1997 Monte Z34 w/strange overheating problem

Or something as simple as a huge air pocket, the hot air passes over the guage sensor and reads really hot and the coolant cools it back to normal. The temp sensor for the ecm is lower so the air pocket never hits it. With how touchy the v6 is with the air pockets it wouldn't be a surprise.

I did replace the radiator cap - i bought one that has a temp. gauge in it so I could cross check the temp at the cap against what's happening at the gauge and at the computer - not surprisingly, it always reads low. For a head gasket breach to be involved, wouldn't there be some signs of coolant loss somewhere? Either venting to the outside of the engine, and dripping - or into the combustion chamber and showing as white smoke in the exhaust? None of that is apparent, and the coolant level stays constant. I've heard of the air-pocket problem before, but I don't know how to get rid of it. I tried letting it run with the radiator cap off, and then topping it off when the bubble dropped the level after reaching that point - but it never happened. Also, the fill neck is angled such that coolant tended to spill out. There's a vent fixture at the top of the thermostat housing, but it leaks fluid as soon as I open it. Is there some other way to bleed the air out of the cooling system?

Head gaskets aren't generally where coolant enters the combustion chamber. Coolant usually gets in around the intake. A leaking head gasket will either dump coolant into the crank case, dump coolant outside, or the hot combustion air will enter the coolant passages causing a very quick boiling over of the coolant and extremely overheated engine, from running normal to guage is pegged in about 10 seconds max. What condition are your radiator hoses in? Mushy or kinda stiff. Check while engine is cold.

These cars are prone to leaks at the intake manifold gasket as well as at the timing belt cover. It could be either one. Do a pressure test of the cooling system. That should tell you exactly what the problem is.

That's what I'd suggest. You can also take the gauge out to be tested at a specialty shop if you don't trust the dealer.

Your answers are well and good, but I had the same problem last 2 yrs, and it drove me crazy. I ended up re-building the Engine and the problem came right back. Overheating, gauge would stay in one place running normal then spike and sometimes it would drop. Short story is that I had replace 6 thermostats and finally I put a thermostat made in ISRAEL. That really helped, but, every now and then it would climb and not overheat. So just two weeks ago my A/C pump died and I went threw the process verifying that it was dead, jumpered the connectors at pump, and when I went to the Pressure Switch, I found that the 3 pin connector had either a bad pin or broken wire. I replaced the connector and now THE HIGH SPEED FANS WORK AND I HAVE NO MORE ISSUES WITH OVERHEATING OR GAUGE FLUCTUATING ALL OVER. IT STAYS AT 182 DEGREES AS THAT IS THE THERMOSTAT I PUT IN. So it seems the GM had bad connectors. Actually I love my Z34 but I would kick the ENGINEERS ASSES IF I COULD FOR ALL THE STUPIDITY PUT UNDER THAT HOOD. WHY DO ENGINEERS GO TO SCHOOL? TO BE STUPID? GOOD LUCK.

GM engineers make enough money to pay someone to handle there problems so they never see them first hand or work on them. The higher ups get give back the company car for a new one so they don't get the experience of having anger issues.

Roger that. I should've recorded audio and video for all the swearing and comments directed towards them and forwarded it too them, so they could see first hand. And with your answer I worked years back for FMC CORP. as a Radial, CNC Machinist and it was constantly revisions on Military Equipment. I'm surprised that it worked, Missile Launchers, Gun's etc.

When I was in the Marines, carpet bombing was close enough. God was busy sorting them out back then. Now they just enter through the smoke stack or front door. It still takes feet on the ground to get that accurate. I calibrated equipment that calibrated instruments that went into the jet aircraft. It is funny how little it takes to give a false reading. We had to test even the springs that went into the gauges. I was introduced to the "Cannon" plugs, and they were a bitch to fix when one pin went bad, or one solder wire was corroded or lose!

So the Monte's been sitting at the base of the driveway for nearly a year. I've been using alternate vehicles. But last week, my wife totaled her Prius so I had to resurrect it. I pulled the upper radiator hose, and the thermostat housing. Removed the thermostat (left it out), noticed that the housing to engine gasket was missing, and replaced it; Installed my 'franken-hose' (upper hose sectioned to splice in an autometer temp. sending unit). Installed a new battery (what a pain!). The car doesn't overheat any more. I had hoped that the removal of the thermostat would give me an extra 10 minutes of road time, enough to complete the commute to the office. But it's just running cool, after about 20 minutes, it starts to move up into the normal operating range. Haven't tried to take it out for any long trips yet, maybe this weekend.

Older cars you can get away with that type of a fix. If you live in the colder part of the US of A, in the winter time you will need the thermostat or cardboard over the radiator.

If you're Z34 is overheating and you're scratching your head. Look no further as Like I said before. Check the 3 wire connector at the Pressure Swithch. Wiggle it and your most likely to hear you're High Speed Fan kick in. The connectors are bad apparently because there are so many posts on this same issue. I've offered my advice so you can have a normal running car again. And I live in MN and I never had too put cardboard in my Z34 with a 180 degree thermostat, had plenty of Heat during the very very long cold winter we had, It ended on the 2nd or 3rd of May 2013. The next day on the 4th it was 70 degrees. Amen. So I didn't get what you were getting at Carl? Because you need to read what I wrote about the real problem. And that would be the CONNECTOR AT THE PRESSURE SWITCH. FIX IT AND IT WILL END THE PROBLEM.

Thanks, I will try this.

I had the same problem right after the dealship replaced the water pump. Took the car back the next day after noticing the temp gauge going crazy up and down. The dealership couldnt find a problem or reason. Well...years later that water pump went bad and what I found was it wasnt the correct pump for my 1997 z34 3.4L monte carlo. That pump (part no.89017269) had only 6 impellers, and the correct pump (part no. 12371989) had 9 impellers. Installed the correct pump and problem solved. To be fair, I found the incorrect pump listed incorrected in GM parts list as the right part for my car. Lots of pumps will "fit" but are not the correct one. Apparently the wrong pump with only 6 impellers was not moving the antifreeze fast enough to cause the gauge to operate properly.

What does the outerseal of a thermostat look like on a 98 Monte Carlo

What does the outerseal of a thermostat housing look like on a 98 Monte Carlo?

Quad ring it fits around the themostat