96z28 Overheating after new Waterpump install and sitting for 1 year ---

[alwayscode390]

I had the opti, waterpump, and heater hose changed at the dealership a week ago.

The car will idle for hours and be fine, but the SECOND I hit the highway (50+mph) the thing shoots up to the red (250 degrees).

I tried bleeding it today from the bleeder screws, and by leaving the cap off the radiator. I bled from one screw to another back and forth, and in about 30 minutes both were peeing steady. Do you have to bleed it after multiple drives to get everything out?

THE SECOND I put it up to 3k rpms the temp goes straight down to 180-190 degrees....so I have been driving it home in a lower gear at 65mph.

I am going to guess its a 1/2 stuck thermostat. Blocking 1/2 the flow, so when the waterpump kicks it in high gear it makes up for the blockage? I saw the upper hose move the coolant the other morning with the cap off the radiator, so I know its atleast opening up a bit.

This just goes against everything I understood about engines, usually they overheat at idle ... and stay cool at 50+mph hahaha.

Time to swap out the thermostat. If its not that, I guess the next step is to clean out the radiator?! The car sat for a year and a half before the parts were installed.

I thought since a lot of you guys are pretty talented mechanics, I would ask for some advice here, THANKS ---

 

[Junior Samples]

1. weak lower radiator hose collapsing at highway speed.

2. plugged radiator.

3. If clutch fan...the tension of the clutch can weaken causing the fan to turn too slow at higher rpm's.

4. blown or leaking head gasket. causing vehicle to overheat when operated at higher rpm, the crack doesn't expand until temperature is excessive.

5. a mouse or rat may have built a nest in the muffler or exhaust pipe. T causing back pressure to build up at highway speeds but not at short low speed in town driving.

just a couple of guesses anyway

 

[alwayscode390]

AWESOME, thanks for the post.

I am going to try a new radiator cap and thermostat for the quick/easy/cheap fixes. Then I am going to go for flushing the radiator.

Its electric fans, and I hear them kick on.

I dont think its a blown head gasket (yet) ... the fluid level stays high, there is no smoking, and the engine runs GREAT ... very smooth ---

 

[Junior Samples]

If its at operating temps at idle then stat is open.. flowing providing its been completly burped. heck could have a plastic bag stuck to outside of radiator.

My first guess was weak lower hose, open hood and rev up and check to see if hose collapses.

The rest of the options I just googled/copy/pasted for you. 20 secs of search beats 20 mins of typing with two fingers.

Gluck

 

[Rodney87GN]

Do you still have the plastic air dam underneath the radiator support? It will run hotter at highway speeds with that air dam missing.

 

[DailyDrifter]

Do you still have the plastic air dam underneath the radiator support? It will run hotter at highway speeds with that air dam missing.

+1, those cars are "bottom" feeders.

 

[Chuck Leeper]

YEP! On the air dam deal....

 

[aminga]

AWESOME, thanks for the post.

I am going to try a new radiator cap and thermostat for the quick/easy/cheap fixes. Then I am going to go for flushing the radiator.

Its electric fans, and I hear them kick on.

I dont think its a blown head gasket (yet) ... the fluid level stays high, there is no smoking, and the engine runs GREAT ... very smooth ---

My sister had an 85 Monte with a blown gasket. Never lost a drop of coolant but it would overhead at highway speeds.

 

[joemama]

I bet it is a radiator that does not work efficiently at highway speeds.

 

[alwayscode390]

The collapsing hoses MAY be the deal.

I also keep hearing to burp the system, drive ... burp the system ... drive.

I may need to keep bleeding it. It was too late the other night to keep working on it after I bled it the first time. LOTS of air came out that time, so it may still need work.

THANKS GUYS ---

 

[TurboBob]

THE SECOND I put it up to 3k rpms the temp goes straight down to 180-190 degrees....so I have been driving it home in a lower gear at 65mph.

trapped air will airlock the cooling system, this may explain the RPM sensitivity. so burp it and heat cycle it several times.

LT1's are very prone to trapped air.


I assume the waterpump is installed correctly. (could be a defective water pump, it is the last thing you changed......)

Bob