My 98 4spd vette idles at 187 but runs pretty hot driving locally like about 220-230. Driving in bumper-to-bumper traffic is fine but once I get to an open highway driving a constant 60mph for about 10 mins my temps jump up crazy to about 240-250. Once I start letting off the accelerator temps starts dropping and when I get off the highway back to driving locally temps returns to about 220-230. But temps would jump up the second I pushed it a little.

Low coolant message but the reservoir is filled appropriately.

My radiator fans run. The radiator surge cap holds pressure as well.

No air dams bought the car like that. (I've read that it does not affect temps, but some people say it does, idk)

Recently had to top off the coolant bc it was below the full mark(didn't flush it tho, but burped after topping off), no apparent leaks except for water from the AC on the passenger side. Not losing coolant after topping off.

Blown head gasket?
Bad Thermostat?
Hoses?

 

A common reason for running hot at speed is lack of air dams. Hundreds of posts on that FACT. Get new ones including all three and put them on!

 

I agree with Jim. The airdam needs to be in place on any bottom feeder car.

Also, I wouldn't be surprised if your radiator and condenser are clogged with all kinds of debris.

 

Install AIR DAMS!!!

 

 

Airdam absolutely a must have on these cars and sounds like the reason for the heating issue.

 

Yep, airdam. Need that low press area under the eng to pull out the under-hood heat and increase flow across the radiator. My vette sucks up tons of leaves that accumulate on the face of the A/C condenser and will block airflow if left in there, especially this time of year.

 

No air dams=running damn hot. If you look at your situation, it is obviously airflow related. 220°-230° around town/stop and go is totally normal, and your fans are obviously working fine. When you get on the highway with no air dams, you're not getting enough airflow. IIRC, the C5 is engineered to cut off power to the fans at speeds of 35mph, and higher. Not sure if the higher temps would override the programming, or not. You may also be confusing people saying they don't run the radiator cover (I would leave it on, GM engineers put it there for a reason) with not running the air dams. You should have both the cover and all 3 air dams in place. My opinion.....

 

All great advice again guys. Hopefully the OP will take it and reports back.

 

Saccity corvettes has a remote switch for the fans that work real well also

 

Air dams ARE important and aid in getting air into that big air entry under the front facia!
220-230 is absolutely NORMAL! DESIGNED to run at that temp. Running hotter at speeds greater than 35 mph is NOT normal and indicates a lack of proper at flow at speed when the fans are not running.

Getting air through the AC Condenser & the Radiator is important. High mileage / Well Aged C5/s frequently suffer from partially clogged radiator/condenser cooling fins. There is a 1 inch space between the condenser and the radiator that is very difficult to see/inspect. That space frequently collects grass, hay, leaves ,sand, paper etc... Recommendation: Remove the air bridge, Remove the plastic cover over the radiator/condenser. (four fasteners) Inspect the space between the radiator/condenser for dirt and debris collected on the face of the radiator. Use a garden hose on full stream and back flush the dirt out/off the air fins of the radiator and condenser. (NOTE!! WARNING!!!) DO NOT USE A PRESSURE WASHER!
Install those air dams!

How are you burping your cooling system?
Have you actually measured the water to antifreeze percent? What is the actual freeze/boil over reading.

GM change the recommended expansion tank cap pressure from 15 lbs to 18 lbs. Recommend upgrading to the new cap.

IF, you check/ do all the above and no change, I would drain the system, flush, change the T Stat, burp properly and see where that gets you.

QUESTIONS?
COMMENTS?

BC

 

When I acquired my 2K auto it ran hot. I noticed in my parents garage an air dam looking thing on the work bench (I inherited the car from my parents). This forum guys like here pointed out the air dams and radiator cleaning. Sure enough my condenser was covered in leaves, and 2 plastic bags from Publix along with the missing air dam. Cleaning the debris and installing a new air dam solved my running hot issues, this was when I lived in Florida.

 

12/27/24 UPDATE:
I just installed air dams as well as cleaned the radiator from debris and now temps don't go over 187-189. Thank you everyone for the advice.

 

 

Question, what does changing the tank cap from a 15 lb to an 18 lb cap do?

 

Well,
1. Most old aged caps don't hold pressure well or vent properly when the engine is cold. So you probably need a new cap. Why not up-grade to the new one with a higher pressure?. .
2. If you live in a VERY HOT CLIMATE, or stress your engine, the added pressure inside the cooling system will allow the engine to reach a hotter coolant temp before the coolant boils. Under normal operating conditions, "NOTHING"
3. Just a little more flexibility in keeping the engine coolant from boiling over and turning into steam. With the correct water to antifreeze solution, good hoses, belts and a good Cap , a LS engine can endure a lot of punishment when it comes to keeping the engine coolant from boiling over.