Hi guys,

I'm the one who recently replaced my 99 V10 with a new Motorcraft engine. It now has close to 2000 miles and everything is going fine except for something weird with coolant temperature indications.

I've only seen this happen when towing. I tow a big enclosed trailer that weighs around 8000LBs loaded the way it is now and is very tall (about 10 feet from the ground to the top) so there is a lot of wind resistance as the wind hits the front of the trailer. Recently while towing the trailer, I accelerated pretty hard going up hill on a bridge. When I got to the top, the coolant gauge pegged hot. I shut er down and drifted to a safe spot on the side of the road. Everything seemed normal so I restarted and temperature was fine.

On the same trip approximately an hour later just cruising on the highway (still towing) I saw the temp guage peg hot. The first time I didn't see if it was a gradual move or it just jumped. This time I saw it jump from normal (about 1/3 up the gauge) to full hot. I shut the engine down and restarted it without stopping. The temperature was fine and it didn't do it again that trip or since....until today.

I was towing the trailer empty today in forth gear at around 55 MPH. Winds were very high so I was watching traffic and not the gauges. Going up a hill the truck stumbled so I immediately looked at the gauges and saw it was pegged hot again. Once again, I shut it down and restarted and it was fine. I believe the stumble was the ECU going into safe mode where it shuts down some of the coils and injectors like the manual says. My check engine light came on which I think is because of the safe mode.

Any thoughts? I don't think it is the sensor as it wouldn't reset when you cycle the key. With the recent engine replacement, I'm thinking maybe air in the cooling system causing an occassional bubble at the sensor. I was careful to fill the system without the thermostat so I could see the water flow up through the engine and try to get all the air out. Is there a good way to bleed it?

Maybe something electrical but what?

Thanks!

 

Interesting problem.

All I've ever done is disconnect the heater core hoses and leave the thermostat cover off, and fill until it comes out both (all three) places. I think my V10 actually has a rubber nipple on a small fitting where one of the heater hoses it - presumably to bleed it.

As to the actual false-overheat, I've heard of this before. But I don't remember if a restart would fix it. But a few people have had false overheats, and everyone had a bad sensor.

BUT - one particular person had a bad PCM causing the false overheat and going into "limp mode".

I'd check grounds, and then the wiring itself going to the sensor (might be pinched), and then maybe, just maybe, replace it ...

I've no idea what the PCM input looks like for the cylinder head temp, but electronically, it just could be a sensor problem and have it go away on a re-start.

 

Check the thermostat, could be sticking.

 

I don't think it is the thermostat. The thermostat is new and is the fail safe type. Also the temp change is instant. One second fine and the next full hot. Resetting the key resets the temp to normal.

I'm 99% sure it is electrical but I'm not sure what yet. I cleaned all the grounds when the engine was out but I'll make sure they are all good and tight. I guess my next step would be to replace the sensor.

Am I correct that the only sensor is the one in the top of the intake manifold? I know that some cars have separate sensors for the ECU and the gauge but I don't remember seeing that on the truck engine.

Thanks!

 

The cylinder head temp sensor is on the back of one of the heads - I think. That's the one that triggers "limp" mode. No idea if that's also the one the gauge uses. Guys?

 

Have you been able to establish if the coolant was really very hot causing the temp gage to peg?

My experience is these motors always vent fairly violently past the pressure cap into the reservoir and always is loud and noticable...you can usually smell a over temped motor also...

In the absence of these indications ofr "real" over temp... and in light of the recent engine swap...I serious suspect an electrical problem...

The 2002 and newer motors all use Cylindar Head Temp (CHT) as the feed to the PCM to drive the Dash Cluster "semi" idiot gage...Your's is a 99 model year vehicle and I suspect the water temp is a true analog water temp sensor in the water jacket and the signal drives both the PCM logic and the dash gage...

A water temp gage is a thermal resister that has high resistance when cold and decreases resistance as it heats up. If you take the temp sensor wire and touch it directly to ground bare metal the gage should peg high or HOT.... perhaps there is a section of this sensor wire that got damaged during the motor change and is chaffing on bare metal and giving the false HOT

A broken or disconnected wire will make the gage inoperable or LOW

Some of the things a NEW motor swap that usually are related to over heating are not indicated because you have been successfully driving the vehicle for 2000 miles.... trapped air and poor circulation could be the cause by 99% of the time this would have showed up during normal driving.... as would improper routing of all the different hoses from the motor swap....

Air bubble is possible, but I am leaning to an electrical issue unless we get more info

 

Fred, my 2001 service manual CD shows a seperate CHT and Coolant Temp sensor.

 

Thanks for the advice. At least you have given me something to troubleshoot. The potential ground scenario sounds like the most feasible. I will experiment with grounding the sensor and see what happens at the gauge and warning light.

I have not smelled, seen, or heard anything that would indicate and actual overheat condition. That and the fact that it is not a gradual rise but an instant peg of the gauge says to me that something electrical is an issue.

It may be purely coincidental but the only time this has occurred has been while towing. It happened again yesterday when I was coasting to a stop while towing so load wasn't a factor there. I have driven the truck a whole lot more not towing after the engine swap and haven't seen it once.

 

Art you are correct but... the 99 through late 2001 use the water temp signal to drive the gage and do a comparison of the CHT and water temp in the logic for the limp home mode and a CEL.... I am not sure if this is entirely true for a 99 MY truck...none the less I think electrical grounding is the issue here be it CHT or water temp sender


after late 01 they did away with the water temp as redundant part of their save money by eliminating systems process...same idiots removed the dual visors, door lights, inner fender shields, under hood shield, and the nice glove box light

 

Crap, now you're gonna make me go back and see if the temp gauge is actually connected to the PCM somewhere.

Aw crap, there it is, pin 45 on the PCM.

You GOT me...