Truck Overheating

[Ikeece]

Hello, I’ve got an 82 scottsdale with the 305 that’s currently experiencing an intermittent overheating issue. It commonly happens when at idle or at low speeds but sometimes it’s just random like for example, i was making a 45 min trip which was mostly downhill sloped freeway and holding about 70mph, and a little over half way there my temperature gauge started to creep up to 5/8ths (mind you it had been holding steady at 1/2 for quite a while no issue). So then i think no problem i’ll just let off the throttle on this big downhill portion and let the momentum carry me and the wind cool down the engine. but instead it keeps rising up and by the time it got past 3/4ths i ended up pulling over and waiting on it to cool down. once it did i ended up just turning around and came straight home and on the way back it held 5/8ths the whole way. Once home i let it sit for a minute and came back out, started it up and sat at idle for about 10 minutes and it didn’t read a bit over 1/2

What’s confusing me is the randomness of it, For example sometimes i can sit at idle fine, and other times it starts to overheat. Sometimes it’ll run cooler on the highway sometimes it’ll overheat on the highway. Sometimes the heater on full blast will bring the temp back down and sometimes it won’t.

Things i’ve already done:
replaced the temp sender,
replaced the temp gauge,
replaced the thermostat (195 degree),
bought a new tube connecting from the air breather to the grill to intake cool air (old one was missing),
burped the system & topped off coolant,
and put some coolant in the reservoir.

I know it’s actually overheating because if it sits high for too long it begins to smoke from under the hood a little bit & the hood gets so hot you could probably fry eggs on it Lol.

The radiator seems to be relatively clean inside and out (fluids bright green & fins don’t appear to be gummed up)

the fan & relating components are all original though but from what i’m able to tell, the fan feels like it still moves air around pretty good.

Any advice on where to go next?

 

[Edelbrock]

1) Check H2O Pump belt tension.
2) Pull radiator cap and check for good coolant flow at "above idle RPM".
3) Check fuel ratio (running lean?)
4) Pull the thermostat out just to confirm that this COMPLETELY solves the problem and runs cold.
5) Reinstall thermostat, and continue trouble shooting.
6) Check to make sure that your engine fan is actually spinning in the correct direction.
7) Check your radiator fan clutch to make sure its not defective /wobbly / leaking oil.
8) Check for radiator blockage (leaves, dirt etc).
9) Try keeping the A/C off to see if that helps.
10) Install an electric pusher or puller fan, which is cheap and easy to do.

 

[fast 99]

While you suspect overheating is it actually doing so? Didn't mention pushing water out or knocking after shut off. Nearly every time an overheating motor will do both. Infrared heat guns are inexpensive today. Get actual temps not "the hood is hot "as a diagnostic conclusion.

 

[Sad Sack]

here is some good extra info:

https://knowhow.napaonline.com/radiator-hose-collapse-diagnose-fix/

 

[79SierraClassic]

Have you checked the fan clutch. Might be intermittantly free wheeling and doing no cooling.

 

[DoubleDingo]

I'd suspect, if it's original fan clutch then it needs to be replaced. Also, did you check the radiator fins to see if they're blocked by bugs and debris? It wouldn't hurt to collect the clean coolant, remove the radiator and clean it both inside and carefully pick everything from the fins, then replace the fan clutch, and then re-pour the coolant into the system, adding some to the overflow bottle, making sure it's topped off, and then properly burping the system, and then if it can idle for 30 minutes or more without overheating you are golden.

 

[Moodyalaskan]

Do normal things as mentioned above like coolant level, radiator exterior clean and belts tight. Did you come back from your trip and drive up the same hill? If so, did it overheat on that trip? Going downhill with loose belts and backing off the throttle lowers the rpm and the water pump doesn't turn as fast lowering the flow of the coolant. Was the prevalent wind from the rear, could also be a contributing factor. Has it over heated since? I was taught to increase flow by increasing rpms to see if the temp drops.

 

[CountKrunk]

Be a good idea to drain out the coolant and flush the system while youre at this.

 

[Ikeece]

While you suspect overheating is it actually doing so? Didn't mention pushing water out or knocking after shut off. Nearly every time an overheating motor will do both. Infrared heat guns are inexpensive today. Get actual temps not "the hood is hot "as a diagnostic conclusion.

Typically when it’s overheating and i go to shut it off, it sputters for about five seconds or so after i’ve shut it off. I’m pretty sure it actually overheating because of what i just mentioned and the smoking once the gauge reads much past 3/4ths

 

[Ikeece]

1) Check H2O Pump belt tension.
2) Pull radiator cap and check for good coolant flow at "above idle RPM".
3) Check fuel ratio (running lean?)
4) Pull the thermostat out just to confirm that this COMPLETELY solves the problem and runs cold.
5) Reinstall thermostat, and continue trouble shooting.
6) Check to make sure that your engine fan is actually spinning in the correct direction.
7) Check your radiator fan clutch to make sure its not defective /wobbly / leaking oil.
8) Check for radiator blockage (leaves, dirt etc).
9) Try keeping the A/C off to see if that helps.
10) Install an electric pusher or puller fan, which is cheap and easy to do.

I’ll run through this on my next few off days.

 

[Hunter79764]

Did it do this before the new thermostat? Did you do all these things at the same time, and is there any difference in how it acts before and after?

Sticky thermostat was my immediate thought, and I've had new ones that were sticky too, so just because it's new doesn't mean it's perfect, but two in a row doing the same thing isn't common.

 

[fast 99]

Typically when it’s overheating and i go to shut it off, it sputters for about five seconds or so after i’ve shut it off. I’m pretty sure it actually overheating because of what i just mentioned and the smoking once the gauge reads much past 3/4ths

Yes a hot motor can do that. Called run on and can be very damaging. There are many other causes as well, low octane, high idle speed, if equipped idle solenoid adjusted incorrectly, vacuum leak, just to name a few possibilities. Until the problem is fixed shut the engine off in gear [auto] or let the clutch out to stop the engine.

Quality of replacement thermostats has been questionable for years. Actually many parts. Besides the cost, that's one reason it's not a good idea to shot-gun repairs. A better procedure is actual diagnosis. Try to find an AC Delco, GM, or Delphi stat.

 

[xm20k]

195 seems a bit high I run a 180 in all my stock small blocks and 160 in my race engine

Some thermostats will also open slow a 180 for instance will not fully open till say 190-200 and then rapidly cool down to 180 once fully open

 

[Edelbrock]

Thermostats eh?

Fun fact of the day:

Engine thermostats don't work the way that many think. I used to think that the spring that holds it closed relaxed when it got hot, which allowed the thermostat to open. Nope.

Turns out that the spring ALWAYS tries to close the thermostat and the spring is not really effected by temp.

There is basically a piston that presses against the spring, which overpowers it - forcing the thermostat to open.

What forces the piston? Wax.... There is a wax pellet under the piston, and the different types of waxes melt at different temps. When the wax melts, it expands - thus driving the piston up. When the wax cools and begins to harden, the wax shrinks in size and the spring helps force the wax back into a pellet shape as the thermostat closes.

Is this the way that absolutely all engine thermostats work? No.
Is this a precise description? No.
Is this an odd fact about thermostats? In my opinion - yes.

 

[edgephoto]

195 deg thermostat is fine. Going with a lower temp. to fix a problem is a band-aid. You need to diagnose the underlying condition.

As someone asked. Did this problem appear only after thermostat replacement or did you replace it try to fix it?

You could have an air pocket and that will give you this exact set of symptoms. Did the replacement thermostat have a "jiggle pin". This allows a tiny bit of coolant to bypass the closed thermostat. If no "jiggle pin" did you drill a 1/8" hole in it? If not, this makes it much harder to bleed all the air out, although it sounds like this has been happening a bunch and should have purged all the air by now.

The randomness is troubling. At speed the fan does nothing so if you are driving 30MPH+ and it overheats you can rule out fans.

My money is on the radiator. They get restricted, clogged and even get a coating inside that prevents heat transfer.

If it was me I would do a chemical flush on the system, then flush it with clear water. The citric acid based flushes work very well. After this I would replace the water pump. I would take out my thermostat and test it on the stove. Put it in water on the stove, have a thermometer in the water, watch what temp it is when it opens. If it is at spec. then reuse it. If not replace it with a good quality. Lastly, I would replace the fan clutch. There is no real good test for these. I have seen many that seem good cause issues.

When filling the cooling system use a vacuum filler. They work great and will help prevent air pockets.

If you have the original brass/copper radiator I would consider having a radiator shop clean and check it. If it is plastic and aluminum replace it. They are disposable.