2000 OBW 240k miles. 

Burned an exhaust valve so a friend and I pulled the motor and had the heads professionally reworked. New MLS gaskets

Car runs great now, but it'll get to temp then every so often the temp (needle on the gauge) will climb almost to the second line above the halfway point, then drop to normal. Never overheats, just climbs a little past half before dropping to norm. 

I thought it might be a sticking t-stat so I picked up an OEM one from the dealer and dropped it in. Its still overheating. No air in the system. I've gotten really good at burping air from these cars. Fans come on. Timing belt and wp were replaced about 50k ago but again the problem only occurred after the engine repairs. 

All grounds are connected. 
Not sure what else it could be. A flakey temp sensor?? I've never seen those fail in this manner. 

Edited March 1, 20233 yr by 211

Radiator isn’t blocked?

How many miles has it driven since the work? 

Did the symptoms start immediately or after a month?

How often does it do this - once a day or week?

Look for a pattern when it overheats. Coming off a long sustained drive (like exiting an interstate or driving back roads and stopping more in city)?   Going up hills or coasting or night and day or ambient temps. 

I've had that exact issue on an 87 Mazda B2000. It would only happen after the engine had cooled fully overnight and it would only do it one time during that trip and rarely ever again that day unless the engine had fully cooled. The needle would sweep slowly up as expected but then continue past normal up to almost the overheat mark and then suddenly drop to normal readings in about 1 second. The engine could not have really been overheating as it happened before enough miles had been driven and the heater was only putting out medium temperature air. After several burping procedures and a new T-stat the fix ended up being a new temp sensor.

Do you see your gauge needle move like that over and over on a single trip or only during engine warm up like I experienced?

5 hours ago, azdave said:

 

Do you see your gauge needle move like that over and over on a single trip or only during engine warm up like I experienced?

My commute is mostly highway for 20 miles, once off the highway, the temp will climb, then drop... it'll do that a couple times before getting to work which is another 10 miles of back roads

7 hours ago, idosubaru said:

Radiator isn’t blocked?

How many miles has it driven since the work? 

Did the symptoms start immediately or after a month?

How often does it do this - once a day or week?

Look for a pattern when it overheats. Coming off a long sustained drive (like exiting an interstate or driving back roads and stopping more in city)?   Going up hills or coasting or night and day or ambient temps. 

Radiator isn't blocked that I can tell. 

I've driven the car a couple times after the work. We noticed the overheat the first time we ran the engine for break in. Called it an air bubble and thought it would work itself out. It didn't. Replaced the Tstat and re flushed the coolant. still doing it. 

Symptoms start once the car has been at operating temp for a while. So far every time i've driven it. Seems to do it regardless of driving conditions although I can expect to see it happen after I get off the highway. Its been chilly here and I suspect the airflow at highway speeds keeps the temps in check until I get off..  

Is the coolant-level in the overflow reservoir behaving as it should?

i.e. is the coolant-level when cold lower than when you stop the hot engine at the destination? And does some coolant then get sucked back into the radiator, as the hot engine cools down to cold (say overnight)?

With multiple occurrences during a single driving event then I doubt it is a bad sender. Do you see any other signs that it is actually running hot or is it only because what the gauge reports? Do you hear detonation or pinging or any other signs that it is truly that hot? 

Do you have access to a cheap BBQ thermometer? You could wrap the sensor around a heater hose and then watch the temps while you drive as a second way to confirm that the temps are really rising and falling that much. I use a unit that I found at Wal-Mart for $15.

This one has 2 channels so you could put the other sensor on the radiator hose to document the temps there too. I sometimes put the sensor on the inlet and outlet hose of the radiator just to observe the temperature drop I'm getting while city or freeway driving.

https://www.amazon.com/Wireless-Thermometer-Digital-Instant-Grilling/dp/B093KNZ3X9/ref=sr_1_17?crid=2QYAI5WOYEC4E&keywords=remote+bbq+thermometer&qid=1677766260&sprefix=remote+bbq+therm%2Caps%2C144&sr=8-17

Edited March 2, 20233 yr by azdave

Coolant in the overflow doesn’t move at all. 
 

Car doesn’t appear to be running hot other than the gauge indicating otherwise.

I did notice this morning (cold) it ran noticeably rough till the t-stat opened then smoothed out. 

also it warms up incredibly quick (according to the gauge). I don’t really remember it warming up so rapidly. Less than 5 mins and the needle has climbed to just below horizontal. 
 

heater works as normal. 

any chance the car ran straight water in the past? maybe the 'vanes' on the pump's impeller are corroded? (really reaching but...?)

6 minutes ago, 1 Lucky Texan said:

any chance the car ran straight water in the past? maybe the 'vanes' on the pump's impeller are corroded? (really reaching but...?)

Negative. 
I've owned this car since mid 2000 and have done most of the maintenance myself.  

Is the radiator cap new and OEM? Maybe it's not quite holding pressure, gets hot, seals and cools back down?

I had a similar issue with a bad water pump gasket though my issue impacted my cabin heat more than anything. Engine temp did fluctuate a little bit.  Best I could tell is system was sealed when everything was expanded and then would suck air in when it cooled down. I didn't really have much of a coolant leak so I never had to add coolant.

Are you needing to add coolant to the system at all? 

Grounds on the engine good and wiring to the sender unit not damaged? You could try wiggling it and watch what the temp gauge is doing, this will show up any wiring issues. 

Nothing near the sender unit that could ground it to give the false reading? 

Old mate that did the HGs - did they put the painted head bolts in the centre two holes? The other four are unpainted (on the head of the bolt) and these go in the outside headbolt holes. Asking as this always catches ppl out the first time they do a HG on these engines. 

Have you done an exhaust gas in coolant check just to be sure this isn’t the issue? 

Cheers 

Bennie

I would be ripping the centre from the old thermostat just to run it without that interference for diagnostic purposes to see if it does the same thing. I ran EA82T for years without thermostat and have had no issues with temps or fuel mixtures - none of what nay sayers have claimed will occur. Things may be different if I drove it in traffic.