Car's temperature gauge goes up while waiting at a long light but goes back down to normal when driving?
Posted by RacerSpeedyboi@reddit | askcarguys | View on Reddit | 32 comments
Hello everyone, my car's temperature gauge goes up to hot when waiting at a long traffic light but goes back down to normal when driving. Is it safer to drive on the highway where the car is constantly moving but at higher speeds or drive the car in city roads but with many traffic lights to get to my destination? Which would be safer to do until my mechanic can find out the root cause of the heating? I have had the radiator replaced and the fluids are all topped off. Thank you.
bigbrightstone@reddit
What car is this? Old mercedes?
RacerSpeedyboi@reddit (OP)
2003 Subaru Outback ll bean edition wagon with the H6-3.0 engine.
BoondockUSA@reddit
Those are pretty solid engines, and probably the best that Subaru ever made. That means it’s likely an affordable repair. It’s likely just an electric radiator fan that isn’t working, or the radiator fins being plugged up by cottonwood seeds or dried mud, or low coolant from something like a pin hole leak or a weeping water pump. There’s a chance that it could even be the water pump if the vanes are eroded by cavitation. Either way, it shouldn’t need major engine work.
I’d tell you to expect worser things if you had a four cylinder Subaru. Thankfully, your past self did your present self a solid by buying a Subaru with the H6.
blur911sc@reddit
You've obviously not tried to access the water pump on the EZ30, it's internal. Anyhow, it's most likely the radiator fan not coming on.
BTW, I had one EZ30 blow a head gasket an another jump timing and smash valves (previous owner of junkyard engine seemed to not believe in oil changes)
porcelainvacation@reddit
My 2004 Honda went through about 4 radiator fan motors over the 280k miles I had it. Its a pretty common failure.
Gubbtratt1@reddit
Almost all older cars does this. The cooling system is designed for driving, and 50 years of sludge in the system doesn't help. Only exception I can think of is old Land Rovers. The cooling system was designed for the extreme heat of various deserts, so even a hot summer day won't cause them to run warm.
HotmailsInYourArea@reddit
Nah my 1998 Land Rover Discovery, also made for off-roading like a loon, had a sludge-filled, overheating-causing radiator by 2011. Loved that thing though, warts and all.
The cooling systems are also very much designed for idling, hence the fans, and cop cars not melting to slag after an 8 hour shift. But you're correct that over time inefficiencies build, and some vehicles are more resistant to the effects of that than others.
In OP's case though, almost certainly their electric fan has died
Gubbtratt1@reddit
I'm talking about 50+ year old cars. A D2 isn't made for extreme heat like a Series was.
Catzsocks@reddit
I had this problem and there was a tiny crack in my radiator and I was low on fluid.
thymewaster25@reddit
It is much better to keep moving so the airflow through the radiator keeps the temperature down in the normal range.
You can turn on the heat at full blast to cool the engine a bit.
On some cars, the temperature sensor that turns on the fans is on the radiator. Maybe they forgot to plug it back in when they changed the radiator.
Gunk_Olgidar@reddit
Fan operation, relay/fuse. Damaged or missing shroud. Low coolant. Plugged radiator. Improper bleed. Wrong replacement radiator. Bad thermostat.
JustAnotherDude1990@reddit
Sounds like the fan isnt running. Turn the ac on at idle and see if the fan kicks in behind the radiator.
RacerSpeedyboi@reddit (OP)
Ok I'll check that thanks 👍
minnesotamoon@reddit
If it’s not running check the fuse. Might be an easy fix.
SonOfMotherlesssGoat@reddit
Not disagreeing that it’s likely to pop again but if it’s 20 years old and popped it’s worth the $0.25 to buy a fuse and see if it happens again. Might get lucky or have been a fluke for it popping.
Elitepikachu@reddit
Replacing the fuse isn't a fix though you still need to diag and find out why the fuse blew. Otherwise you're not actually fixing anything.
jules083@reddit
Sometimes. Ive had fuses pop for presumably no reason before as they get older. Probably 5 or 6 times in my life I've had to replace a fuse on something then had the new fuse last for years without me ever fixing anything else on the system.
The best guess I've ever had is that the connector wire in the fuse breaks down over years of heat cycles and slowly weakens then finally can't handle the voltage spike when starting. It's almost always been on a motor of some sort, like a fan or a fuel pump.
RacerSpeedyboi@reddit (OP)
Appreciate the info
LeadfootYT@reddit
Impossible to say without knowing what car and where on the gauge it sits. If it has a dummy gauge, it would indicate a problem. If it’s something older with a real temperature gauge, entirely normal.
Bi-mwm-47@reddit
The symptoms you describe are consistent with an electric radiator fan not working. When driving at speed, the airflow over the radiator is enough to keep the coolant at the correct temperature, without needing the fan. When you’re stopped in traffic, or a red light, not so much.
Candid_Ad5642@reddit
This!
When stuck in traffic and the temp is rising above a certain temp it is usually possible to hear that fan kicking in if you have the windows open and the music off
And this can be caused by the radiator fan not working, some temp sensor (probably on the radiator) not working, no power to the fan (blown fuse), or some logic somewhere being bugged
And the above really should have been checked before replacing the radiator
Concrete_Grapes@reddit
One, fans need checked. Two--your AC is on, isn't it? Drive with the AC off and see if it still does it. It may simply be too dang hot, and the AC thing is in front of the radiator, so, if the fans are working, they can be pulling 250+ degree air off the AC part, into the radiator.
Can check the AC charge, sometimes they get too hot due to that being low.
Often, far more often, this is a sign of a coolant pressure problem.
So, your radiator cap has a pressure rating, and, that pressure is added to the coolant, to keep it from boiling. If, anywhere in that system, there is any kind of leak, and the pressure can't build, it will boil. This is most likely when at a light, because the air flow is less, and, the water pump speed is a third or less of driving speeds, so less fluid moves.
So, check the mix on the coolant is correct for best boilover protection. Then, pressure test the coolant system. Pressure test the radiator cap.
Then move to the AC charge. But, fans first.
trish828@reddit
check fan, fan clutch
BroccoliNormal5739@reddit
Fan clutch. (In the old days)
DCHacker@reddit
If Original Poster has an electric fan, does it turn on and off? If it is a mechanical fan, is the clutch engaging and disengaging? Does the fan have a shroud.
do2g@reddit
Like others have said, the AC fan/fan circuit is a strong contender and good place to start. Check to see if the fan turns on with the AC and runs when the engine is warm. If not, I'd test the fuse and relay for it. If those check out, it could be the fan itself or the temp sending switch.
NoRegret1893@reddit
Given the symptoms, you have an AIR circulation problem, not a coolant circulation problem. Hopefully your mechanic will address it from that perspective. Malfuctioning electric cooling fan or fans, missing fan shroud perhaps.
sandisc731@reddit
As most have said, it’s probably the fan not turning on. But why is it not turning on? It might be the coolant temperature sensor or something to that effect. Still should be an easy fix.
Iamthewalrusforreal@reddit
Faulty water pump. Or loose serpentine belt. It's not working properly at idle, but when you're at higher RPMs it's functional.
Old_Confidence3290@reddit
Probably normal. It will heat up a little bit before the fan comes on. It's very common to see this when you are not using the AC.
Blu_yello_husky@reddit
When youre moving, there's constant airflow to the radiator keeping it cool. That's why they say if you have another overheating issue with hour engine, you can limp it home as long as you stay moving and never stop. Older cars you'll see a slight fluctuation in the temp guage as you stop and go (especially on hot days), but with more modern cars, you really shouldn't see any noticeable change in engine temp as you drive through stop and go traffic. Id check to see if your electric fans are running, or get your cars fan clutch checked if it has one.
ready2xxxperiment@reddit
Had a similar issue and there was a pinhole leak in the radiator. Also check for low coolant or evidence of a coolant leak.