1996 Monte Carlo overheating/air in cooling system

I have a 1996 Chevy monte Carlo Z34 3.4L DOHC

I replaced a leaking water pump and flused my coolant and it seems all hell has broken loose.

I stated to run hot 3/4 to red line while accelerating in town and driving on the higway. I would put the car in nuetral while driving and rev up to 5,000 RPM and the temp gauge would immediatly go to 1/4 or 1/2. I would shut off the car for a minute and then the start up to find the the temp went down to 1/4 or 1/2 way. I started to open the bleeder screw on the thermostat housing (while the engine was off and hot) and notice that only air was coming out. I was advised that the intake manifold gasket was bad. the intake air was escaping into the coolant passage due to a bad seal around the coolant passage (i have pictures). Air was getting trapped around the temp sensor and not sensing the temperature of coolant. It was sensing air temp; there not made to sense air temp. So, my fan was never turning on. when i removed the air it ran at normal operating temperature (good fan Cycles) at idle speed in the driveway until i put a good amount of load and we would start all over.

After replacing the head gasket; Cylider heads (because of valve guide sliping), intake manifold gasket, temp sensor, radiator hoses, thermostat.
I have reversed the problem. Know the engine runs hot after 10 min from sitting over night on the road the fan will never turn on. I will purge air out of it and it will run and normal operating temp the rest of the day, fan going on and off when needed. After sitting over night the problem will repeat itself. I have been doing this for a week and I figured that thier is just air in system from filling after fininshing the repairs.

I am not having any coolant looses either

Is any one having the same problem.

That sounds typical of entrapped air in the system. There should be at least one, probably two, and possibly more air bleeds in the cooling system. These bleeder valves need to be opened when the system is filled to prevent air entrapment. There is usually one on the thermostat housing, one near the water pump, and may be one on the intake depending on the year.

Sounds like you replaced a lot of stuff for nothing. I don't doubt the intake gasket was bad, but I do doubt air was entering the coolant system thru the intake port. If anything, it'd be the other way around since the coolant is under pressure and the intake air isn't(unless your running boost).. The next time you park the car for the night, (do this while the engine is still warm)fill up the coolant overflow tank and let the coolant system purge/fill itself overnight. When you walk out the next morning, the coolant level in the overflow should be lower than it was the night before. You might have to do this a few times to purge all the air..

their is only one bleeder location on top of the thermostat housing. I have been bleeding air out of this for two weeks.

Here is my procedure
i check the radiator level while cold. it is always full even with the bleeder open. run the engine util it get hot and starts runing 3/4 on the gauge. I turn off the car and open the bleeder until all of the pressure is releaved. I check the fluid level in the radiator and add if need.

As stated before there is a new head gasket, intake manifold, intake manifold gasket, reman cyliders, upper and lower radiator hose, water pump.

This problem started when i replaced a leaking water pump. Has any one heard of problem with autozone water pumps. Instead of the cast fins, this pump had four sheet metal fins. I am starting to wonder if this pump does not have the same efficiency as original. It may just recerculating air.

this intake gasket is junk. I have pictures of the seal being sucked into the cooling passage. It is my understanding while traveling on the expressway the air pressure is greater than the coolant pressure.

There's around 14" of vacuum in my intake while I'm running down the highway.. Yes, those waterpumps are junk, but they sell both kinds still, i believe. What do you mean by remanufactured cyl? Did you try topping off the overflow as I suggested? Tested the radiator cap?

I have tried leaving the bleeder valve open overnight (when it hot) and fill the overflow tank to the top. It only dropped about a half inch and i beleive it was due to the coolant leveling out. the container on the ground caught a good portion of the fluid. I am pretty sure that the coolant was hot enough to leave the thermostat open. (i have change the thermostat twice).

My next move is to change the water pump. I must not getting proper flow when putting a good load on the engine. the very little air that is left should be pushed to the radiator with no problem. Also, i will try changing my serpintine belt. Maybe it is slippin. that is bit of a stretch though.

Any other thoughts.

Your not supposed to open the bleeder valve. When the system cools down, it creates a vacuum in the system. The radiator cap allows the coolant in the overflow to be sucked into the radiator, and flush out any air bubbles remaining. When you open the bleeder, you never get the vacuum. You have the classic signs of air in the system...

So I figured out the problem.

The water pump that was installed was not producing enough flow. When I removed it had sheet metal fins. This look complete different than the production one. The production had cast fins and three more fins. Since i have this water pump my temp gauge runs about 1/4 occasionally running close to 1/2.

Here is my theory for the air. The old pump was not have high enough flow to cool down the engine. So the coolant was coming to a boil leaving it to surface behind the thermosat. This is forces the temp sensor to read air, which means the fan will not turn on. When i purged the air this allowed the thermosat and temp sensor to work normally for the rest of the trip.

Lessons learned don't always assume that new parts are the proper parts for your engine.

I had a similar problem on my 03. No heat, over temping. I did a pressure test and it held for 10 min., but the system didn't seem to draw coolant in from the tank. After replacing the radiator cap which was bad, the water pump, and thermostat, I found a small leak in the straight hose from the bypass pipe to the throttle body. I never saw fluid leak, but it seems air was able to get in under suction, allowing air to get into the top of the engine and the cause of my problems. After replacing this 50 cent piece of hose, bleeding the air from the 2 bleeds, there is suction from the tank and I have heat!

96 monte with 3.4 motor .Had same problem the temp gauge would pin, however with a scanner hooked up the coolant temp sensor would read normal. Long story short, it was an aftermarket water pump with a sheet metal impeller. I replaced the pump with a delco that had a cast iron impeller end of problem

Same problem here. My story is better. lol

Thought maybe new water pump was defective so I got a replacement made by "someone else" still had the problem. Check Radiator. 1/3 obstructed. ahh-ha..installed "new" not reconditioned radiator..still had problem. Tried two thermostats. settled on backing down to a 180 instead of the 195 required.
helped but did not fix.... Replaced the Temp sending unit..didn't change anything.Started testing cooling fans. replaced relays. It didn't help. Bench tested fans. one was drawing a bit too much power than it should have. replaced it. Didn't change a thing. During the process the heater core developed a pin hole or less minor leak. (lost a cup of coolant over a week) found heater core issue. replaced it. still didn't solve the "mystery" temp spike. Flushed and bleed system time and time again. Even had a shop mechanically flush and fill. Didn't help. All this time Mechanics a scratching there heads just as I. So I ran the ac all the time. Presto the additional cooling fan kept engine temp at a normal safe level. But still actually had the original problem. Then I ran in to a shade tree mechanic that talked of all the same stories. Air, head gaskets, ect ect. And then he said "It was and is none of the above". go get an A/C delco water pump. cast iron fins and not pressed steel. To quote him "I had the same problem". "It was the replacement pump the entire time". It works fine on the 3.1-3.4 simply doesn't like the DOHC design. Well,Lesson learned !! So, there is something to "genuine parts and giving up the extra $3. unbeleivable. Total count three different shops. alot of money. More head scratching.. And it is the strangest thing I ever seen. Guess what? Someone is buying one of those after factory replacement pumps as you read this. There in for some fun. I'm still wanting to throw something. But all those new parts under my hood are Pretty!
So, Who has a better tail chassing story? lol

having almost the exact same problems, finally replaced the poopy autozone pump with an ACDelco replacement that looks like the stock water pump. However my temp gauge will frequently jump sometimes all the way to full temp. would this be due to the thermostat reading air???

having almost the exact same problems, finally replaced the poopy autozone pump with an ACDelco replacement that looks like the stock water pump. However my temp gauge will frequently jump sometimes all the way to full temp. would this be due to the thermostat reading air???

No its not. I had the same problem once, and what i found out is lower grade water pumps cause to much ai rin the system witch makes the thermostat sick open at times. I would replace it.

this post is over 2 years old... this is over and out..

Yes, this post is old. However, still Interesting the problem continues as the manufacturers doesn't seem to stop selling pressed steel impellar pumps that do not work. One "other" thing I've found is alot of guys are using Chiltons and Haynes reference material for a guide. Those books reference the "one" bleeding point above the thermostat. Look on the firewall in the area of the brake booster. There is a second bleeding screw on the line running to the heater core (Haynes doesn't mention that one chances are Chiltons likely doesn't either) At least I can tell you that the second bleeding screw is located there on the 96 Monte Carlo z34. This one is the key to filling the system and getting most of the air out when you fill the system. If you don't open that one you will be messing around for awhile bleeding the system of Air. I've been there done that. Also, removing the steel impellar water pump and replacing the pump with the ac delco pump (or a pump with the cast impellar) doesn't solve the problem immediatly something else is happening obviously. Pressure testing the system seldom lies. About the only thing pressure testing doesn't cover is the radiator cap. (unless the kit your using does check caps) I've seen a resevoir line plugged before and that can give you some issues during a refill. This obviously doesn't allow a place for any air to escape during initial warm up or allow the system to pull coolant back into the system during cool down. Not trying to sound elementary here folks. I simply understand your pain. This problem drove me nearly nuts untill I came across the fix.