False overheat problem solved

There are quite a few posts on this site about Ford truck engines giving a false overheat indication. Some posts suggest that the wiring harness for the trailer can cause the problem, and this was true for my truck. I will summarize for people who see false overheat warnings and wonder how to fix them.

I have a 1997 Expedition with the 5.4L Triton engine, replaced with a rebuilt Jasper after 190K miles. The rebuilt engine has 30K miles on it. The truck has a factory-installed towing package including the wiring harness.

While pulling a 5K pound trailer, I noticed that the heat gauge was pegged on hot. I quickly pulled over and checked the engine, it did not seem to be overheating. I shut the engine off and restarted it. The temp gauge went back to showing a normal temp!

I continued on my way, keeping a close eye on the temp gauge. It was normal, but after a bit of time it jumped to peg on hot. Again, the engine did not seem hot when I stopped to check it, and restarting the engine solved the problem. Sometimes the oil temperature warning light would also come on, and eventually the check engine light as well.

My next step was to get an OBD sender that I could read the codes and monitor the engine temp. Using the OBD I could see that the engine temp was running about 210 - 220 normally and would spike up to over 250 when the engine was under a load. At about 253 the temp gauge would peg, and at about 257 the engine would go into safe mode, running on 4 cylinders to help cool the engine down. Stopping and restarting the engine fixed everything. The check engine codes included some that were eventually traced to a bad spark plug coil.

My truck has two heat sensors, one for the heat gauge and one for the computer. The second one measures the cylinder head temperature (CHT). A number of posts suggest replacing the CHT sensor once you are certain that the engine is not actually overheating. I replaced the CHT, and it helped by showing that the engine was running 10 - 15 degrees cooler than before. However, with a load on the engine the OBD showed that the temp was still getting above 250, and the heat gauge would still peg.

Several people have said that the false overheat problem can be caused by a bad ground connection, and that the wiring harness for the trailer could be at fault. In fact, my trailer brakes have quit working several times and I had to add jumper connections. The 12V connection to the trailer has not been working for a while. Taking a closer look at the harness, I found that the wires go past the fuel filler and a canister with what I assume is an oxygen sensor. The brake and 12V wires were badly corroded right near the sensor. I replaced both wires with new 12 gauge wires, starting just in front of the rear tires and going back to the trailer socket. I also cleaned the socket contacts and added dielectric grease to the contacts. The false overheat problem is now totally fixed. My OBD now shows that the CHT gets up to 240 when pulling a load up a hill, but it cools promptly once you get past the grade. The nominal engine temperature reports as 200 at freeway speeds with no grade or load.

I assume that the 12V and electric brake lines are more likely to corrode compared to the trailer light wires, since they have constant voltage on them. They carry more current than the lights, and I'll bet that they arc to ground, causing noise on the oxygen sensor that gets carried back to the engine computer and confuses the temp sensor. The temp sensor responds to the CHT but gives inaccurate (high) readings, leading to the false overtemp warning (the heat gauge pegs on hot). I am guessing that exposure to exhaust and fuel fumes degrades the wire insulation over time (my truck is almost 20 years old).

I hope this helps others solve similar problems. One thing to be very clear about is that you first want to be sure that the engine is not actually getting too hot. You can buy a temperature monitor to directly measure the actual CHT to be certain that you don't actually have an overheating engine.

 

sounds like a 10th generation problem ... us 9th generation guys just have to change the oil regularly and buy new tires every few years.

seriously though, good write-up; should save somebody a lot of time. well done !

 

Thanks for the write up.

 

Your assuming a bit too much for the cause of the original issue..
As I know the system operations, there is no direct connection or inferred association to the CHT circuit, noise or otherwise unless there would be an actual cross in the harness at some point..
The CHT is an input sensor to the PCM and does act as you have seen for limp mode.
The CHT is purely a temperature dependent variable resistance and not very prone to a noise interference.
However, running at 240 under load suggests your short on cooling capacity and may have a plugged/restricted radiator or other cooling system issue for the age of the vehicle.
If you observe the temperature dropping quickly after a high load, the CHT circuit is working ok.
The load on the cooling system has reduced so cooling recovers even if restricted. Would seem that there is not much spare cooling capacity left.
You must remember to include transmission temperature rise in the mix as well as A/C if you have it on.
Anyways, just be awhere of this possibility.
Good luck.

 

Bluegrass, you are exactly right! Turned out that the temp spiked when pulling the trailer again, and I replaced the thermostat and flushed the cooling system. Problem solved!

So it looks like 3 problems occurred and each contributed:
1. bad sparkplug coil
2. bad CHT sensor
3. short in the trailer wiring harness

each of these was done on my truck, and each time the false overheat problem got better;

But in the end, there was a true overheat happening as well.

Hope someone else learns from this saga . . .

 

Great to hear of your final success.
I've seen it happen before.
The bottom of the radiator collects the silt, restricts the cooling capacity and helps the thermostat fail sooner.
The coils also take a beating from the extra heat developed.
In some circumstances the radiator hoses can suck closed from not being able to pass coolant that normally keeps them open and in shape.
Good luck.

 

With a new thermostat and flushed cooling system, the CHT reads out at 188-192 most of the time, where before the fix it ran at 200-210 on level ground without a load. Pulling a load up a mountain pass, the CHT maxed out at 203, where before it would go over 260. Since the thermostat should open at 190, I should have figured there was a cooling problem.

Bluegrass is right about an underlying coolant reserve problem that was there all along, but the diagnosis was complicated by the ghosts in the electric system. Once the three problems I mentioned in the previous post were fixed, the overheat condition did not happen again until I was pulling a trailer. Importantly, the cylinder heads were running hot without a load, but I would not have known that without the OBD since the engine coolant gauge read out as normal. I would also note that infrared temp readings on the engine would not have helped as much as the OBD, since you have to stop and open the hood to use the infrared unit and by the time you do that the cylinder heads have cooled down again.

works great if you need something very basic. It cost 35 bucks and it connects to an iPhone. It is great for a consumer unit.