My 2004 Montana had a rad leak and its now fixed . That was approx a month ago . The temp gauge runs steady all the time but when I slow down the temp from the heater drops , and when I speed up again it goes back to hot .Today after a short drive and I stopped , shut the vehicle off , went into the store ,when I came out and restarted my van the temp gauge was almost to the red . And it would not go down . It appears to have air in the system , because tonight when I left work to go home it ran the normal temp but done the cool air when stopped again . How do I make sure the system is bleed properly ?

engine oil level at right spot on dip stick (oil also cools the engine)
all that and no proof of overheating, the gauge sensor is bad.
step 1 for me would be a new gauge sensor, or scan it first for sure.
or the thermostat is bad.
or RAD cap bad.
what does this mean

"had approx 4 of antifreeze in it" ??? please elaborate this?
i guess,drained it and measured how much came out? in quarts or liters? means the test tool, concentration is 40% not 50% test in car
coolant look rusty color of have oil in it or carbon black dust floating it the coolant?
16 quarts (4 gallons). (ok 4 means gallons i bet) and is.


rad, dead cold.. must be 50% no more , less,
best is when cold , check rad level read cap off, and full
if not full we find out why first. is overflow side tank bad? hose missing or fell off?
Evidence LIST mine.:
overheat has many symptoms. not just the funky dash gauge.
fan belt bad, serp belt damaged or tension'er springs dead.
leaks to the road,
leaks to the passenger well <(not you it was bypassed)
leak to pan,(oil will look like milk chocolate. drink !)
leaks to combustion chamber spark plug tips go toa super ugly nasty mess. 1 or 2 can ..
Engine pings, "aka Knocks /detonation) Gas engines.
Next-Generation engine.
4.7-Liter Magnum®, SOHC 12-Valve SMPI V-8
Loss of power hot. for many reasons but ping is top cause.
noise from engine increases.!
Check engine lamp comes on.? (CEL)???

to prove it overheated , do and scan it, see ECT readings
ECT is the engine coolant temp sensor. it will be 180f to 200F
on a hot engine, and way more hotter.
the rad cap raise the boiling point via pressure. 15psi to
from 212° F water and 50% AF 223f to 268° F
never run a thermostat over 7 years, that is there life span.
(tests before, during or after)
scan tools can be used parked, and running or driving and logging
or parked later, scan and see stored DTC or pending DTC
DTC are errors. and can be very helpful on any failure
like DTC telling ECT readings wrong or too slow.
DTC= diagnostic trouble codes.
also learn that if the coolant level in the RAD is allow by you drop for any reasons, the ECT can lie to you with air pockets at the ECT
or even cause a gauge surging reading, up and down , madly.
learn now to check the RAD level properly
some of these cars the engine has, 2 coolant sensors.
ECT and the Gauge sensor , the later is 1 wire sensor and can fail.

so it is overheating or the sensor lies. or coolant levels were low check cold next morning at the RAD cap off, (buy a new cap)
a 19 year old cap or thermostat is very wrong, that old.
Hoses must not leak hot engine , hood up, seen with eyes.
there is simple IR GUN test for the RAD to find bad clogged RAD core tube. be shame top change every related part on the engine
and not do the IR Gun test, to find cool running tubes, that are in fact clogged up.(deep inside)

the only hard problem to diagnose this is water pump impeller blades rusted down 1/2 way, ouch

there is RAD pressure leak down test.
there is RAD test device that discovers exhaust fumes and has blue/green chem reaction to tell the fact.

this is my 20 year old car answer.
there is more, some RADs have trans cooler inside and when rust out and coolant is sent to the transmission(omg), so trans oil is checked for the milky effect. (dip stick looks)
I hope our sender was bad. (gauge) and 1hours work and $5 cost; DIY

Frost plug, water pump leaking?

I am a little confused here, you say the temperature gage is in the red but the engine is only at 140-160? Is that Fahrenheit?

With the car running because it reduces the back pressure have someone that knows how to do it remove the cap from the radiator and see if the radiator is full, not the bottle but the radiator itself. If the antifreeze is not moving from the bottle to the radiator it will overheat. Not having heat unless you rev the engine makes me think the radiator is either not circulating the water or it is low.

A fan is on or off not slow do any fans in your house run slow because they are old?Your thermostat fan or temp sensor is bad

more probable is fan clutch

Yes that is cool if it's correct. 195 F is approx.as low as you should have. I'd change it. Be sure to run with cap off, and heater on (not defrost) while refilling system. Wait for rad fan to come on 2 or 3 times before you put cap back on, then add coolant to return bottle as needed. Make sure rad fan only comes on when up to normal running temperature. If it runs all the time, it may be causing low coolant temperature too.

You need to check coolant level in rad, when cooled down, and top up your return bottle to marked level.
Start by having coolant (antifreeze) ready, remove your rad cap, and top up the rad level, then start the vehicle, and turn on your heater (not defrost) . Let it run with cap off for a while, and keep an eye on level of the rad & top up as necessary, but leave an inch or so from top.Let it run & observe your guage while doing so, and when gauge gets to normal operating temperature, your rad fan should come on, run for a while then shut off for a while, then back on again after little while, and off again. Let it do this about 3 times, top off the rad as described above, put cap back on tight, and top up your coolant return bottle to mark on side. If your rad fan doesn't come on around normal operating temperature, and gauge heads too much towards hot reading, shut vehicle off, that's an indication that your rad fan fuse has blown, or relay, or fan itself is defective. If all works well & you now have heat, great, but keep an eye on your coolant return bottle level for next while to see if it keeps going down, and add as needed if it is. If the level in bottle continues to go down, you've got a leak in the system somewhere & need to address it a.s.a. p. to hopefully avoid bigger problems, but we'll get to that only if we need to. Do what's up above and let us know the results.

I had the same problem with my 2001 Pontiac Montana and the intake manifold gasket. This is a major problem with these 3.4L engines, if you are in the states I'm pretty sure there is a class action law suite against GM for this. I had my manifold gasket replaced and it was about $1000 CAD. You need to get your vehicle looked at right away as this could cause major engine damage, coolant will leak into the engine and soon enough you will be replacing that too. Good Luck