VT V6 Supercharged Mystery Overheating Problem

[Shebang]

Hi guys, this is my 1st post here I usually just read but this time I have a problem of my own that so far no mechanic can find so hopefully somebody here might be able to help. I have very basic knowledge of cars so I will try my best.

The car is a 1998 VT V6 Supercharged Auto.


The Saga:

The car overheats when you stop at the lights or in stop and go traffic, I can drive on the highway for as long as I wish and the temp stays on around 88c-90c but as soon as you stop and idle it creeps up to over 110c and displays "hot" and I end up turning the car off and creeping home..

About 2 months ago the overheating began and I took it to my local mechanic who pressure tested it, flushed the system and replaced the water pump (old pump was OK but I said just replace it anyway just incase) but he basically said that it may have had an airlock in the system and seems OK now so see how it goes after the flush and it should be fine. 3 weeks and the car is running fine sitting on 88c-91c above 60kmh and reaching 96c in traffic triggering the fans and coming back down to 88c until one day when it decides to play up again and reach 110c+ and display "hot". Once I can get back up to 60kmh+ it will go back down to 88c and stay around there until I hit traffic again.

I took it to a radiator specialist this time and they said that it had an airlock in the system and they bled it and said to see how I go with it and hopefully it will be OK. He also pressure tested it and it was fine and he flushed the system. I mentioned could it be the thermostat and he said the thermostat in the s/c V6 is a big job and thermostats either usually work or they don't so it is highly unlikely. That was about 1 month ago and since then the car is running great up until yesterday in traffic same problem 110c+ "hot" etc etc.

After it is flushed and bled the car is fine for a few weeks but after that the problem seems to come back.


Radiator was replaced about 3 years ago and mechanic says there are currently no blockages.

Both fans always come on at 96c every time.

Water pump has been replaced.

Radiator Cap replaced.

Chemical test came up fine.

Pressure tested at 2 different mechanics and was fine.

Cooling System Flush twice at 2 separate mechanics.

System bled by 2 separate mechanics.


Over $500 spent.

If anybody has any ideas it would be much appreciated and thanks in advance! I do not know what to do next :s

 

[Pollushon]

Sounds like a lazy mechanic, it's not a mystery if all avenues haven't been explored. The thermostat could absolutely be causing this issue and it's the most likely candidate next in line. From what I've seen from the L67 guys here, it's not the end of the world to replace, just a bit awkward as it's up the back under the intake elbow.

 

[beaumontsl]

yea id go a thermostat but have you considered it might have a dodgy sender at all and it is randomly playing up and sending wrong messages to ecu

 

[Cashman_309]

Pollushon and beaumontsl are right. The 2 things to look at replacing would be the thermostat and the coolant temp sensor.

You could also try bleeding the system again, try doing it with the car facing up a hill then again facing down the hill, and make sure the radiator is full up to the cap.

 

[Shebang]

Thanks for the replies.

Can I open the radiator cap and top it up without letting air in? I was told only to fill up the coolant via the overflow, which is showing as full.

I would have thought if the sensor was playing up it would overheat daily rather than 2-4 weeks after I get it back bled and flushed? I am not sure?

Cheers for bearing with me with my limited knowledge of mechanics.

 

[chargedvx6]

Intake manifold gaskets may be a possibility. To subdue the problem you can turn the heater on high to help take some heat away from the coolant circuit in the mean time. A mechanic should be able to work it out. Try a different one for a different perspective maybe.

 

[Shebang]

Can I top it up via the radiator cap or will it need to be bled again if I pop the cap?

 

[Pollushon]

Depends on how low it is. One trick is to use an uneven surface and park the car nose up like Cashman said. Then top up. If the rad is really low, then bleed it. Repeat the process over a couple of days if needed.

 

[VK SL 3800]

I would change the thermostat, they do get sticky and only half open and carry on. Especially in if there older they can get covered in rust and other crap in the cooling system and no amount of flushing will clean that off unless you physically pull it out. I would also check that the fans are working on both high and low speeds.
Bleeding is important, i changed the coolant in mine, started it and had the fan on and it overheated just sitting there idling before i got all the air out.

 

[monkeys437]

Might sound stupid but was it bled properly? I've come across mechanics that didn't know there were three bleed valves or what the correct order was. It wouldn't hurt to top up the coolant then bleed it yourself properly just so you know it's done right