2003 7.3L Excursion, no mods other than Bullydog GuageTuner. Never had any real mechanical issues. Last month I was driving through Colorado and had some weird overheating issues that neither I, my friends nor the shop in the town could figure out.

Going through the mountains any time we hit about 10,000' it started to overheat. I first suspected that it was a stuck thermostat so we replaced that (twice)and it had no effect. Put the stock map back in the ECU and that had no effect. Replaced the fan clutch and it had no effect. Flushed entire cooling system and replaced coolant with no effect. Every time we started going up the mountain and got up high it would get hot. Ended up having to drive 1/2 mile at a time and stop for it to cool down to get over the mountain then it's fine on the way down.

To Summarize:

• Overheats only at altitude going uphill
• not the thermostat, fan clutch or ecu map
• No overheating issues at any other time near sea level, even when pulling a heavy trailer.

It drives and runs fine all the rest of the time but I want to address this prior to making the same trip next year. neither I nor anyone else I've talked to has a clue.



PLEASE, let there be someone on here who has had this issue and figured it out in the past.

 

Kinda unusual since these things rarely overheat. First thought given it is altitude related is the barometer died. Truck might then think he's at home at sea level and over fuels at altitude. At the time did it have lots of black smoke in the exhaust? Testing the baro requires reading the PID on something like AutoEnginuity ($$$), or the Torque app on an android phone with a BT Elm OBDII reader ($). The barometer is inside the PCM and non serviceable.

You have gauges like EGT, boost or trans? What did they show?

Another idea would be partially clogged radiator. Zap with a temp gun looking for cooler spots. Everything else I think of would have loss of power issues.

Welcome to FTE!

 

I vote radiator.

I think of was Pete that fought this all the way from RRE and by the time he hit Denver the only thing left to change was the Radiator, and why do you know that fixed it.

I had a partially clogged radiator that only showed up towing heavy above 6 or 7000 feet, slapped in a new NAPA radiator for around $200 and not an issue since.

 

I think the first question is how are we assuming its overheating... There is NO way for the factory PCM to read coolant temp, so one would have to have a aftermarket temp gauge to gauge if this was an issue.

 

Supercracker, did you measure oil temps?

I'm also thinking radiator, or a restriction of airflow in front of the radiator. At high altitudes, the drop in air density exposes cooling issues that would never be seen at lower ones. My off-road 4runner never overheats on a 100* day in Moab (4000ft), but will at 12000ft, even when driven easily, and with ambient temps in the 50s.

Mark

 

There is a troubleshooting process for high-altitude Overheating which is:

Water thermostat
Water pump
Radiator
Cooling fan
Pressure relief cap
Fan belt
Restricted airflow

The PCM does monitor the ECT Sensor Voltage. The cold PCM Strategy On Automatic Transmission Equipped Vehicles does not use it for the first 600 Seconds (10 Min.) of operation or until the engine oil temp reaches 130*F - 150*F. When the ECT reaches its pre determined temperature, the warm strategy is employed. Another function of the ECT is If the Transmission Fluid Temperature (TFT) Circuit encounters an issue with it's circuit, the O/D Light Flashes and the PCM uses the ECT Circuit to alter output based on temperature (cold/warm strategy) until the TFT is repaired.

 

A 7.3 PCM does not know the coolant temp (unless you have a manual truck)... Watch it on a scan gauge... or any scanner period.

 

AT and MT trucks are different. AT trucks have a real old-school coolant temp gauge on the dash. Not PCM related, that gets temp info from the oil temp and the IAT. As I understand it, MT trucks use coolant temp in the PCM, not oil temp, so the dash coolant temp comes from the PCM on a manual truck.

 

Thanks for setting me straight.

I will remove those pages I cut and pasted from in both my 99.5 Factory Workshop Service Manual and 99.5 Powertrain Control Workshop Manual.

 

Perhaps you posted gas engine equipped superduty information?

 

Thanks guys. It does have a WT gauge in the dash and I don't have an aftermarket one installed.........yet. We flushed the radiator and it didn't appear to be clogged or to have any cool spots where water flow would be restricted. Not saying it couldn't have been restricted but it seems like that would cause issues regardless of altitude.

I suspected the barometer, but I didn't notice any excessive smoke while it was overheating. Also, if it were going full or excessively rich wouldn't that work to cool it and cause some rough running? My only other theory was/is partially clogged injectors leading to a leaner mixture than intended. Injectors are on the menu for this truck any way.

Part of why this is extra frustrating is that I have no way of knowing whether or not I've fixed until I'm going back up the mountain next year.

 

The water temp gauge on the dash is nothing more then an idiot light. It has no connection to accual water temp at all...

What symptoms were you experiencing to lead you to think it was overheating?

Put a water temp gauge on it, even a cheap one for temp use...

 

Which PCM pin receives the ECT sensor signal on a truck with an automatic transmission? My pinout diagrams specify pin 37 gets the ECT signal only on trucks with a manual. On trucks with an automatic, pin 37 gets the TFT sensor signal, and ECT signal doesn't show going to any PCM pin.

 

If you have an automatic transmission the dash gauge is real.


Good news!

That's gasser thinking. Over fueling a diesel results in (some) more power, more black smoke and higher exhaust gas temps. More fuel = more power if you have the air to burn it. No such thing as clogged injectors with 20k psi blowing the fuel out.

By then you will discover the joys of modifying your rig and have PMS.

 

One unlikely possibility which I have not personally experienced, but if nothing else works--if the water pump has a worn or damaged impeller (poor coolant maintenance can cause an electrolytic loss of metal in the cooling system).

The reduced flow and pump generated pressure could theoretically allow an air bubble to reside in the coolant system. At higher altitude, this bubble of air would expand and create a more complete blockage of the coolant flow causing overheating.

I know this is a far-fetched idea, but desperate times sometimes call for crazy ideas. Waiting for this idea to be shot down!!!
Larry