Greetings! 95 LT1 'vert, 6 spd, 69k miles. Yesterday took first long highway drive since purchasing ~ a yr ago. I95, RTE 9, I84 in CT. Rolling hills, 50-70 degrees and sunny, spds 65-80. In 6th gear, coolant temp increased to a max 0f ~235, shift to 5th, @ same speeds, coolant temp lowered to 212-218. Oil temp pulled exact reverse, 220-230 in 6th, 235 in 5th. (I'd expect oil temps to go down as engine rpm decreased). -Once I figured this out, I lowered speeds to 65 for rest of trip and stayed in 5th to keep temps down.
Once off highway, coolant temps back to normal 205 to 220-230 as fans came on and went off. Around town, car is fine.
Nothing wedged in radiator/ac fins (checked twice at rest stops), and I had washed out the core area with water pressure early this past summer. Coolant level is fine, Penzoil 5W-30 synthetic in pan. I have not flushed the system however, my next step.
I've had many GMs over time, but this is my first reverse-flow. Any corelation, or is the extra rpm at spd making up for gunked-up core tubes, or...?
Otherwise, the car is wonderful and has required nothing other than regular maintenance (yeah, I know flushing is maintenance).
Appreciate any thoughts!

 

Flush and fill with water wetter would be a good start. In my 95 the coolant temp on the highway is 159 about 170 in town. head temp stays around 185-195.

 

Do a complete system flush and replace with a 50/50 mixture of distilled water and dexCool or your favorite ant-freeze. Check your t-stat (submerge in boiling water). Check your hoses. Consider buying a "Big Mouth Air Dam" for about $100. They really work well (keep temps down) at speed, 40 mph +. Also, you might want to pull the radiator and clean it (between the fins) with a good hose.

Do the above for starters.

 

For a stock car, your temps are perfectly normal. My car runs the same temps.

 

Those temps are fine given the driving and air temps. The later C4's (and most all cars after 1990 or so) ran "hot" using a 195 degree thermostat in order to help with emissions. Higher engine temps help with more complete burning of fuel and reducing certain gasses.

6th gear is about .50 to 1 so driving where you put a load on the motor like hills or accelerating in 6th can cause a rise in coolant temps. Lower RPM's causes the water pump to run slower.

At 69k miles and 12 years old, you should do a coolant flush and refill. GM used Dexcool as the factory coolant. Dex has an effective life of about 5 years, so if has been that long since the last cooling system service, it needs to be done.

If the prior owner did not have the cooling system serviced, there may be some clogging in the radiator core. Is the coolant still a bright reddish color or is there some brown in it? Dex will turn towards a brown color if it has been exposed to too much air in the system or other contaminants being introduced from lack of maintenance.

Replacing the thermostat wouldn't hurt either as would new upper and lower radiator hoses. You could go to a 180 degree 'stat, but that may affect emissions testing if you have to go thru that.

Oil temps at cruise speeds will typically be 10 to 20 degrees above coolant temps. It takes a little longer for oil temps to drop compared to coolant temps. Synthetic oils will handle temps above 280 so that should not be a big worry.

 

235 on the highway seems high to me. Is your front spoiler OK? If its damaged and not deflecting enough air into the radiator the car can run hot at higher speeds. Check for crud between the radiator and the AC condenser. Flushing the radiator is definitely a good idea too.

Good luck, Paul

 

Thank you for the quick and thorough replies! (car is stock btw). Flushing needs to be done anyway, will do a thorough job, check the stat, replace the hoses, then re-evaluate. Interestingly, it is running standard green coolant, not dex-cool.

 

'95 was a transition year for coolant. Early ones had the green, later ones had Dexcool. Mine was made in April and had the green stuff.

 

Apparently somebody didn't like DexCool and installed the green ethlyene glycol coolant. The two coolants are not compatible and if somebody did not do a really complete flush of the OEM DexCool, there could be brown gunk in the radiator or even in the block and head passages.

There is no easy way to inspect the radiator coolant tubes but it might be worth removing the radiator and have it professionally cleaned. The radiator is not too difficult to remove and you would be removing the radiator hoses anyway to replace them.

With the green coolant already in the car, you can stay with that. Use a 50-50 mix of good quality green coolant and distilled water. To do a complete flush, you will have to remove the knock sensors from the bottom of the block as that is the best way to get all of the old coolant out. Messy job, but worth doing. Don't forget to clean the overflow tank!

 

pm454,

Our 94 (100k miles) does the same thing (temp up in 6th, drops in 5th), and A/C on doesn't make a difference as the ECM commands the fans off at highway speeds.

However, I too am not quite ready to accept it as 'by GM design'. I would like to think that when our cars were new they did not react this way.

Therefore, giving consideration to age, wear & tear, I have speculated it could be caused by:

1. Original water pump with poor blade design and slightly erroded blades from either electrolosis or cavitation reducing coolant flow through the engine. I am considering replacing the water pump as there may be a small oil leak on the seal anyway.

2. Upper radiator hoses' angles restricting coolant flow as the rubber has aged and is slightly restricts coolant flow. I have new OEM hoses which appear to be slightly shorter and should reduce the angles, especially on the hose closest to the engine.

3. Coolant flow from the throttle-body bypass hose as it enters the uppper radiator hose 'T' (which has build-up on it), may slightly 'interupt' the flow from the engine and/or causes some air bubbles which in of themselves somehow affect coolant flow. Considering replacing the 'T' with more of a 'V' connector so that the bypass hoses entry angle is not perpendicular to the radiator hose flow.

4. Radiator core is slightly clogged and restricting coolant flow. Will be pulling radiator to clean the fins, and may take it to a shop to have flushed, but since it is an aluminum core, shop selection may be difficult as it cannot be flushed with solutions used for normal (copper) radiators. So alternative is to just replace radiator with an OEM (< $200), might be easier/cheaper in the long run.

5. Air flow to Radiator changed due to either bumper height changes (slightly springs/shocks), air dam support brackets changes (bent brackets) or debris in the A/C condensor fins.

6. Design change to coolant component which did not get to after market companies (ACDelco included). For exampe, what if the thermostat's opening 'spec' was supose to be larger. Or the radiator cap was supose to be a higher pressure or not allow the initial pressure to escape until the coolant closes the metal seal.

The things I know it isn't are:

1. old Anti-freeze or air in system -- flushed system with 50/50 'green'/distilled water and purged all air in various ways (although the 'race engine' does not work on our expansion tank cars).

2. Thermostat -- new ACDelco 'stock' unit tested and installed.

3. Radiator cap -- new ACDelco model installed.

4. Radiator fins clogged -- frequently clean condensor and radiator.

 

It makes sense for the coolant temp to be less and oil temp to be higher when driving the same speed in 5th as opposed to 6th. the engine turns faster hence coolant flowing faster and engine running harder

 

I don't think they ran that hot on the highway either.
I would think that the '94 is not much different than my '96, and at highway speeds (5th or 6th, it doesn't matter), the temps. stay in the 195-197 range all day long.

Of course, if I stop and let it sit in traffic I get the very normal 220-230 temp. range.

I think if your running hot while traveling at speed you have more of a coolant flow issue, instead of an airflow issue.

 

Thanks for all the additional thoughts and analysis. It's all been very helpful. Can't set aside a day until late next week to pull her down, but am leaning toward replacing the radiator (with the hoses and stat, etc). Based on other alloy/plastic radiators I've had, I just know the tank/core seal will just start leaking sooner or later anyway. The car is worth the expense . I will also look at the water pump. I hadn't thought of that angle.
It is interesting that the engine turns about the same rpm @ 70 in 5th (or 6th now, I don't recall) as around 40 in 4th where, w/out the fans running, it runs cooler. That also leads me toward coolant flow rather than airflow as the potential issue. And if after all the work it stays as it is, well, we'll figure that out if it happens.
I'll post an update when I have data to share.
Many thanks!
-pete