Overheating- all of a sudden
 

I have been driving my truck [69 GMC w/early 68 396, double electric fans with adjustable thermostat, shrounds] for a few months now, with some work breaks in between.
It has always run very well with no problems.
The fan most always comes on after engine shut down- as it should- to deal with the temp from stopping the water circulation.
Today when I got home from work steam started blowing under the hood. I hit the manual override for the fans, but it didn't help.
The fans come on with the override.
I let the truck cool for about three hours, refilled the radiator and started it up.
I kept watch on the fans- they didn't come on. The truck got hot enough to blow some steam again, so I am parking it until the weekend.
The fans still come on with the override.
The radiator is less than a year old, 4 core.

Is there a logical answer to this or are there two problems out of the blue?

In which order and how do I narrow this down to the problem?

Engine Thermostat?

Water Pump?

Fan thermostat?

Something else?

I did a search on overheating and didn't find anything like this- running fine, now not.

How do I check each item? I don't want to throw new parts at it until I get lucky...

Thanks,
.

Re: Overheating- all of a sudden
 

Did the temp drop the second time when you used the override?

Re: Overheating- all of a sudden
 

Wire around the themo switch and run it with the fans on. If it run cool, change switch.

Re: Overheating- all of a sudden
 

My first guess is thermostat, where is the sensor for the fans?? If the thermostat is stuck, the water will not circulate through the rad. Where is the steam coming from?? Just my first guess.

Re: Overheating- all of a sudden
 

You could get a cooling system pressure tester and follow the instructions to help detect a combustion leak into the coolant. Or, let it run with the cap off and look for bubbles in the coolant. Not saying it is a blown/leaking head gasket, but suggest ruling that out before you start guessing and blowing money on things that won't fix it.

Perform a pressure test on the cooling system to be sure that it is intact, i.e., no leaks....just to be sure.

Fan thermostat working? Should the fans come on while the engine is running....like periodically?

There is a logical reason for the trouble.....but I don't do well on logic problems. I'm a nuts/bolts/show-me kinda dude.

Re: Overheating- all of a sudden
 

I agree with above. I've had head gaskets leaking on several vehicles in the past and that is what happens. Hot gases get into the radiator causing havoc.

After checking the obvious; thermostat, radiator cap, fans not working, etc.,
also check the bottom return hose for collapsing under pressure. I had one do that and caused my vehicle to overheat.

Re: Overheating- all of a sudden
 

The steam is coming from the radiator overflow.

I will try turning on the fan with the override Saturday and just let it sit and idle and see what happens.

Thanks,
.

Re: Overheating- all of a sudden
 

did you check the front of the radiator to make sure its not clogged with ice or snow

Re: Overheating- all of a sudden
 

if the fans aren't coming on automaticly, but they are with the over ride switch, and the rad is getting hot, then the problem will be in the sensor for the fans, or the wires, or a relay.
Why do you have it wired where the fans come on after the engine is off? That makes no sence at all.

Re: Overheating- all of a sudden
 

The engine bay has no debris.

The fan is hot all the time to prevent overheating, like when you turn it off after driving. When you stop the water from circulating the temp climbs. Instead of blowing water through the relief valve it just continues to cool as required, until the water in the radiator is at a safe temp. I have noticed that the folks here on the forum are about 2-1 against the fan running after the motor is shut down. I don't mind- I like it the way it is :)
Will check back in over the weekend. It kills me to let it sit, even for a couple of days. This is my daily driver.
Thanks,

Re: Overheating- all of a sudden
 

I came across this part of an article on safety when working on a car engine- it mentions that most newer cars come with the full-time fans.

"Heed these warnings even when the car is not running.

Remember, modern cars have electric fans that can turn on at any time."

The link is here: http://www.troubleshooters.com/toverheat.htm

Re: Overheating- all of a sudden
 

I've been a mechanic all my life, and have never seen a single car wired like that. That is a generic warning that was probably required by a lawyer who doesn't know whats up.

Re: Overheating- all of a sudden
 

LH, I think you missed it - the fan comes on when the temp gets high enough - then, when you turn the car off, if the temp is still high (or rises), the fan stays on or starts until the car cools down. Ever walked past a car parked at the corner store - the cars off, the drivers inside but you can hear the fan running still? That's what he's talking about (and he has a manual override to beat the temp sensor). Hope that all makes sense.....

Re: Overheating- all of a sudden
 

many many of the modern vehicles for the past 15-20years are wired for constant hot circuit of the fans

Re: Overheating- all of a sudden
 

Well, I have never seen one, and I've been turning wrenches for 17 years. Granted, only the last 5 or so on cars and trucks, the rest on heavy equipment... and what ever junk is in my driveway or my 2 sisters driveways, or mom and dad's driveway.
From an engenearing point, I still don't see any advantage, and a few reasons not to do it. But, I guess it is a moot point, he wants it wired that way.
Back to the original question, The problem is still going to be in the fan circuit, on the automatic side.
The t-stat is fine. He said the coolant is coming from the overflow tube. That meens the coolant in the rad is hot. If the T-stat was bad sticking closed, then the hot coolant would stay in the engine, if the t-stat was stuck open, then it would probably not get up to temperature in january.
The fan motors are good. They come on when he hits the override.
There is no reason to suspect the water pump here from the description given.
No reason to suspect a coolant leak as the problem... but possibly a result of the overheating. Even so, there was no mention of water in the oil, or oil in the water. If you own, or have access to a pressure tester, I would go ahead and do it. If you don't have one, I would hold off on buying one unless there are more symptoms after the fans have been fixed.
A bottom hose would cause overheating on the open road (highway) not in the driveway when the RPMs (and water pumping suction) are at a minimum.
A clogged rad externally would cause major problems, mostly at speed. Lotsa RPMs + no airflow = overheating much quicker than once he pulls into the driveway.

As for the temperature climbing when you shut the engine off, that's the coolant temp in the engine... not in the rad. The coolant in the rad has no way of getting to the engine if the pump isn't pumping... when the engine is off. The coolant can be 300 degrees in the rad, and cause no harm to anything at all.
Once the engine is turned off, the engine, will warm up a bit upon shut down, be it an electric fan running, an electric fan not running, or a mechanical fan, since, as mentioned, there is no coolant flow.
But, I'm sure the battery companies like the idea of a high amp motor running with no alt running... as do the alt companies since you'd be working the alt harder to recharge the slightly weakend battery upon starting it.

Can anyone point out a car built in the last 15 to 20 years that is wired liek this?
I'm not trying to start an arguement... just trying to find out if this is a common thing that I have never seen.

Re: Overheating- all of a sudden
 

What about a stuck thermostat? Steam coming from the overflow tube says it's steaming inside the radiator, which says to me something like the thermostat being stuck. I've seen them stuck, engine is hot and radiator is not. If the fan thermostat is located to pick up the radiator temp before turning them on, and the radiator is cool enough to not trigger them, even though the engine is hot.....thermostat may be stuck. You do have a thermostat, right? Not having a thermostat will not let the coolant pass through the radiator slowly enough to cool the coolant. However, that being the case and the fact that your fans will not run automatically unless the coolant in the radiator is hot enough, still points to the thermostat. I've seen top hoses cool, bottom hoses hot in situations like this. The bottom hose on these radiators is on the same side as the side tank with the radiator cap. The steam, being a gas, would rise from that bottom hose, and pass out of the overflow tube. That act may be enough to keep from heating the radiator, yet not cool the engine.

Re: Overheating- all of a sudden
 

I'd go with pulling the thermostat.

Frank

Re: Overheating- all of a sudden
 

Friday update:

Yes, the truck has a thermostat :)

I honestly like that you guys are assuming I know nothing about engines- one of you might ask about something I have never heard of :) Please continue to do so. I appreciate the help and the suggestions.

I have a recirculating burp bottle, but it is in the garage. Because I am going to install that bottle there is currently no hose on the overflow fitting.

Picked up a thermostat, thermostat gasket, radiator cap and magnetic oil pan plug today. A pressure tester was a hundred bucks so I passed on that :)

Will install them tomorrow.

For tonight, I filled the radiator [the truck sat all night], and left the radiator cap off. Turned the manual override for the fan ON. Started the truck and smoked two cigarettes. The needle never moved.

Turned the heater, the defroster and the heater blower motor all on high. Smoked another Camel. No change.

Put the original cap on and tightened it down. Turned off the manual override for the fan. The needle climbed slowly to the mid-range on the gauge [normal position for this truck] and stayed there.

Got out to check if the thermostat had triggered the fans [mostly deaf, here]. The fans were NOT running.

The overflow on the radiator, however, was puking water like crazy. The water coming out was nowhere NEAR boiling.

The radiator was not hot. It was not cold, but certainly not hot enough to be spewing water.

Maybe the fan's thermostat is fine, and just the radiator never got to a temp to activate it?

Read a lot of info on the net last night and learned that way over half of the cooling problems are caused by the water thermostat and the radiator cap. So for $16 I am going to install them.

Any other ideas about what [and how] to check?

Thanks,

Re: Overheating- all of a sudden
 

hondas in the 1990s were that way..... saw it with my own two eyes on more than one car...... My 2005 honda dosnt do it

Re: Overheating- all of a sudden
 

It is not unusual for the car fan to run after you turn the car off.
http://www.hondacarforum.com/acura-t...ing-noise.html

95 Honda Accord cooling fan runs after car has been cut off for hours.
http://answers.yahoo.com/question/in...8213155AA89rAF

93 Cadillac Coupe DeVille fan motor runs after car is turned off
http://en.allexperts.com/q/Cadillac-...pe-DeVille.htm

Re: Overheating- all of a sudden
 

you can check the thermo sensor with an ohm meter and a heat gun or a pot of boiling water

Re: Overheating- all of a sudden
 

First thing I will check tomorrow. Thanks,

Re: Overheating- all of a sudden
 

If you look at those links, the first one says it is normal on the acura, the second one everyone tells him to go get it fixed, and the third one they are talking about the heater blower motor.
We hardly get any acuras, but the hondas and a hand full of caddys that come in aren't wired full time hot.

Re: Overheating- all of a sudden
 

Re: Overheating- all of a sudden
 

It's a common thing on newer vehicles.
After the engine is shut off, you will get some coolant flow due to thermosyphoning. Just after the engine is shut off, the water temp. in the block will go up & the hot water will flow into the radiator & be sucked back out the bottom of the radiator. That's what causes the fan in a parked car/truck to cycle on & off w/ the engine shut off.
The volume of coolant flowing is certainly not even close to that of the water pump running, though.
Crap. I can't believe it. All them years of working in a refinery finally has some practical application.

If I stepped on anybody's toes, I'll apologize ahead of time. That certainly wasn't my intention.