I've seen a lot of threads on here with people having similar issues. My temp gauge one day showed about 3/4 then the following day red lined. Pulled off to allow it to cool down several times water not boiling, no steam, doesn't gush coolant out of the reservoirwhen you loosen the cap (no real physical sign of overtemp) IR thermometer shows upper rad hose 180 degrees. Overheat only occurs at a steady road speed. As soon as you slow down or stop it drops right back to normal.

Truck has damn near 200,000 miles, no coolant leaks that I can detect, holds some pressure when hot, idk exactly how many lbs BUT consistently holds some pressure anytime it's hot. There is no oil in the coolant nor coolant in the oil, there is no evidence of steam from the exhaust nor evidence of exhaust in the coolant. I run straight yellow AF w no water mixed. Fan clutch cycles as it should. I keep the AC condenser washed out. The lower rad hose is in good condition and at least in the driveway at high rpm it's not sucking flat. There are no DTC's

1st things 1st, change the thermostat, (192 deg napa). Appears to open ok, after truck gets warm upper rad hose gets hot. still overheats.
Ok next change the water pump, flush both the radiator and the engine block with the garden hose, water flows good w no restrictions, still overheats.
Hmmm, change the head temp sensor (for those needing to find this lay under the truck on the drivers side, just in front of the bell housing on the back of the head. It's right under a freeze plug & maybe 3" above the exhaust pipe. ALSO this is a dry sensor. Don't drain your coolant before pulling this sensor.) replaced, seen some difference. With just the truck w the AC on it'll idle at 212 and run around 236. Tried pulling a trailer, went straight to red line. Temp readings taken with a nexiq prolink iq, and readings from the ECM consistently match the dash gauge.
So what's left? Next I change the radiator (for those wanting to do this Ford has tied the oil cooler lines in with a quick connect fitting. A standard disconnect tool will not work. They're too long so get out the grinder & shorten it only to realize it's too thick. These tools are available thru Amazon & eBay but no parts store has them, no ford dealer sells them ((OTC 6593, OTC 307-569, Lisle 39960)) don't waste your time or $ on the napa SER 3893. It will not fit. Changed the fittings. Same result on temp.

I'm not sure where to go next, possibly an issue caused by a fault in the ECM, or an issue with the exhaust causing an abnormally high ambient temp at road speed that's affecting a nearby DRY head temp sensor. I appreciate any input, I've got to have this truck right for work so I'll be scheduling a pressure test and getting it to a muffler shop to check for exh leaks & cat inspect next week.

 

Replaced the fan clutch today just for good measure. Still showing overtemp.

 

Thinking I should have done this on a different forum lol. Got it to a muffler shop, they did find a small exhaust leak toward the rear of the truck, should have no effect. Also the cat they said is within normal temp. According to them should the cat get plugged up it'll misfire & run like crap before it causes an overheating back pressure issue. The guy at the muffler shop recommended I look at the ECM next.

 

I recommend taking it to a Ford shop. They will find exactly what is going on with it in no time. Saving you money and anguish.

 

I read, and 're read your post- you stated that you run straight yellow antifreeze with no water..... do you mean the 50/50 premix or the full strength uncut coolant? If it is the latter, that may be the root of your problem. Coolant needs the water component to dissipate the engine heat. Have you run the truck with straight water to test what it will do? Water alone is the best heat transfer/dissipating media, but won't work due to long term rust formation or naturally freeze up in cold climates. Straight uncut coolant can't dissipate the heat quick enough. I hope this helps, and if I mis read what you meant, my apologies, good luck. P.S. only use tap water for testing, not mixing with coolant, use distilled water( $ 1.00 a gallon) to prevent mineral contamination formation in the cooling system.

 

I appreciate your reply Mark K, you're correct I was running full strength uncut antifreeze. I ended up giving up on this truck & traded it on a GMC Sierra lol which really sucked because I had a ton of lights, corner strobes, diesel transfer tank to switch over, not to mention a bunch of work on the cooling system just thrown down the drain.

 

I know this is an older thread and you no longer have the truck. But I'm curious, why were you running straight antifreeze instead of mixing it 50/50 with distilled water? That could have been the problem.

 

Right. That's what I had on hand so that's what I put in it. If the 4.2 is so finicky that it'll overheat by having the wrong water / coolant mixture it's not the motor for me! I really liked the older ford motors, 5.0 & 5.8 were always kinda bulletproof for me, as is the 5.7 Vortec I run now. It doesn't care what sort of coolant / mixture you put in it as long as it's wet it'll pull a heavy load & never overtemp.

 

It's not just Ford engines - 100% ethylene glycol MUST be mixed with water in any engine. The heat transfer properties of 100% are very poor. That's why 50/50 is recommended. Not only that, the freeze temperature curve starts coming back up after about 70% - 50/50 freezes at -34F, 70/30 freezes at about -80F, 100/0 freezes at +20F. The only antifreeze that can be run at 100% is propylene glycol, and it freezes at about -70F.

 

I just don't understand, with all the parts you threw at it, that you didn't use 50/50 water / coolant to make sure that wasn't the issue. There's a huge difference between 50/50 and concentrated coolant that's meant to be mixed with water.

As glc stated, it's not that the 4.2 is so finicky, they all need the correct mixture.

If all you had around the house was diesel, would you put that in a gas engine?

 

Of course not, that's retarded. I'm satisfied that the problem was head gasket or head or water jacket blockage related. It was a 4.2 with 200,000 miles it was worn out & certainly not one of fords finest marvels of engineering lol. Again I've had plenty of (water cooled) V8's that ran at normal temp regardless of weather they had full strength coolant, just water or a mixture thereof.

To put this thing to rest I do believe now that I think about it I test ran this using only tap water and had the same overtemp issues.

GLC good info! Thanks!

 

Mister, you need to get your head checked. Straight antifreeze?! You ruined your truck. Hopefully you at least learned a lesson from this and aren't still doing this practice. Damn, that was dumb! Especially for someone who seems like an above average DIYer at diagnosing and repairing.

 

Nice first post.........

That post you replied to is 2 years old, and straight antifreeze WILL NOT, I repeat, WILL NOT ruin an engine. It's just not ideal from a freeze point and heat transfer standpoint.

 

just a after thought and I can see it's a old post but did you ever pull the water pump I have seen them break the impeller or whatever it is called off and you can't tell unless you remove them they don't make any noises just stop cooling I think it's a common problem with them

 

I know this is an old thread and really just wanted to add a little trick to removing the cooling lines from the radiator. Instead of buying the quick disconnect tool, I used the plastic kids medicine syringe you get from any pharmacy and cut long ways then cut it to length and it popped them loose with no problem. Saved 24 bucks.