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I have a '94 3100 Z26; I am having a problem with high temperatures while driving.  
I already have ran the car for about 30 minutes with the radiator cap off to see if any air bubbles come out and didn't see any.  I just now drained as much as I could get out and put new fluid in with no luck.  
I'll go over all forms of the car running to help in getting a solution.

When I am driving freeway speeds in overdrive, the car temp goes over the 3/4 mark.  As soon as I tap the gas, the temperature drops down to about half.  That would give me the sign of low coolant or a random air bubble.  

Driving city in OD, I have the same problem as above.

On the freeway in drive, Temp rises to about 3/4 and doesn't go down unless I put the pedal to the floor.

Driving city in D, Same as above

Last, Idle- Temp goes a LITTLE above half and that is it.  It never gets to the 3/4 mark.  I let the car run in idle for 45 minutes and it peaked once less then half way between the 1/2 and 3/4 mark.

I do have a 25 minute drive to and from work each day so any help would be appreciated.

Could be thermostat.

maybe a problem with your guage

It could be the headgasket too the system becomes unprimed and when you step on the gas it primes again.
A friend had this sort of problem before.

(revenger @ Mar. 08 2006,21:24)Q
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EIt could be the headgasket too the system becomes unprimed and when you step on the gas it primes again.
A friend had this sort of problem before.
Was there any smoke or coolant loss?  
When I got home last night after driving 25 min straight, I took a flash light to the side of the exhaust exit and couldn't see even the slightest of a smoke.  There are no strange smells anywhere, nothing other then the gauge to indicate a problem.  
I can't see it being a faulty gauge cause it acts so normal under other situations, raise the rpms, temp drops a tiny bit when in idle.  I have heat inside the car when the temp gauge starts to move up.
Thermostat is hard to believe because the car can sit in one spot, no air running through, and hovers around the half way mark,  coming off the freeway is like this too.  The gauge can read above 3/4 mark and when the car slows down enough to be in the 7-800 rpm idle range, the temp drops down to normal.

Any other ideas?  I really don't want to be buying another Beretta engine this shortly after getting the car.

Could be a faulty Rad cap.
Water pump worn out or gummed up.

I am now guessing that this is an isolated problem with very odd explinations.  I have received every option of a problem EXCEPT for a clog somewhere in the system.  

I just tried running it at about 2200 rpms (the same as it runs when driving in OD @60-65 mph) in park but nothing, it remained at half or a tiny bit above and would only drop if I pushed it to about 4K.  

It is hard to explain really because it will only really get hot while moving, I have tried everything I can think of to get it up to that same temp when in idle but to no luck.

I geuss I have no other option other then getting everything that would be affected by the cooling system checked out.   I will respond back to this if I find the problem to let everyone else know wha to expect if they have these issues.

Mine was acting like this just before my lower intake manifold gasket dumped. It 'heating up' was pockets of air in the system because all of my coolant was flowing into the oil. You do know there is a bleeder screw for getting air out of the cooling system, right? Its that bolt sticking straight up out of the black tube running right in front of the power steering pump.

The problem seems to have been found, or at least corrected... well kind-of.
Since I am having problems getting the back 3 plugs and the rain starting in mid change, the front 3 have been changed from a basic AC Delco spark plug over to AC Delco RapidFire plugs.  Already running late to an eye test, I didn't see any harm in driving with 2 different types of plugs.  Jump on in the car and head down the road....

Not really paying attention for the first 4 or 5 miles I looked down and the car was running at the 1/4 mark.  Now paying attention I jumped on the freeway, Gauge now reads about 1/3.   Kept on watching the gauge and maybe twice I noticed the gauge over the half mark, once just after I floored it and another slow creep up but dropped back to about 1/3rd area.  

After seeing this drastic change in temperature readings after changing only 3 plugs, I am even more confused on what could be or have been going on with it.

I guess it would only be wise to do a complete tune-up on the car and go from there.  Time to pull out all the synthetics!

Check for mayo on your oil cap. You may have lower intake manifold gasket going out. You don't want that in your oil as your bearings will be shot fast.

I'd try thermostat next.

There isn't tranny fluid floating in your coolant tank is there? Had that happen to me (radiator busted internally)

Oh yeah and get all the plugs the same. you can un-do the 2 bolts in the passenger side wheel well, then un-do the slide bolt, and then pull on the mount to move the engine forward, and then re-tighten down the slide bolt to hold the motor forward. Then you can get to the rear plugs. I can do it and I have a strut tower brace. Don't pull on the motor by the intake manifold, as that could cause the lower gasket to go (it's just plastic)... To re-align the motor I take a rubber mallet and bang on the mount back and forth until the 2 bolt holes align in the wheel well, then put the bolts back in and tight it all up.

I have yet to be able to get it in to have this checked out but recently I have noticed something.  I can get it up to 70-75 mph; drive stop and go; on and off; but as long as I speed up very slowly keeping the RPMS below or at 2500 the temperature gauge stays at about 1/4.  Drive time does not matter as long as I keep the RPMS down, but as soon as I hit the gas and had it downshift, up to the 3/4 mark within seconds and would not go down.
I have also noticed that as long as the temp gauge is at the 1/4 mark, I can squeeze the upper radiator hose.  Yes, they need to be changed soon, but when the car reads 3/4, it almost feels like a there is a piece of PVC tube inside it.  There is A LOT of pressure when the car shows its running hot.  Time of driving does not change how this is too, I can get the car to read hot within a few minutes and the pressure is way up, I can drive the car for hours with the temp showing 1/4 and there is little pressure in the system.
I’m just confused. ÂÂÂ
Two questions, do I have to pull the exhaust header off to get the thermostat out? ÂÂÂ And, how difficult is it to get the lower intake gasket out? ÂÂÂ Any tips?  

Edit:  No signs of any fluids mixing.  All fluids are the correct color and texture.

you dont need to take the xover off to change the thermostat, just think flexible.  The lower intake gaskets a pain.

Sounds like a intake gasket or a bad water pump.  but check it with a temp gun or probe just to make shure that the gauge is correct..

Mr Goodwrench