Pontiac Repair: 2002 Montana temp gauge up and down on idle w/o fans???, cylinder head gasket, pontiac montana

Question
Hi there,

I have a used 2002 pontiac montana (59000kms). The normal operating
temperature was one click above the 1/4 mark. Then about 3 weeks ago the
van started to gradually rise to the 3/4 mark on idle, then WITHOUT the help
of fans, it immediately falls back to 1/2 in one drop... and then it starts
again.

I bought the car in Feb/2007 at 56000kms, it had been on the GM dealership
lot for one year (not sure why it hadn't sold?) and I was told that it had the
cylinder head gasket was replaced at 55000kms.

I've had it to a local mechanic who purged the tank and replaced the rad cap.
He said the van was only functioning at 8000lbs and now was back up to
16000lbs (?). This worked for one day and then the same problem started
again.

Now I just took it to the local pontiac dealer ( who kept saying it was just air)
and they changed the thermostat. I was told they test drove it and that it only
functions at one below 1/2, steadily. Which was true... when I'm driving,
however when it's idling it STILLS bumps up slowly to 3/4 the quickly drops
to 1/2. The dealership claims this is well within normal operating parameters,
however I this issue came out of the blue and I have own half a dozen cars
and NEVER heard of this problem... as well I can't find anything about it on
the internet?

Please help me, my warranty expires tomorrow.
Thanks in advance.
Tracey
: )

Answer
Hi
first I'll  answer this:
"Now I just took it to the local pontiac dealer ( who kept saying it was just air)
and they changed the thermostat. I was told they test drove it and that it only
functions at one below 1/2, steadily.
<This is because of the constant air flow across the radiator>

Which was true... when I'm driving,
however when it's idling it STILLS bumps up slowly to 3/4 the quickly drops
to 1/2.
<This is a true statement DEPENDING on the gauge accuracy and the actual temp of the vehicle.>
I do not understand the 8000 and 16000 lbs UNLESS he meant the cap was only holding 8 pounds and the cap was rated as a 16 pound cap. which would make sense.
I would take it BACK to the Selling dealer and have them pressure test the system and check for leaks. That is to MAKE DARN SURE there is no external leaks. Have them put Dye in the system and check with Black light.
I would also ask them to record the SCAN TOOL temp at the time the fan comes on and then goes off. Should come on between 210deg and 235deg and go off 185deg to 200deg. That is a GM standard and GREATLY varies from vehicle to vehicle.
The original complaint , to me, sounded like a thermostat.
The thermostat SOUNDED like it was sticking and or stuck occasionally.
I would also have the radiator checked for flow. A partially restricted radiator will create very intermintant problems.
These are things I would have checked. Check on your local implied warranty laws, you MAY be able to extend until problem is diagnosed and repaired properly, if at all maybe not. I am NOT a lawyer you'll need to check that.

The main item I would have recorded is the Temp at fan on and fan off AND the relation of those to the gauge. That will be MY determining factor.
I hope this helps.
This is JUST advice from experience.
Good Luck
Paul