What is causing my 40 to overheat? (1 Viewer)

[Turkleton]

New guy here - both to this forum and to FJ40's in general - so apologies in advance if this is a naive question. I recently inherited a 1976 FJ40, and I'm experiencing some inconsistent overheating issues that I'd love to get some guidance on. For reference, my temp gauge has two lines - one at the ~1/3 mark, and one at the ~2/3 mark (I'm guessing this sounds familiar to most of you). Anyway, since acquiring it, I've been making mostly short trips (<10 miles), and it was doing fine for the first few weeks. Then I took it on a longer drive (surface roads, averaging 40 mph), and I watched the temp gauge steadily climb until it hit the 2/3 line after about 20 miles, at which point I pulled over to let it cool down (coolant was spewing out of the overflow tank at that point). I decided to bring it to a mechanic that I trust about 30 miles away to have a look. Strangely, I didn't experience anything close to overheating on my drive to the mechanic (the needle hung out at about the halfway point for the entire drive), and my mechanic couldn't replicate the overheating issue during his test drives or extended idling either. Nevertheless, he recommended flushing the coolant and installing a new thermostat and radiator cap, which I had him do, hoping that would solve the problem. Unfortunately, on my drive home from the mechanic (same roads, same speeds, even the same weather), the engine overheated again, and I had to pull over twice to let it cool down on my drive home. To be clear, I've not let the temp gauge go much above the 2/3 line, for fear of causing damage to the engine.

Any thoughts on what might be causing this? The fact that the overheating seems to be inconsistent is what is particularly confusing to me. I don't know how long the vehicle was sitting before I acquired it, but it had a relatively recent sticker when I got it, and passed inspection without problem this spring, so my friend didn't let it fossilize. Based on my limited research, I'm starting to wonder if the radiator needs to be recored...however, if coolant is not moving efficiently through the radiator, I would expect the engine to overheat in a predictable and repeatable way, rather than seemingly randomly. But I don't know much about this stuff.

Thanks for any thoughts you folks may have.

 

[bhsdriller]

What kind of fan/condition does it have on it. Fan Shroud?

Wonder if he removed the block drain and did a complete flush

 

[1969FJ]

You need to do a complete flush as said. If it were me, I would completely go over the coolant system, radiator, thermostat and housing, most important, the block.

 

[Turkleton]

What kind of fan/condition does it have on it. Fan Shroud?

Wonder if he removed the block drain and did a complete flush

I believe it has a fan shroud. There's also a small supplementary fan (about the size of a table fan) blowing over the engine block, controlled with a dedicated switch on the dash. This fan seems to have little effect on the temp, though. I'll snap a few photos tomorrow.

I'm pretty sure he did a complete flush. He said there was some rust discoloration in the old coolant.

 

[Ranger Jeff]

That secondary fan tells me a previous owner had a problem that might be the same you’re experiencing. A proper 2F cooling system does not need that. Also, did mechanic know there’s a block drain that need to be opened to flush the system thoroughly. Also, on my ‘77, I have to burp the system of trapped air after full drain. This involves filling radiator, running engine until thermostat opens (radiator drops from full right away) then adding coolant again until full while circulation under way. Next thing I would check is the fan clutch (if it’s not engaging properly or consistently, it could explain the intermittent overheating.)

 

[Casey E]

That secondary fan tells me a previous owner had a problem that might be the same you’re experiencing. A proper 2F cooling system does not need that. Also, did mechanic know there’s a block drain that need to be opened to flush the system thoroughly. Also, on my ‘77, I have to burp the system of trapped air after full drain. This involves filling radiator, running engine until thermostat opens (radiator drops from full right away) then adding coolant again until full while circulation under way. Next thing I would check is the fan clutch (if it’s not engaging properly or consistently, it could explain the intermittent overheating.)

The secondary fan is stock and blows on the carb/intake to cool it.

 

[Ranger Jeff]

Just learned something

 

[reddingcruiser]

Did the mechanic install the o - ring on top the thermostat? Was there one originally? It's almost acting like there isn't a thermostat. Did he check for pitting and corrosion on the housing that can prevent the thermostat o-ring from working properly?

 

[1969FJ]

If your block has a lot of crud, it is not so easy to clear. There is a plug on the rear of block jacket, open it, see if it leaks out or not. My bet not.

 

[bhsdriller]

Block Drain and Drain Plug Removal

OK -- the drain plug was easy to get out but no fluid came out on my FJ40. Wanting to flush the system. Next step.... remove the Block Drain but appears to be a bit tight. 1.) Are the threads reverse or standard? Figure the drain plug was normal thread (lefty lucy and righty...

forum.ih8mud.com

 

[Turkleton]

Thanks to those who have replied. Just called my mechanic, and he did NOT drain the coolant from the block plug. Do you think that would explain the overheating issue? If there is crud in the block, wouldn't that cause more predictable, regular overheating, rather than intermittent overheating?

Also, I just want to check on something: is it normal for the temp to regularly reach the 2/3 mark on the gauge for these vehicles? The first time I hit that mark, I pulled over to let the engine cool and noticed coolant spewing out of the overflow tank, leading me to believe the engine was overheating and that I shouldn't let the temp go higher than the 2/3 mark. But on my drive home from the mechanic yesterday, I pulled over once again when I hit the 2/3 mark, only this time there was no coolant spewing out of the overflow tank. This difference might be explained by the new radiator cap that the mechanic installed, which could be helping to maintain better pressure. So now I'm wondering if the 2/3 mark is in fact within normal operating temp, and whether I should keep driving when I hit that mark and see if the temp levels out around there. Any thoughts?

 

[bhsdriller]

Mine has been operating around the first 1/3 mark, a little above when idling or below when driving

 

[erics]

I would check the actual engine temp with an IR thermometer so you know exactly what's going on - gauge could easily be lying to you.

 

[Ranger Jeff]

Mine also runs around 1st 1/3 mark unless rock crawling on a 100 deg day, then it might make it as far as midway. If mine hits 2/3, it’s boiling over.

That said, I only trust the gauge enough to tell me to check something as I don’t trust its’ accuracy related to actual “numbers”. I second the suggestion to get some real numbers related to your gauge for reference.

 

[reddingcruiser]

Buy or borrow an infrared heat gun to see how temps are dispersed around the block. This should give you an idea of how coolant is moving around and if there is a blockage or sediment build up or areas not cooling properly.

 

[1969FJ]

Mine is always 1/3 and never higher. I agree, get a Harbor Freight IR and see what is happening.

 

[Turkleton]

OK, based on the advice of @erics, @Ranger Jeff, @reddingcruiser and @1969FJ, I borrowed a thermal camera from work to see what was going on. I drove for a couple miles until the thermostat gauge was a little over half-way (closer to the 2/3 mark than the 1/3 mark), then took a few shots of the engine.

Passenger side:

Driver's side:

Grill/radiator:

Clearly coolant is not moving through large sections of the radiator. I let the engine idle for a little while, and noticed that the temp dropped a bit - settling around the 1/3 mark on the gauge. I then snapped another image of the radiator, and sure enough, coolant had spread into more regions of the radiator:

This probably explains why I was getting intermittent overheating - the temp is higher when less coolant is moving though the radiator. Though I'm still not sure what causes more vs. less coolant to enter the radiator at any given time.

So does this confirm that I'm looking at a radiator rebuild or recore at this point?

 

[erics]

way to double down on technology with the cool toy!! Why not just grab a FLIR camera vs a $30 cheapie IR temp gun?

I'll let others advise on radiator coolant flow but your assumptions seems likely to me... I'd still want to know actual temp the coolant is reaching (and coresponding gauge levels) - I assume the temp in the upper left of the image is the reported temp at the "crosshairs"? If so then your head was running at 79.9C = 176F. If coolant is there as well then certainly well under 190-200 normal operating range.

excellent pics - would be cool to see same of known normal operation!! I'm happy to take/post if you will loan me the camera

 

[Pighead]

Maybe just have the radiator shop "rod" out your core, see if that helps, before you spring for a new core.

 

[Casey E]

Can you get a pic for us of what fan you’re running and does it have a fan clutch ? I think it doesn’t hurt to base line your radiator and whole water system. All of this is money well spent and will keep you of the tow trucks bed.