Running hot FJ40

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Buffchief

SILVER Star
Joined
Aug 28, 2014
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45
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534
Location
East Texas
My 1980 fj40 seems to be running hot!!! Dash gauge shows hot so I replaced the thermostat with a new OEM from Toyota with no change. Added a under dash temp gauge wire into the temp sending unit with ZERO improvement!!! Took a reading with a infrared gun and the lower hose reads about 80°f and upper about 130°f.
I recently installed a new rebuilt head.

Any ideas??
Thank you

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Are you sure its running hot? Maybe you got a vacuum leak, that could make it run hot. Maybe they didn't get the new head cleaned out good so it has poor coolant flow. Head Gasket could be blocking ports. Crap got into the cooling jacket and is restricting the water pump inlet/outlet or the cores in the radiator. Big air bubble not allowing coolant flow
 
Are you sure its running hot? Maybe you got a vacuum leak, that could make it run hot. Maybe they didn't get the new head cleaned out good so it has poor coolant flow. Head Gasket could be blocking ports. Crap got into the cooling jacket and is restricting the water pump inlet/outlet or the cores in the radiator. Big air bubble not allowing coolant flow
I had the front end up on a 10° incline and a "bubbler" installed in the radiator cap and let it run till full. Heater valve front and rear open. It doesn't take long to heat up. It's not frozen (I'm in East Texas) Not 100% sure about water pump or radiator! Fan clutch seems to be good as well
 
Since you have replaced the head and head gasket, with a rebuilt head….and your issue started after the replacement…..
1st thought, is it the same as the original? There are several iterations of 2F heads. Then I would double check all my vacuum lines (as noted above).
If all looks good, I would use a combustion gas checker. On Amazon, they’re about $35. They use a chemical that changes color if combustion gas is present in the coolant.
It’s been many years , but I replaced a head on an old Chevy 6, ended up there was a hairline crack that everyone missed when it was rebuilt.
 
And another quick check….remove the sender and place it in a cup of boiling water…..see how accurate it actually is…..since you’re at or near sea level….should show approximately 212*, depending how fast you are between the microwave and the driveway 🤣🤣🤣🤣
 
I even checked the temp at the temp sending unit and around the head and it never showed higher than 170°
And another quick check….remove the sender and place it in a cup of boiling water…..see how accurate it actually is…..since you’re at or near sea level….should show approximately 212*, depending how fast you are between the microwave and the driveway 🤣🤣🤣🤣
I was actually curious if the sending unit can go bad! I didn't think
 
Every pound of radiator cap pressure raises the boiling point of the cooling system by three degrees. So, a 13-lbs radiator cap (stock) raises the boiling point by 39 degrees before it discharges into the expansion tank. However, a 50/50 coolant mix normally boils at 222 degrees in normal atmospheric pressure, so it is definitely not going to boil-over. The internet suggests that 220 is normal for a running motor.

I'm running a 2F without the smog pump. It is critical that the tension for the belt running the waterpump is correct, as the Owner's Manual tells me to check it in the event of overheating, but, because there is only one belt running the waterpump (smog pump delete), and it is at an obtuse angle on the 2F, it is probably more significant of a check. Maybe a radiator flush is in order for this engine? I've never done a flush, or chemical clean-out, to my system, but, I do a coolant changes, before they are due on the maintenance schedule. I don't think that your motor is running hot, but, I'd imagine that many of these 2Fs are subject to well or aquifer water that can clog radiators, or ruin seals with chlorine or chloramine, or the coolant is just old (two years). Use pre-mixed coolant, or use distilled water to fill or flush is my approach.

The infrared sensor seems difficult to interpret, at best.
 
Every pound of radiator cap pressure raises the boiling point of the cooling system by three degrees. So, a 13-lbs radiator cap (stock) raises the boiling point by 39 degrees before it discharges into the expansion tank. However, a 50/50 coolant mix normally boils at 222 degrees in normal atmospheric pressure, so it is definitely not going to boil-over. The internet suggests that 220 is normal for a running motor.

I'm running a 2F without the smog pump. It is critical that the tension for the belt running the waterpump is correct, as the Owner's Manual tells me to check it in the event of overheating, but, because there is only one belt running the waterpump (smog pump delete), and it is at an obtuse angle on the 2F, it is probably more significant of a check. Maybe a radiator flush is in order for this engine? I've never done a flush, or chemical clean-out, to my system, but, I do a coolant changes, before they are due on the maintenance schedule. I don't think that your motor is running hot, but, I'd imagine that many of these 2Fs are subject to well or aquifer water that can clog radiators, or ruin seals with chlorine or chloramine, or the coolant is just old (two years). Use pre-mixed coolant, or use distilled water to fill or flush is my approach.

The infrared sensor seems difficult to interpret, at best.
Thank you! The radiator is an unknown for sure! I bought it years ago on eBay and just now using it. It looked great but the insides the question . Can the temp sensor go bad?
 
Yes they can go bad. With todays lack of quality control they can be born and sold bad far too often.
 
Which temp sending unit did you use and where is it mounted to the engine?


When I installed my Dakota Digital dash I broke the adapter for the sending unit they sent so I used a sender that I had (same style). It was reading about 30 degrees low and one of the guys here on MUD caught it in a photo I had attached. It had been about 3 months between when I had installed it and when the engine was first started. I had forgotten about it.
My point is that the temperature sensor has to match the gauge or display as stated above.
 
Since you have replaced the head and head gasket, with a rebuilt head….and your issue started after the replacement…..
1st thought, is it the same as the original? There are several iterations of 2F heads. Then I would double check all my vacuum lines (as noted above).
If all looks good, I would use a combustion gas checker. On Amazon, they’re about $35. They use a chemical that changes color if combustion gas is present in the coolant.
It’s been many years , but I replaced a head on an old Chevy 6, ended up there was a hairline crack that everyone missed when it was rebuilt.
Actually it started with the first head cracking on the top dead center under rocker rail from overheating. Never figured out the reason. The engine came from a wrecked 1985 fj60 . Drove it one time about five years ago down a country road for a shake out and noticed some smoke. Noticed the gauge high then as well.
 
When I installed my Dakota Digital dash I broke the adapter for the sending unit they sent so I used a sender that I had (same style). It was reading about 30 degrees low and one of the guys here on MUD caught it in a photo I had attached. It had been about 3 months between when I had installed it and when the engine was first started. I had forgotten about it.
My point is that the temperature sensor has to match the gauge or display as stated above.
Engine from an 85 fj60, gauge is 1980 fj40
 
One more thing to check is your ignition timing, particularly the advance mechanism on the distributor. Retarded timing will make an engine run hot.

Not long after I bought mine, it started running quite hot and boiling over on the highway (on a trip to east Texas BTW). Turns out my stock original vacuum advance had rusted up and was not functioning, resulting in retarded timing at highway speed.
 
I didn’t read all your details, but I may have had a similar problem. I have a newly rebuilt FJ60 engine and head (Thanks FJ40Jim!). This was installed in my ‘76 FJ40 this fall ‘24. The FJ60 OE temp sender (top rear driver side cylinder head) was reused during the rebuild. FJ40 factory gauge was reading abnormally HOT and didn’t seem right. I ended up pulling the old sender from my original FJ40 head and swapped it onto the new FJ60 head. Works great. I never researched whether or not this is a sender/guage incompatibility from the 40 to the 60 series, but this may be the case. Whatever your setup is, try a different sender before you start digging too deep.
 
Fan clutch, or older style water pump? Also, I have a used water pump on the bench. Precipitate collects on the impeller, can that reduce efficiency?

I'm still thinking that 220F isn't that hot, based on a few web searches.

Oil viscosity changes a couple of things. Thick oil adds to friction, despite the lower levels of wear, and it transfers heat slower. That might not affect thermals in the head, but it reduces power output. But still, oil is an interesting topic to me; are you running 20W-50?

More coolant means better corrosion and freeze / boil-over protection. Water is better at heat-transfer than ethylene glycol coolant. 50/50-mix?
 
I didn’t read all your details, but I may have had a similar problem. I have a newly rebuilt FJ60 engine and head (Thanks FJ40Jim!). This was installed in my ‘76 FJ40 this fall ‘24. The FJ60 OE temp sender (top rear driver side cylinder head) was reused during the rebuild. FJ40 factory gauge was reading abnormally HOT and didn’t seem right. I ended up pulling the old sender from my original FJ40 head and swapped it onto the new FJ60 head. Works great. I never researched whether or not this is a sender/guage incompatibility from the 40 to the 60 series, but this may be the case. Whatever your setup is, try a different sender before you start digging too deep.
That's next. Thanks for that info. The gauge I bought for under dash came with a sensor to install in the head. I didn't swap it yet hoping it was just the 40s temp gauge.
 

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