I just bought a 2002 f250 with a 7.3 a couple days ago and I’ve noticed that the coolant temperature gauge registers very cold, if at all. Even after driving for a while it doesn’t get up to operating temperature, usually either completely cold or maybe an 8th of the gauge from the bottom. Oil temperature and transmission temperature both look healthy and the truck runs and drives perfectly. Is this a bad sensor or gauge, or something else?

 

It's probably the thermostat that went bad and is stuck open, but it could also be a fan clutch that has gone crappola and is spinning at full engine speed all the time. Usually you'd hear some clues that was going on since moving that much air makes a lot of noise, but another thing to check is how much resistance the fan blades give while the engine is off. You should be able to turn the fan but with a heavy-ish viscous resistance. That one is kind of hard without a couple other fan clutches that are operating normally as a reference but it's another clue. Also worth mentioning is that these engines tend to overcool themselves under light load and can be hard to get hot especially this time of year. You can run one with no fan at all in the middle of summer and it wont overheat...the rad by itself is efficient enough assuming its not clogged or obstructed. I would put a new thermostat in and also get one of those aluminum housings and new bolts, and hope the old bolts don't break off in the water pump.

 

A first step for you is verifying whether the gauge is correct or not. Does your heater blow hot air? If so, then most likely you have a gauge problem. Another thing to check is get an infrared thermometer and check the temp on the thermostat housing. You may just have a malfunctioning gauge. If you verify that the gauge is operating correctly, then we can start working through potential issues.

 

How does the heat feel? If the gauge is telling the truth you probably don't have very hot heat. I would guess the thermostat as well and replace it asap as cold engines wear faster. I had a 6.9 beater that ran too cold for years, should have fixed it but instead just used cardboard in front of the radiator in the winter to get heat. That engine ended up burning oil although I can't say for sure if it was excessive wear or valve guides which had a habit of wearing on those engines.

 

Bearing wear goes up on motors that cant stay warm enough, especially in cold climates. I picked up on that myself through oil analysis of cold weather oil runs vs warm weather oil runs. The warmer weather intervals always report drastically lower lead ppm. I believe thinner weight oils help in winter but this just seems to be the unavoidable nature of it. (Not to stray too far from the topic). I forgot the heat from the HVAC vents is also a good diagnostic clue.

 

The heat seems to blow hot as normal. I had to drive to town for about five minutes and checked the coolant reservoir and it was maybe a quart low. I don’t think that would be enough to throw the gauge off but I’ll fill it to see if it makes a difference. I’ll try to get a thermometer reading when I get home but from what I can tell it is getting hot

 

Welcome. You would also benefit in more than this to check out the technical section in the stickys at the top of the forum.

it would benefit you a TON to get a free app on your phone and a Bluetooth dongle to plug in your truck.
FORScan lite and a quality dongle will also tell you values and give you LOADS of other diagnostic and monitoring capabilities.

this is the one I and a lot of others have I think. Man these have gone up… hasn’t everything.. 😣



https://www.amazon.com/OBDLink-Bluetooth-Professional-Grade-Diagnostic-Performance/dp/B07JFRFJG6/ref=mp_s_a_1_3?adgrpid=87149155149&gclid=Cj0KCQiAw 9qOBhC-ARIsAG-rdn7-YJ6ppvMskOZMyKXpChyyVjB7Olx_oqE_GndDDqzm1dLrR-U2aXkaAl3CEALw_wcB&hvadid=408680286899&hvdev=m&hvl ocphy=9025610&hvnetw=g&hvqmt=e&hvrand=160734220015 38473255&hvtargid=kwd-904679985767&hydadcr=25609_9900498&keywords=obdlin k+mx%2B+obd2&qid=1641525264&sr=8-3

 

Thanks, I’ll look into that. I put coolant in it and drove it around trying to get it to warm up. I was able to get it to warm up to just under a quarter of the gauge and it started to bounce a little bit which if I’m not wrong means that the thermostat is opening prematurely. I’ll replace the thermostat and see what happens tomorrow.

 

A stuck open thermostat (under normal driving conditions) only delays the engine coming up to temperature as the coolant is constantly flowing. A thermostat is designed to close when cold, as everybody knows, to speed up the heating of engine components and when open, acts as a restrictor to slow flow through the radiator enough to allow heat transfer to the radiator for air flow to remove the heat. That is why running without a thermostat does not fix an overheating issue, it usually compounds it. Like said, if your heater is working fine I would be checking temp with something as the factory gauges are not known for accuracy.

 

If it turns out you need the temp bulb then make sure you get motorcraft. Hears a link to a thread with some info as to why

https://www.ford-trucks.com/forums/1...ct-sensor.html

 

The problem with that is the cooling system in these trucks is so oversized that (A) it will take forever to climb to temp or (B) may never reach full operating temperature when operating with a small or no load. Also can't necessarily tell much by how the heater is working. I doubt you could really tell the difference between air heated by 160 degree or 195 degree water, and if you aren't in an area that goes into the single digits much you probably don't spend much time with the heater running full.blast anyway.

I can say that I'm about 95% certain the the t-stat in my truck is stuck open. The heater works good. It never overheats, but it also doesn't seem like it gets to full operating temperature. Not going off just the dash gauge. I've also run data logs. The engine oil temp never gets up to full operating temperature, it should run at same or slightly higher temp than coolant temp. Highest oil temp I recall seeing was in the 180s, thermostat should have the coolant temps in the 190s.

I've been driving like that (not daily) for 6 months and no adverse affects yet. Don't know how long it has gone before I got the truck. Hope to fix in the next few.

 

Thanks, that’s all good help. I couldn’t find an infrared thermometer so I don’t know the exact temp, but I drove it for about an hour trip today and driving it pretty hard I was just about able to get it to operating temp, but when I slowed down at the stoplights in town it cooled down again. I’m starting to think that the thermostat that is in it is too cold. According to parts websites I need a 195 degree thermostat but the forums say 203. Which one is best for it if I’m not pushing it hard very often and in below 50f weather most of the time?

 

https://www.ford-trucks.com/forums/1...-opinions.html


For the can of worms you opened...the can I opened a few months ago!

 

My truck did this exact same thing when I first bought it. Wouldn't come off of the 2nd line. Eighth of the gauge was about all I could get. Once in a blue moon I would get it to come slightly above an eighth and then it would drop back down again when I stoped. What it ended up being was my thermostat seal was sitting crooked after being blown through. Which, as you probably guessed by now, means constant water flow. After I replaced the thermostat I had no issues and the truck sounded and felt so much better. It felt like it came to life instead of being tired and sluggish.

 

I live in a very hot climate through most of the year so I put the 195 thermostat in. If it was somewhere that got below double digits I would probably have put the 203. Personally I don't think the 203 is all the hype. But I don't want to get involved in that can of worms. It's really hot where I live so I put 195. Just my two cents!