oil temperature warning light. Here are two reasons why I believe this:

1- After watching this problem develop consistently from the beginning at least 3 times now, I can confirm that this oil warning light comes on only when the temp gauge needle hits the red zone ... and it seems to come on EXACTLY when the temp gauge needle first touches the red zone. It never comes on before this, and it is already on by the time the needle is completely inside in the red zone.

2- The oil pressure "gauge" never changes position when this problem occurs. This would indicate that in fact there is sufficient oil pressure all the time, even when the oil and/or cylinder head temperatures are (supposedly) too hot.

Basically I now believe that when the CHT sensor reports the beginning of an overheat condition the computer will start pushing the temperature gauge needle higher. Then if the engine continues to overheat (or if the sensor mistakenly says it's still overheating) the needle will hit the red zone -- and it's at this specific pre-programmed temperature that the computer switches on the oil warning light and kick the engine into limp mode.

If these assumptions are correct, then all these issues are related to a failing CHT sensor, which is what jbrew told me earlier in this thread, and what you also confirmed.

I might suspect a bad electrical connection if the problem occurred at different times, such as immediately when starting the engine, or before it warms up, or before about 5 minutes of driving. But so far this problem has occurred ONLY after the engine has warmed up to normal operating temperature, thus suggesting that the source of the problem is most likely the sensor -- which fails when it gets warmed up.

Jbrew

Oil light reflects a high temp warning as well. When the engine heats up the PCM's (executive) programming isn't advanced enough to determine the exact root with that sensor malfunctioning. You have a cooling issue, but no coolant DTC grouping. You also do not report an oil pressure gauge issue so I'm guessing there isn't one. The oil pressure gauges (idiot gauge) you get with these trucks do work. They work very well in fact. If your losing oil pressure it would reflect on the gauge. You can count on it.

With this in mind, it's a rather sure bet it's the sensor itself. Your description (well done) also adds more weight or supports that. You know, with the way it's occurring.

What's nice, - slipped my mind at first, you can R&R that sensor removing the alternator. That makes it an easy fix.