hey guys I'm new but i've been enjoying reading this forum for a few months now. It has helped me fix a few problems with my car lately...
anyway today we took my 02' TJ for a drive down the highway and we got caught up in roadworks. while we were stopped for a good half hour in roadworks i noticed something weird with my engine temp. usually my temp is dead on half way once that car has been running for a bit and i've never seen it go over. but today when stopped it moved a good 1 - 1.5 marks above halfway (still no where near H) but it got me worried. soon enough both thermo fans kicked in and it went back to halfway. we were stuck in roadworks with the car in neutral and the ac off. now i've never seen it do that in the 10 years we have owned the car. sure enough once we started moving again the temperature remained constant. but I'm freaked out over it. at the lights on the way back it did the same thing. I'm a bit worried.

i did recently bypass the heater core as it was leaking onto the ecu. but i checked the hoses and they were fine so I'm stumped. as far as i can tell everything is fine. its due for it's next service soon so i will probably get the cooling system flushed as i have no clue what is in it at the moment and i would like to know. but does anyone have any suggestions?

cheers

EDIT: could it possible be an air pocket from when i bypassed the heater core?... i see this all the time on my mates VS commodore but not on any car we have had

Could be a number of things. Check the thermostat / CTS. Its quite unusual for a genuine overheat situation in a 3rd gen. Have you checked the radiator filler cap is on nice and tight and has a good seal?

It is completely normal for the temperature to rise above half when stopped and idling due to no airflow through the radiator....it will keep rising till the cooling fans kick in and then cool back to normal - it will then cycle in this manner till the car moves again. Bypassing the heater will make the coolant heat up much quicker when stopped due to a reduced amount of coolant in the total system....you have 1 - 2 litres less coolant to absorb the heat.So I'd say you have nothing to worry about. However, I'd do a cooling system flush, new cap and thermostat and possibly new hoses all as preventative measure.
Good Luck.

It is completely normal for the temperature to rise above half when stopped and idling due to no airflow through the radiator

This is not normal. I have never once seen any 3rd gen Magna I have driven ever go over half way in any given conditions, even stop-start peak hour traffic, or spirited driving in 39°c heat.
But then again I tend to go overboard with preventative maintenance.

If it goes over halfway there is something not quite right, maybe a drop in coolant level, a small blockage, fan issues, thermostat issues, water pump / pressure issues etc.

First recommendation here is double check the radiator is full when cold and top up if required while the engine is running for a few mins (up to normal temp). But a good flush with fresh coolant/quality water mix won't go a-stray, and thermostats are cheap so get it replaced and go from there. Failing that, check water pump.

My KH Xi always used to do this. I replaced my thermostat which fixed the problem right away.

my first gen does the same thing because the cap is absolutely rooted. it will heat up, pressurise then force the cap out and loose all pressure because it doesn't seal properly. when it looses pressure it looses its cooling ability effectively. remember its not how stiff the cap is but rather the condition of the rubber. if its anything but totally round and flat replace it. thermostat is a good idea and is easy.

if your prepared to do it ide flush the system (thermostat out, top hose blocked with a rag and run the garden hose through it till it comes out clear. new coolant, then bleed all the air out. if your changing coolant remember to flush and refill the recovery tank).

if you still have problems after that then its probably wise to let the mechanic look at it but really the only place you can go wrong is if you dont bleed all the air out, so ide look up the procedure on that as im not aware of how the third gen systems work. may also be dickey temp switches or relays but that comes later.

ditto, NOT normal!. i would check the bypass you have made. is it kinked or restricting flow? you could chase allthe usual cooling system faults, but if you altered the stock system then this is your best bet for fault finding.

This is not normal. I have never once seen any 3rd gen Magna I have driven ever go over half way in any given conditions, even stop-start peak hour traffic, or spirited driving in 39°c heat.

I think I'd be ok to assume in 30+ heat that the A/C would be on so both fans would be running.

I've had the car go over half in traffic often and if it concerns me I throw the A/C button on to keep it cool.
Over the years I find the changing the coolant helps .... for a while but eventually it goes up and goes down.

I suggest a full flush as suggested.

When I first got my TH, throughout Summer it recieved a decent amount of lead-footedness. Not once did it creep up from 1mm below half way. To compensate for the engine being hotter (for example, 45 degree heat, or bashing it off the limiter) the fans run for longer and cycle more often.

i fixed it or something... i had a rugged check over the cooling system and dug into my Gregory's manual and checked absolutely everything, bled the system etc etc... and it seems to be fine now. drove the car a bit before leaving it on the driveway running for a bit and it stayed at halfway. i will be completely flushing the system soon. cheers for your help guys. i checked the bypass its fine btw. when i do the flush i will check out the thermostat as well. got into an argument with my mate about it though , he thinks its normal and i argued that every car we have ever owned hasn't done it but he wont listen, and yet his commodore broke down today :P.

i have a question though. i check the engine bay relays today and relay letter G isn't present, as far as i know it has never been there though so im wondering what it does. the car manual and greg manual don;t tell you hat the relays do.

Well, we're all entitled to our opinions....and I gave mine. I still think that the problem mentioned by the OP is "normal". I find it hard to understand how people can say that their temp gauge never rises above half even after spirited driving and stop start driving. I cannot understand how the radiator can cool the engine when the car is stationary and there is no airflow through the radiator. I cannot understand how reducing the total coolant system size by 10% (heater) will not result in altered cooling capacity.

Well, we're all entitled to our opinions....and I gave mine. I still think that the problem mentioned by the OP is "normal". I find it hard to understand how people can say that their temp gauge never rises above half even after spirited driving and stop start driving. I cannot understand how the radiator can cool the engine when the car is stationary and there is no airflow through the radiator. I cannot understand how reducing the total coolant system size by 10% (heater) will not result in altered cooling capacity.

Its a magna thing. Generally, they dont budge once up to temp. I have had mine climb up one more notch after very hard driving

I cannot understand how the radiator can cool the engine when the car is stationary and there is no airflow through the radiator

Fans....

My TJ always sits just below half and I have never seen it move from there, not even on a really hot day. Also it only takes about 5 minutes to get there even on a frosty morning.

Well, we're all entitled to our opinions....and I gave mine. I still think that the problem mentioned by the OP is "normal". I find it hard to understand how people can say that their temp gauge never rises above half even after spirited driving and stop start driving. I cannot understand how the radiator can cool the engine when the car is stationary and there is no airflow through the radiator. I cannot understand how reducing the total coolant system size by 10% (heater) will not result in altered cooling capacity.

If your radiator is in good condition (no blockages, restrictions or damage to the core for air flow), it is amazing just how effective the fans are at pulling heat from the radiator, this is what they are designed to do (and do well). When the thermostat is wide open, the water pump moves coolant around the system rather rapidly and the fans rotate at some hefty rpm. Everything works together well to maintain coolant temp at approx 88-92°c.

Bear in mind that it is common for the thermostats to deteriorate over time and can be sticky and not open fully/correctly or efficiently. A lot of people opt to replace their water pump when the timing belt is changed every 100,000km too.