Hi!

Got the Firebird out of its winter storage and lo and behold the temp issues did not vanish in the meantime
(When I got it, it was way worse with the temp going up well above 240)

My Setup:
89 Trans Am
305 LB9 (0.060 overbored by the former owner around ~8 years back and runs well now, still that might contribute quite a bit!!)
Champion 3 Core Aluminium Rad with Shroud and two electric fans (2400 cfm according to the spec sheet)
Extra Oil Cooler in front the of the Radiator
Airdam present
New Water Pump (High Flow from Rockauto, GMB if I remember right)
New (also tested with other ones) Thermostat (160)
Tried to burp the Radiator yesterday but it seems like theres no trapped air
Aftermarket Temp Gauge (and crosschecked)
Headgasket seems fine (Oil looks ok, Water looks ok, pressure test also ok)

Situation:
(Car always ran quite hot and generally a lot of crappy work which we remediated in the last few years)
-Takes quite a while to get (normal amount of time) to get hot and will creep up to around 185 before getting to the highway
- On the highway it will get up to around 220 (it gets a lot hotter if the heater is not turned on a little bit!!!)
-Only after driving slower again (through some towns etc with ~40 mph) the temp will get down again to ~200

My Theory:
-Not enough airflow at higher speeds to get the built up heat out of the radiator
-Worst Case: Blocked Water Passages in the Block or issues due to the overbore

Where exactly should the Airdam be mounted? (I used the original mountings)
Due to the thicker Radiator it now sits a little behind it, from a gut feeling I would say it would need to be either flush with the Rad or a little ahead?
The Radiator is slanted a little backward to clear properly and that means the fans can probably suck quite a lot of hot air from withing the engine compartment?
Why does the little Heater Core have such a huge effect on the temperature?
Are the electric fans too puny? Do they even have that much effect on the highway? (would be a pain in the *** to get the Radiator out again, the cooler lines for the trans are extremely stubborn...)

 

Did the car cool properly prior to the installation of the High Flow water pump? The reason I am asking is those pumps are made for high HP race applications and you could be pumping the water through the radiator to quickly and not allowing the coolant enough time in the radiator for it to properly cool the coolant. This could contribute to the engine not heating up properly, the water is flowing through the block too fast when cool. The more engine RPM and the hotter it will run as the coolant will flow through faster which could contribute to the higher temps on the freeway.

 

There are a million reasons why a car might run hot, but looking at your list the one thing that jumps out at me is that it runs hotter at highway speeds than lower speeds.

In my experience, cooling issues are usually a result of insufficient airflow across the radiator, but it sounds like you have everything in place to make proper airflow happen and shouldn't be seeing it hotter at highway speed when the airflow across the radiator is at maximum levels. This makes me wonder if maybe there is a fuel / timing adjustment issue, or maybe even something like a clogged catalytic converter.

I don't have a solution to your problem and hopefully these ideas don't make your diagnosis more complicated, but rather give you ideas for things to check that maybe you haven't considered.

Here are some ideas to mull over:
-Check the fans. Make sure they are blowing air the right direction. Aftermarket fan blades can be installed to push OR pull, depending on whether they are mounted in front of or behind the radiator. Some fan blades have to be re-mounted directional, depending on which side of the radiator (push vs pull). Some fans can even change direction based on which wire is used for hot. If they are blowing air forward toward the front of the car you could theoretically have good cooling at low speeds but the opposite at highway speeds with air speed being counteracted by the fans and essentially no air flow across the radiator.
-Check timing and fuel adjustments.
-Check catalytic converter. If it or the exhaust manifolds are turning reddish or seem excessively hot this can be an issue. I had a V6 Camaro that ran great, but the cat was clogged and caused a massive amount of heat from the manifolds. I noticed everything under the hood seemed way too hot and changed out the cat to fix it.
-High flow water pump probably not doing anything for you. Already mentioned in a post above. I have never used a high flow water pump but read a LOT of mixed opinions on them online. Either way, this engine doesn't sound like it has any need for anything other than a stock pump so this might be something to consider.
-Blocked water passages. This seems so unlikely I wouldn't consider it until every last possibility has been ruled out, UNLESS someone has used block/radiator sealer at some point. I have heard too many horror stories of guys using that junk and it plugging up the whole cooling system. I still wouldn't think it would have too much affect on the block itself because it usually seems to plug small openings like those found in radiators and heater cores.

In answer to some of your questions:
-I'm not sure why the little heater core seems to have a noticeable affect on the temperature. My dad showed me that trick when I was growing up and he thought it worked, but I have tried this on numerous overheating cars as an adult and I have never seen it make any noticeable difference.
-I can't quite picture what you mean by the radiator slanted and possibly allowing the fans to suck heat from the engine compartment, but the radiator should ideally be covered on top (between radiator and body) to help keep air flowing straight through it (just like a shroud). I wouldn't think this is really an issue for you though, especially if your fans are mounted almost touching the radiator or have shrouds.
-Your fans might be too puny, but the overheating at highway speeds causes me to think the fan CFM is not your primary issue. I have tried aftermarket s-curve blade fans on cars and have never been impressed with them. Not sure what you are using. The GM stock electric fans work very well, even without much of a shroud. My 1985 has dual fans from a Toyota (did an install post about it a couple years ago), and those along with a Dakota Digital controller completely ended my overheating issues.

 

That's a myth. Coolant does not need more time to cool before re-entering the engine. The radiator is a heat-air exchanger. Constant liquid flow and air flow is what carries heat away from the engine. The reason a high-flow waterpump can cause coolong problems is because of aeration and cavitation, not "time".
Think about it; the more time the coolant is in the radiator, the more time the coolant in the block is getting even hotter.

Additionally, the more rows a radiator has, the more air required to cool. People should be less focused on rows, and more focused on the size of tubes, and the amount of airflow. I would have gone with a 2-row radiator with 1-1/4 inch tubes...

 

@vinny R Way worse I would say before the install of the new Radiator and the new Water Pump (but since Water Pumps are cheap I already was thinking of ordering another one yesterday )
@Aaron R.

• I've already checked the timing, actually it was running way lean when I got it (had to install a new fuel regulator), currently its running a little bit too rich when idling, I will get that fixed also.
• The catalytic converter is new (about a year old) as well as the O2 Sensor (but I have a new sensor already in my trunk)
• Fan Direction is ok, I was wondering if I maybe should use a second set in a pull config? (have a second set lying around)
• The problem is that these aftermarket Radiators are a little bit too tall, that means they are not installed at 90 degress but probably ~80 (the bottom is nearer to the front of the car by maybe an inch (2 cm)

@T.L.
In hindsight you are right, I wanted to get the "better" part (more expensive is not automatically better!)

Made another test run today in a very hilly area with lots of serpentine roads up mountains:

• Heater only on "Low" with the Heater Fan also on Low, went up to 225 on one hill at the end of driving around for ca an hour
• Heater on Full with a few sprints up the road and some cruising in between (same route, just reversed) --> Couldn't get it to 200 (granted, the air was a little bit cooler by then but not by more than 5 degrees more)

Just shows that the issue is to get rid of the heat after it has built up? (I really have to change the position of my airdam or maybe make myself a taller/wider one!?)

 

I did not see it in your original post but I may have missed it. Have you done a pressure test on the cooling system? Air leaks have been known to cause it to run hot. I agree with moving the air dam, the size should be fine.

 

Myth... not for me. I have a stock cooling system and was having my intake replaced. No issue prior to. My mechanic forgot to put my thermostat back in. The car took awhile to heat up on the drive home but never stopped climbing until the car went over 245 and the ECM went into limp home mode. I called him the next day and we where talking about why it would act like this. long story short, my T-stat was under his bench, he never put it in. I asked what would that have to do with it and he said it was because the coolant, was moving too quickly through the radiator and not exchanging the heat into the rad. Simply dropped the t-stat back in and the car has been perfect ever since

 

Actually yes, I did let it get checked because I was not trusting the rebuilt the previous owner did to see if the headgasket was ok --> according to the mechanic everything was fine.
So no resizing, just a move of the Airdam, will do so

 

It should. If it is the original it worked for a smaller cooling system then what you have now.

 

We were talking about waterpumps, but same thing; aeration and cavitation, not amount of time the coolant spends in the radiator.
You're ignoring the fact that the more time the coolant is in the radiator, the more time it's also in the block & heads getting hotter and hotter.
I stand by what I said...

 

One thing to consider is if you have the correct rotation water pump. Serpentine systems run reverse rotation pumps, but can physically be the same as a standard rotation pumps. The pumping vane in the pump is a mirror image and will try to pump coolant the wrong direction.

 

I have heard the water moving too fast argument before and don't believe in it from a simple physics standpoint. Water and metal don't care about how fast each other moves to transfer heat. For example, I can heat up a piece of metal to red hot and then either dunk it in a bucket of water or hold it directly under a gushing faucet. The faucet actually cools it faster every time for a variety of reasons, but the point being that fast moving water cools as good or better than slow moving water because heat is transferred instantly.
I have read a lot of good arguments about high water pressure causing cavitation and hot spots. I have read numerous articles about removing a thermostat causing overheating problems because of the removal of restriction and resulting cavitation. I have personally run engines without thermostats and never witnessed this, but I'm inclined to believe it is possible. This is why I question using a high flow pump in any application not needing the extra pressure or flow potentially created.