Spontaneous Overheating

[Intentional Wanderer]

Today was the "shake-down cruise" for the newly installed rack and lights (I will show it in the build thread).

I took the Disco up Box Canyon Road from Continental to Sonita. While slogging up the heaviliy washboarded road, on a moderate grade, the engine suddenly - over the space of about 30-45 seconds - went from normal temperature to overheating. I saw a plume of steam coming from under the hood, so I quickly shut the engine off and popped the hood.

Upon closer inspection, the steam was coming from the overflow tank and there were no broken hoses (my main concern when I am 20 miles from the rest of the world), so I cranked the engine back up, turned on the heater and let it cycle the water back into the radiator. Within less than a minute (20 seconds max) the temp gauge was back down to the normal. I turned the heater off, continued up the mountain and out to the pavement... and back home.. with no further issues.

Now, the kicker is that I did have the air conditioner on... but it really should not be so sensitive that I cannot run the a/c while moving along at 20 mph on a slight to moderate grade at 75 degree ambient temperature.

The one thing that I do not know is whether the electric fans were running. When I initally stopped the engine, I turned the switch off. When I checked the hoses and started the engine back, I did notice the fans turning.

Soooo... thoughts? My first thougts are something pointing to a fan... or a fan relay... going internittent due to the bumpy, rough road. But ?????

[muskyman]

well the reason the temp changed so fast is that the gauge in the DII's only has 4 positions. One for cold one for warming up, one for normal and one for overheating.

When it goes from normal to over heating its like click done! Then as it cools it drops from full temp back to normal in one click.

Now back to the condition.

The first thing I would be doing is replacing the thermostat (funky piece that it is) and flushing the system and replacing the coolant. Then make sure to fully bleed the system to make sure it dosent have any issues.

running with a fully loaded rack hard up a grade can heat up one of these trucks but it should still not over heat.

[R_Lefebvre]

David has a D1.

[muskyman]

David has a D1.

well then its a easier answer...

radiator is plugged up...I would bet on it.

[Martyn]

well then its a easier answer...

radiator is plugged up...I would bet on it.

No opinion on the subject, just interested that you mentioned a plugged radiator. If it's plugged how do you explain the quick return to normal temperature?

Did the radiator become unplugged and the fluid circulate normally again??

[muskyman]

No opinion on the subject, just interested that you mentioned a plugged radiator. If it's plugged how do you explain the quick return to normal temperature?

Did the radiator become unplugged and the fluid circulate normally again??

D1's are notorious for having plugged up radiators and they act exactly as he described.

they will cool just fine until you get past the tipping point and then they heat up instantly to the brink of death for these motors. once the load is removed because they are aluminum blocked motors they cool right back down just as fast fast as they heated up.

this is beyond common on the D1's

if he were to pop the tanks off the radiator the core will be all scaled up and flowing really poorly.

The best fix is a new 5 row radiator core installed into the stock tanks, this upgrade will really fix the issue for a long time and add the extra capacity that we demand out of these trucks when we start bolting all the extra weight on them that we all do.

[Michael Slade]

Do all the D1's have the newer clear/yellow reservoir tank? I know with the RRC's the old style black ones were notoriously bad for splitting at the seams and causing overheating. If it were me I'd check out the whole system, rad, t-stat, hoses, clamps, reservoir, reservoir cap...everything.

[Intentional Wanderer]

One other solution... is the radiator cap...

In troubleshooting these things, I like to go from simplest to the most complex. If the cap is bad, then the presssure can immediately rise... based on loosing its seal... which it could going down the bumpy road.

... as soon as I stopped, it reseated, the pressure went up.... and the temperature drops.

I ruled out thermostat because a. thermostat problems usually don't happen that fast... and even when/if they do then b. they don't fix themselves that quickly, if at all.

The other problem is that this vehicle hasn't ever experienced this problem before... so knowing if I "fixed" it... may be problematic...

The fan relay and radiator replumb are things I will fix in time anyway... so... those will eventually get ruled out even if they are not required to "fix" this problem.

[crusader]

If the cap is bad, then the presssure can immediately rise...
... as soon as I stopped, it reseated, the pressure went up.... and the temperature drops.

Hmm...Perhaps my understanding is off...are you saying that you may have a defective cap that stays loose, keeping the system de-pressurized, but suddenly seats and allows the system to pressurize (like it should be all the time) when you go over bumps?

Frankly, that seems like a bit of a long shot to me, but radiator caps are cheap...

I don't think pressurizing the system should cool off the system as you describe. It just increases the boiling point of the coolant, so that it'll stay in liquid form at higher temperatures. So...I don't think a sudden pressurization of the cooling system would cause your sudden drop in temperature. Could be wrong though....

[Intentional Wanderer]

...just saying that the cap HAS been a factor in my mis-adventures in heating in the past... I think it suddenly lost seal... overheated... and regained its seal when I stopped...

Okay, so I am not a physicist....but... I do have the internet...

Pressure Cap
*The radiator cap actually increases the boiling point of your coolant by about 45 F (25 C). How does this simple cap do this? The same way a pressure cooker increases the boiling temperature of water. The cap is actually a pressure release valve, and on cars it is usually set to 15 psi. The boiling point of water increases when the water is placed under pressure.

When the fluid in the cooling system heats up, it expands, causing the pressure to build up. The cap is the only place where this pressure can escape, so the setting of the spring on the cap determines the maximum pressure in the cooling system. When the pressure reaches 15 psi, the pressure pushes the valve open, allowing coolant to escape from the cooling system. This coolant flows through the overflow tube into the bottom of the overflow tank. This arrangement keeps air out of the system. When the radiator cools back down, a vacuum is created in the cooling system that pulls open another spring loaded valve, sucking water back in from the bottom of the overflow tank to replace the water that was expelled

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Pay attention to that last line... that is the crux of the problem... as I hypothesize... the steam almost immediately disappeared from the overflow tank... as soon as it sat still for a moment...

FYI... I do have a replacement tank...not the black plastic bomb variety... :-)

Oh, and the cap is about $10... so what can it hurt to buy a new one?