VZ Overheating Mystery

[Aussie_Dark]

Hopefully someone will be able to guide me to work out what has failed.

I have already had the radiator cleaned, had the cooling system pressure tested, and I have replaced the cap twice as I know the caps are unreliable. Also the fans seem to cut in appropriately. There are no leaks apparent. The exhaust does not appear to blow steam, and there does not appear to be any coolant in the oil, or vice versa. There is no strange residue around the oil filler cap.

What normally happens is that the car temperature gauge stays around the 25% mark for approximately half an hour of driving and then will shoot up rapidly to 100% and the overheat alarm sounds. It doesn't need to be a hot day for this to happen.

On a cool to cold morning the temperature stays just below the 25% mark. On these occasions there is no drop in coolant and the overflow reservoir stays at the same level.

On days where it hasn't overheated, but the temperature has drifted a little above the 25% mark, I have found that more coolant has been pushed out into the overflow reservoir than what is sucked back, but it isn't a huge amount. All checks of the overflow reservoir have been done after the car had completely cooled down.

On a couple of occasions I have had the temperature operate at the 25% mark, shoot up wildly to around 75%+, shoot back down to normal, shoot back up again, then down and stay down until I got home. On another occasion it continued to shoot up and down for the whole journey (but on all of these occasions it was about 30 minutes before the strange behaviour started).

I have removed the radiator cap and watched the water circulating as the car gradually heated up. The first thing I observed is that bubbles appeared every so often as the water circulated. If I slightly pressed the accelerator the water will overflow. Every so often the bubbles became extreme to the point of the water foaming, and then return to bubbling every so often. The water level also fluctuated up and down. After approximately 30 minutes (and rapidly without any real warning) the water appeared to boil with steam and water spurting out. A check of the temperature gauge indicated that it was still only a little above the 25% mark.

So what I have concluded is that with this problem, the gauge may be still be at around the 25% mark with the system being hot enough to push coolant out of the overflow. I'm assuming once enough coolant is pushed out that the temperature will start fluctuating or rapidly hitting the 100% mark.

Any pointers to what could be the problem would be greatly appreciated. I am down to the water pump, thermostat, or head gasket. None of these are things that you want to fork out to fix without some hope of being right.

 

[MR_VX]

well i would be changing the thermostat as they arnt hard to change at all. i think most commodores have a 95* thermostat from factory most people change them to a lower temp one like 81 or lower, it allows the cooler syster to start quicker and start cooling down the motor better.

Have you checked all the hoses are fine? there might be to much air in the water?
maybe the water pump needs replacing? have you taken it to holden maybe the tech 2 scan tool will tell you the problem!
im only putting sugguestions out there for ya.

 

[marcdon]

I would say thermostat for sure

 

[holdenvtman1123]

thermostat is harder to change than old mates vxII.

 

[MR_VX]

really opps my bad. id still say change it tho

 

[Aussie_Dark]

Does anything I wrote, positively remove either the water pump, thermostat, or head gasket as a possible cause? Can I discount any of these three?

 

[Danoz]

replace your thermostat. That is all it could be going by the symptoms you have described. I had the same symptoms in my old sigma... The thermostat worked, but it was "stick". I reckon you have the same prob. Get it replaced!

 

[torana355]

Yep thermostat. To bad it is in a bitch of a spot and some mech's even drop the box to get to it lol.

 

[Sean880]

It may well be the thermo sticking and then releasing sporadically but before I did a change I would be doing another coolant flush and refill and then test it out. This time it's done make sure the system is properly bled of air because from what you have said you may have a hell of a lot of air in your system.

You said:

"I have removed the radiator cap and watched the water circulating as the car gradually heated up. The first thing I observed is that bubbles appeared every so often as the water circulated. If I slightly pressed the accelerator the water will overflow. Every so often the bubbles became extreme to the point of the water foaming, and then return to bubbling every so often. The water level also fluctuated up and down. After approximately 30 minutes (and rapidly without any real warning) the water appeared to boil with steam and water spurting out. A check of the temperature gauge indicated that it was still only a little above the 25% mark."

I assume the bubbles you refer to are air bubbles. If you do have air in your system you will potentially get blockages in coolant passages and you will get an overheating problem.

Before I rushed off to replace the thermo or anything else I would first test to ensure your problem is not caused by inadequate bleeding of air from your cooling system.

I would query who did the last coolant flush and change in the car and whether they used the correct procedure to ensure all air is bled from the system after the refill. Bleeding out the air is critical to achieve unblocked coolant circulation and correct operating temp.

Once you have eliminated air locks in the cooling system as the cause of the problem go on to the thermo. If this was my car this is what I would be doing given that a thermo change is a hell of an exercise in your motor. Just make sure you use the correct GMH spec coolant for your vehicles cooling system (refer owners manual) when refilling the system.

You also mentioned that the problems with the temp changes does not occur until after 30 minutes of operation.

"On a couple of occasions I have had the temperature operate at the 25% mark, shoot up wildly to around 75%+, shoot back down to normal, shoot back up again, then down and stay down until I got home. On another occasion it continued to shoot up and down for the whole journey (but on all of these occasions it was about 30 minutes before the strange behaviour started)".

With a faulty/sticking thermo I would expect problems to show themselves much sooner than after 30 minutes of engine operation each time.

Anyhow best of luck with it.

 

[Aussie_Dark]

Sean880...

Yes I agree it doesn't make a lot of sense. I had forgotten to say that I had the air bled from the system, but unfortunately it wasn't done by a Holden mechanic. This also doesn't explain why it started in the first place. Also the air bubbles just keep coming. I have watched for 45minutes and the rate of bubbling fluctuates as described, but they do not stop. Also the water level does not go down as the bubble are released. I was worried that the bubbles indicated a head gasket problem, but I was told that these systems can create their own bubbles (Would someone like to check theirs to confirm this?).

Another thing that made me think head gasket was that it kept pushing extra coolant out of the overflow. Not much extra comes out (unless it goes really hot), however it will continually loose more coolant than it sucks back in (unless it is a quite cool day).

Something else I forgot to say is that it started overheating only on really hot days after about 1.5hrs of driving. It has been gradually getting worse for the past 2 months to the point that it is only taking about 30 minutes and it only has to be a warm day.

In my head I keep flip flopping between head gasket or thermostat.

Could it be the thermostat sticking so that the engine runs hot enough to push a little bit of coolant out the overflow, and I don't have big problems until a certain amount of coolant is released? The problem with this idea is that the temperature gauge only shows a little above 25% for the extra coolant to be released, and I suspect that not much would have been released in the 30 minutes it takes for the temperature to start going too high.

You can see why I am scratching my head. Most people appear to be saying thermostat. I have to take a punt soon, as I don't have any confidence on what is the route cause.