2001 3.4L Temperature gauge intermittently pegs out at hot - Why?

Any help with this issue is greatly appreciated, I have been dealing with it for a couple of years since it is intermittent.

2001 Doublecab pre-runner with 3.4L, 139,000 miles, I am the original owner.

My temperature gauge will show that the truck is warming up and then will normally read at the 9 o'clock position (half way between the H & C). Intermittently, the temperature gauge pegs out at Hot, this has happened during the winter and on 90+ degrees days. No consistent act or condition happening that I can tell. Sometimes it will go back to normal on it's own but usually if you shut the engine off & let sit for a minute it will go back to normal. I checked the fluid in the overflow & it has adequate coolant in it.

Things that have been replaced and the order they were replaced.

1. Valve cover gasket - had a small oil leak on the right side of the engine. Not sure if the problem existed at this point. Within a couple of months I noticed the problem.
2. Thermostat - aftermarket with a jiggle valve.
3. Temperature Gauge on the dashboard. - New Toyota part.
4. Radiator - aftermarket from Autozone. Original passed a pressure test but the original cap didn't feel like it was getting as tight as it should.
5. Radiator cap - New Toyota part.
6. Coolant - Toyota brand concentrate - distilled water added.
7. Valve cover gasket & cam seals due to an oil leak
8. Engine Coolant Temperature Sensor (located on the front of the engine block - reads to the ECU
9. Engine Coolant Temperature Gauge Sending Unit ( located at the back of the engine - reads to the temperature gauge.
10. Burped the radiator using a radiator burp kit

I have been using an electronic copy of the 2003 FSM to try to diagnose this issue. The only thing I haven't tried is the ECU, it is an expensive part to replace. Thanks, Beach Beagles

 

I would definitely be looking at a connection issue next, inspecting the exact pin to the cluster, and any unions along the way.

It would need to short to ground to trigger a hot signal to the cluster. Low resistance is hot.

I would try disconnecting the cluster coolant sensor next time it "overheats" then see if it drops off. Disconnected is high resistance and should read dead cold.

Are you positive it's not actually overheating? Headgaskets can cause this, are there any coolant stains from a rad vent?

Try turning the heat on the cabin to max and see if that drops the temp down.

Just suggestions.

 

When my VW had this issue, it turned out to be the temp sensor.

 

You can pick up the little infrared temp guns pretty for reasonable money. When the gauge reads normal pick a few locations like the top and bottom rad hoses and the heater or bypass hoses and note the temperature.

Check your benchmark spots a few times for a consistent average. Next time the gauge spikes jump out and check your benchmark points.

BTW, shiny surfaces don't give accurate temperatures, the black hoses will be more accurate.

 

Have you ran any monitoring connected via OBDII?

I ask because the the gauge on your dash is pretty useless.

A single wire sender, connected toward the rear of the motor/intake manifold tells the gauge needle where to sit.

Your actual temps, those measuring your coolant temperatures, is a sensor and communicates with your ECM. It is located towards the front just under the bridge of your fuel rail.

The only way you can see what this sensor is reading is through the OBDII port.

You can have a faulty sender, or a bad connection via the single wire connection and actually have good temps.

 

I’ve seen you cover this several times. Do we know why Toyota went with temp monitors in those separate locations?

 

Why?

I wish I could say.

As stated, a sender can go bad and give you a false reading of how hot or cold your engine is running.

However, a sensor going bad will cause issues in how your vehicle runs while you maintain a good temperature on your dash.

I had a sensor go bad immediately after coming back from a weekend long trip of wheeling in Moab.

My symptoms were high idle, rich fuel mixture (smelled it too), and sometimes not turning on at all. The sensor was bad and would cause the 4Runner to remain in open loop, even if the vehicle itself was already warmed up or after a long drive even.

 

I will say, I didn't really read the OP's post.

I didn't even read the replies that followed.

Just hoped to reveal some information on how things work on these trucks in order to help with diagnosing the issue (based on the thread title).

Suggesting to check via OBDII just confirms whether or not the engine operating temperature correlates with that of the gauge on the dash.

If it doesn't, there's your issue right there.

Saves a ton of time and money before fiddling with anything else.

 

I take this same approach sometimes and focus on keywords I see when I try to help someone out. I always have the best intentions even though sometimes I come off like an ass.

 

Is your fan clutch bad?

 

Really?

I come off way worse, and don't even mean to.

Sometimes I'm just try to be as literal as possible, minimal confusion, straight to the point, but people get offended

 

Thank you to everyone who responded. I have already tried some of your solutions but I forgot to write it in the op. I have disconnected the sensor (engine was cool, hard to reach especially when the engine is hot) and it dropped to cold & when I grounded the wire from the gauge it went to hot. I was holding off on the ECU since the truck has been running fine otherwise. It doesn't appear to be leaking otherwise, and the head gaskets have been changed twice since other work was being done. Turning heat on doesn't make it go down. I have an infrared temp gun but I never had it with me, now it is in the truck.
I have put my code reader on there and doesn't show anything, green light, no codes nothing pending. Both the sensor in the front and the sending unit in the rear have been changed. I have someone in a couple of weeks is going to put his computer on there and give me more info than my little code reader provides.
Fan clutch I have not checked because every time I have it the truck running, even if the gauge has pegged out to hot, the fan is running so I thought there were not any problems with it. I will update when I figure it out, with it being intermittent it makes it hard to know when it is fixed.

 

With all the parts replaced, and no loss of coolant, I'd aim to follow @slander suggestion.

Your fan clutch may be spinning, but you can't gauge how fast it is just by looking at it.

Do the fan clutch test; use a rolled up newspaper.

Make sure your fan shroud is in place; if it's missing that could lead to warmer operating temps as well.

 

You know on a scanner you can run live data and see what's going on...
Everyone should be doing this now and then, to check fuel trims at idle and hwy speed if nothin else..

 

OP states this was done.

But yes, agree, and was suggested above.

 

He said he just checked for codes....

Re:
I have put my code reader on there and doesn't show anything, green light, no codes nothing pending.


I must of missed something......just trying to be helpful

For us who run cold air induction......always have to be checkin fuel trims......extreme lean.

 

Ah you could be right.

Even though it was suggested, twice already.

Who knows.

 

Yeah, sometimes I get a brain fart and dont see the obvious, reason why I keep u guys around....
Or did something wrong in the past, and now a new thread tips me I got to go back and redo right....
Like necessitating a whole rear brake parts replacement....that took 3 trys, until everything was adjusting right.

 

Next time your trucks temp spikes, shut it down and try spinning the fan by hand. If it spins freely your clutch is bad. My truck got weird and would overheat at odd times and it turns out it was a bad clutch. Just because your fan is spinning doesn't mean the clutch is working. Also listen for it to click and engage, and make that deep whoosh sound.

 

Good chance to upgrade to the 'tow rated' fan clutch.

Only 4 bolts and 10 minutes for a total replacement.

Cheap too, under $100 for an Aisin model.