Random Overheating Issues. Please Help!

2014 Toyota Tundra 5.7l 130k miles. Owned the truck since it has had 40k. I have had no issues related to this tell now. Here is the story:

About 5 weeks ago I was driving about 3 hours away from home, the temp outside was around 20 degrees. 1.5 hours in, I hear a beep on the dash, check engine light comes on, overheating symbol is on and warning messages comes up the truck is overheating. The coolant gauge is peg all the way to the right. No heat from the HVAC. I pull over, let the truck idle but the temp is not going down. I shut the truck down.

I check the engine code with a reader. Can't remember the code but it was something about ECT sensor low voltage. I clear the check engine code. I get out and pop the hood. Can't see anything noticeable. I turn the truck back on, no check engine light. The temp has went down, still above normal though. I get back on the highway, and in about 30 seconds the coolant gauge is pegged to the right again, overheating message, but no CEL. I pull over, shut the truck down, call some people. I go in the engine bay, the coolant hose below the thermostat (I assume it's downstream of the thermostat?) is cold, not a bit of heat. When the truck goes back down to the normal temp I start it and take off again.

The trucks temp stabilizes, I get heat from the HVAC. All seems good. But 20 minutes or so later, it all happens again, no heat, coolant gauge pegged to the right, overheating message, but no CEL, same hose is cold. At this point I was able to limp it to O'Reilly's, still overheating.

I change out the thermostat in the parking lot. I did NOT add any coolant. I felt like I didn't loose hardly any when changing the thermostat. I start the truck back up. Level in coolant reservoir barely changes, it's not full but it's not empty. I drive the truck around the block, it immediately overheats. All the same symptoms, no CEL. I then go by a new radiator cap and install it as I was reading some issues with those. It made no change. I then buy a new ECT sensor, install it. Truck then works fine. No overheating, hose mentioned is warm again, heat through the HVAC. Everything looks good. I proceed to drive home.

I drive 100 miles, no issues, then the truck overheats the same way as it has done before. Again no CEL. It never fixes itself. I limp it along for couple miles to make it home. Driving tell it got hot, pulling over and letting it cool.

The next day I start looking things over. The coolant reservoir was empty. I fill it up. I drive it around untill it gets up to temp. All looks good. I then drive it a couple days later and it overheats after a couple miles. I decided to drain all the coolant and fill it back up, using techniques to ensure there was no air trapped in the system. I thought maybe that was my issues, but how did I drive 100 miles with no issues??? I have no idea.

After doing this the truck was fine. I drove it hours away from the house no issues. That brings me to tonight. The last 2 weeks I've drove it everyday to work, 70 mile round trip. No issues. I take some back roads on the way home tonight, and a couple miles away from the house it over heats again in the same way. I checked the coolant reservoir and it did drop to between the halfway and full mark. So maybe I am losing coolant somewhere and it's causing an air lock? I can't see any coolant anywhere. And that doesn't explain how it ran for 100 miles after the oriellys visit...

Please someone help. I have no idea what's going wrong. None of it makes any sense. It's also like the thermostat isn't opening, but what would be causing that? How is it overheating so quickly even when the temps are blow freezing? It's never a gradual climb when it happens. It goes from normal to overheating it 20 seconds or so.

 

Did you replace with OEM parts? I've been bitten too many times by aftermarket. I would replace the ECT sensor with OEM (maybe Denso?, keep all parts you remove!). No coolant in oil? Did coolant get changed at recommended intervals (possible head gasket issue)? Check fan clutch, may be failing, but shouldn't matter on highway, doesn't make sense for short time of overheating. Check for debris in condenser and radiator, make sure air is being drawn in. If you have access to a scanner check/monitor coolant temp and tank. The way you described condition at first, I would have thought air pocket with a coolant leak somewhere and possibly the thermostat. If you have a temp gun check exhaust manifolds for any temp differences. Check for head gasket issue at radiator and also pressurize the system to check for leaks (tool rental?).

 

It was just whatever option o'riellys had at the time. The thing is when I installed the new ECT sensor everything worked. And even if it was bad, why would I have no heat in the pipe below the thermostat? Why would I have no heat in the cab? Should I take the thermostat back out and see if it's working? I've seen some people out then in boiling water?? Never done it before.

No coolant in the oil, I just changed it. The coolant did not get changed at the recommended interval. When I drained it I did not see anything in the coolant that would be concerning. Coolant temps on my scan gauge was showing that it was overheating. Don't have heat gun.

I also have a cylinder 5 misfire randomly come up at start up. It was very random, happening days or even weeks apart. This started happening not long before the overheating story. When the CEL comes on, sometimes the truck is running rough, sometimes it's not. When it is running rough, I shut it off and turn it back on and it runs fine. It's still happening, and actually happening this morning for the first time 2 weeks.

I've read some stuff on a coolant leak in the valley of the engine. Some say you may never see it if it's a small leak.

If we run with that theory things still don't add up. Let's say in the first trip I took a lost just enough coolant for an air lock.... But why did the overheating issues temporarily go away and show back up? Why was I able to drive 100 miles after losing MORE coolant (from the thermostat replacement)

Nothing adds up

I was going to replace the injectors for the misfire. I put new spark plugs just a couple thousand miles ago. And have swapped coils around to see if that was the issues, didn't fix it. While I have everything apart to do that maybe I'll uncover something....

 

I would remove the #5 spark plug (could also read the plug), pressurize the system and use a borescope to see if coolant enters in the cylinder. I doubt it's the injector or any injector problem.

 

Besides the passive rough running that you mentioned, were you having any hot-hot engine symptoms... knocking, boiling coolant etc?

Remove your thermostat entirely and troubleshoot from there. It sounds like you have an air pocket or a dead headed water pump. I say that because you are losing heat in the cab.

 

That's not a bad idea. Curious why it would be so random or intermittent though if there was coolant getting in the cylinder? Wouldn't that cause problems all the time? Guess you could say the same thing about the injectors though. Is 130k alot on original injectors?

No other issues are occuring besides the random misfire. And it's not directly correlating to the overheating issues. And yeah I agree I think that would be a good start. That's what doesn't make a lot of since as it's seems the thermostat is not opening. The other side is cold and no heat in the cab.

Also wouldn't the engine coolant be boiling over into the overflow if the actual coolant was as hot as the gauge was reading(maxed out)? It does seem like when this is happening coolant isn't on the sensor, like the air pocket is there. But what I also don't understand is if there was air in the system would it not get out of the system by means of the expansion tank?

Is there any other valve in the coolant system that could be closed/opened?

 

Alright so I am definitely losing coolant. I just can't see where, my bet is its the valley.

After the last overheating issue the coolant reservoir was down to maybe half way. I parked the truck, took it around the block the next day, then today the coolant reservoir was empty. I ended up having to use a half gallon or more coolant to fill the radiator and coolant reservoir. I'm gonna start by taking the intake off and see if the leak is in the valley and go from there.

 

If your truck is loosing coolant and you can't see leak, also starts in the morning with misfire for couple seconds and than clears up, you are having head gasket problem. (may not be the only coolant lose issue)
Really easy way to confirm that is to take spark plugs out and check with borescope your piston domes. If you find piston without carbon on it like clean top that is your problem cylinder. There are two ways head gasket will affect coolant level:
#1 is coolant going into combustion chamber/s and getting burned. ( small quantities will not be noticeable through color of exhaust gas but will cause short term misfire when started first thing in the morning)
#2 is compression from the engine is leaked into the cooling system, thus pushing coolant out through overflow bottle. You would think, this one should be obvious but while driving you will not see that, but this one will cause your heater core make gurgling sound and no heat.
I hope you don't have this issue but by my own experience and information you provided, ods are not in your favor.

 

You need to diagnose before throwing more parts. Find a service manual and understand exactly what's involved. I just bought a 2020 and haven't found the data however:

On any truck, there is the basic mechanical cooling system ie radiator, waterpump, thermostat etc. These have to function to cool the engine. On newer vehicles, there are sensors that supply information to various computers like ecu. The ecu then responds. Old school sensors just worked the gauges. We're past old school.

You have to make sure that the engine is actually overheating. This requires a thermometer. IR thermometers can be had for not much money and are handy for all sorts of testing. Make sure that the radiator is hot by measuring at different spots. Hot coolant goes in the top and comes out cooler at the bottom. Measure the temp at the sensor and thermostat. I would track down a thermostat from an OE manufacturer because they have better quality control that the cheap ones.

Now, look up the test procedure for whatever sensors are involved. Every manufacturer has a test procedure. Find and follow it.

As a matter of policy, use only OE manufacturer parts. They wll be the same part as the manufacturer with a different brand. I have an old Dodge Cummins. The Cummins thermostat works and NAPA gold work meaning the temp gauge stays rock steady (not sure on NAPA since it's been a while) but the others I've tried all let the temp vary by about 10*.

If you're going to do much wrenching, an oscilloscope it the shitz for finding electrical issues. Prices have come down and are around $120 for one (PicoScope) that connects to a laptop. Think of it as a fast volt meter that graphs. See mechanic mindset on youtube and also go to his site. The site owner is a wealth of knowledge.

Your miss isn't the spark plug unless it's a consistent miss. The oscilloscope will give a direction on that but you'll have to put in the effort to learn how to use it.

I haven't seen a good source for the factory service manual - especially not a free source.

 

That would definitely be a shame for only having 130k and that being nothing for these motors....

 

You can pressure test to find external leaks. Internal would likely show in the exhaust. Do the simple stuff first.

is your radiator clean? Pull the fan shroud and look. That’s a problem on some trucks.

 

Here is pics of the inside of cylinder 5.


Here is the spark plug for that cylinder. It's only got 8000 miles on it.

And here is the inside of cylinder 1 for comparison. It was almost at tdc so I didn't get the same view.


Thoughts??? Cylinder 5 looks cleaner, but it's not like cylinder 1 has a lot of carbon build up either. But seeing one side of piston 5 black and the other shiny may be telling me what I don't want to hear
.... I didn't see any coolant in cylinder 5. This was 3 hours after a 35 mile drive. I'm going to let it set all night and check it in the morning to see if any coolant seeps in there.

 

Compare it to a few other banks. That picture alone doesn’t look clean enough for a coolant wash imo

 

I'll scope a few more cylinders. I let the truck set all night and looked in cylinder 5 this morning. No change from the picture I posted. Looked exactly the same.

 

There isn’t anything major wrong with your engine. You have to test the basics. Understand all of the sensors involved and how they interact.

I haven’t seen 1 reference to one good source of service data for these trucks so we’re basically pissing in the wind as far as real diagnosis is concerned.

Don’t overheat your engine but don’t throw anymore parts at it either.

one of the dumbest things I’ve done is try to find out why a 20 amp fuse kept blowing on my starter circuit. It turns out that the soldered end of the battery cable was making intermittent connection. I’d test voltage on both ends and have full battery voltage. Pull and test the starter and solenoid and both were good.

I finally found it with a friend on one end looking at voltage and me on the other

 

You don't need to understand how sensors and related ECU wiring works to figure out you have coolant leak. I DO AGREE with you to fix something you should know how it is working to begin with!
Coolant lose is purely mechanical failure of breaching closed system, is it gasket (take a pick) or stuck closed thermostat ( causing overheating thus loss of coolant), water pump....

 

This started as an intermittent overheating problem. OP changed the thermostat and some sensor. He hasn’t to my knowledge pressure tested the system or made sure the radiator is clean or measured temps.

I just bought a 2020 and haven’t even picked it up or found the FSM so am just going by how every other vehicle I’ve owned over the last 50 years has worked.

OP has a dash gauge saying it overheated. He’s wandering in the forest pulling random spark plugs and taking pics without doing the basics.

My questions still stand: is the radiator clean on both sides? Is the water pump moving coolant? Are there any external leaks? Are the sensors functioning? Those need to be answered before expecting a cracked head or gasket on a Toyota 5.7 with 130,000 mi that hasn’t been dogged to death.

 

I think that cylinder 5 pic looks plenty clean to make you check further on the head gasket problem. Agree with pressurizing the system, letting it sit overnight and looking for coolant in cylinder 5.

 

I think the head gasket on this truck is toast at this point. Multiple overheats, the head gasket usually fails between cylinder 5 and 7 on the 3UR. Plus as OP stated, cylinder 5 is cleaner than the rest. Looks steam cleaned to me.

Also as a side note, for whatever reason the heater core on these trucks gets an air lock. Can be a bit tricky to bleed it out. Usually running till the thermostat opens and then shutting it off does the trick. Took me a bit to get it bled during my last coolant swap.

 

How about running a compression test on cylinder 5? And then comparing that to a couple other cylinders. I agree that the borescope photos don't scream cracked head or head gasket, but intermittent misfire on cold starts with unexplained coolant loss sure does. Oreillys will rent you a compression tester free of charge

 

Yep, I cringed when reading how many times it was overheated.

 

Same here, you can't do that on all aluminum engines. Iron block and iron heads, sure (to a point). Iron block and aluminum head, maybe. All aluminum, you're done bud.

 

It certainly may be but you don’t jump straight to that conclusion without finding out what caused the overheating in the first place.

We simply don’t have enough information. And a head gasket or cracked head wouldn’t be intermittent. Even if it would have to be driven for a while before it overheats it would be consistent.

 

What caused it to overheat is mostly irrelevant at this point. The engine has been overheated ~7 times per the original post.

To be clear, I'm not saying the head gasket was the original cause of the overheating event. However, the subsequent information provided points to head gasket damage. Even if the original issue is identified and fixed, the head gasket is likely compromised and the block and/or heads are now potentially out of spec.

 

I'm going to pressure test the cooling system tonight while I also looking into cylinder 5.

I also want to reiterate that I am driving the truck everyday 70+ miles with no issues besides the few instances in the original story. And the occasional misfire at start up. The last 2 days it has not misfired.

As far as some are concerned with the overheating. The sensor was reading that it was overheating but I believe this was from an air pocket, not the actually coolant getting too hot to cool down the engine. There was no smoke/steam/boiling. Now how large was this air pocket? And was there cylinders actually not getting cooled at this time? I don't know. But from how quickly it happens I have a hard time thinking the actual engine coolant was getting that hot that fast when it's 20 outside. And the fact that once the thermostat opened it immediately went back down to operating temperature. Just my thoughts and I'm not an expert in cooling systems but Is my logic sound here?

 

I disagree. Even if the head gasket is shot, the original cause has to be repaired and I wouldn’t want simply driving my newly reworked head, block and gasket to be the test process.

 

All those questions are irrelevant besides the leak part. I'm losing coolant, that needs to be figured out first. The first place I looked was inside the cylinder because there is no visible external leaks.

 

Hopefully you'll have clarity after the pressure test.

 

Not meaning to sounds like a broken record, but a compression test would be a highly effective and quick way to rule in/rule out the head gasket or cracked head issue as a possible cause.

I hope you find an external coolant leak that can explain all the coolant loss, but it seems unlikely that'll happen if you haven't seen any signs to this point

 

I would think at this point, you'd see some around the bell housing underneath the truck or smell a strong coolant odor, if it was coming from the valley plate.