Have a question regarding my 455. If my 1980 cutlass only overheats on the highway above 3000 rpms does it have to be a flow issue across the the radiator? This is my 3rd summer of driving this car around and have given it plenty of hot suppers and done plenty of highway driving and it has always ran under 200 degrees . I haven't changed anything it just all of a sudden seems to run extra hot under sustained higher rpm . it will run and drive all day in traffic or just cruising and never gets hot . i will check my vac advance and all in timing and all that stuff when i get time but not sure why it would have changed .I did do a coolant flush and rinse but honestly think i just wasted a lot of antifreeze for nothing as everything was nice and clean.thanks

 

While this can be caused by a malfunctioning advance mechanism, overheating at speed is frequently caused by blockage in the radiator. I had exactly this problem on a truck and it turned out that the lower 1/3 of the radiator was plugged. A new radiator fixed the problem. The diagnosis was simply holding my hand next to the face of the radiator when it was overheating. The lower third was cold.

 

"​This is my 3rd summer of driving this car around and have given it plenty of hot suppers..."

What are you feeding it?...🤣🤣🤣 (sorry, couldn't resist 😁​​​​​​

 

Likely a steady diet of Prius or Domestic Hybrids....
tc

 

The radiator would be my obvious suspect as well, I agree with joe. At first I was going to ask if you were using the original radiator seeing as how Oldsmobile didn't use a 455 by then so there was a possibility that the radiator designed for the small block of 1980 would not be sufficient to cool a big 455 but then I remembered that this is the 3rd year. Unless it took 3 years for the smaller radiator to get blocked up somewhat which puts us right back to where we started.

 

yeah this was a transplant .I’m using a 4 row aluminum I bought at summit when we put this all together it has always cooled nicely I’m going to check the bottom of the radiator as Joe suggested and go from there .

 

Good move.

 

I picked up the car at lunchtime and drove it to work got it good and hot and this is what I found with my cheap ir temp guage holding it 2 inches from all surfaces measured. The temp guage in the car read 180 ,top hose 166, bottom hose 155, top radiator fins162 bottom radiator fins 87 degrees . It would seem that the bottom part of the rad is plugged ( how this happens with a new build and a new radiator I’m not sure ) but I will pursue it from here and post results . Thank you for the replies

 

I did a good flush of the engine with the garden hose until the water was nice and clear then blew out the residual tap water with compressed air . I then backflushed and rinsed out the heater core ( separately from the motor ) followed by compressed air. Finally got to the radiator and filled it with a mixture of clr and water and let it sit for about an hour then rinsed several times . I then put everything back together and filled with antifreeze mixed with distilled water. I only ran it for a short time before it got dark and started raining , but the bottom of the radiator was noticeably warmer .a good run down the highway will tell me if it worked. The one thing I never did on this build was use distilled water in the cooling system . In fact I’ve never used it in anything I’ve ever owned in my 58 years. Maybe my hard well water had something to do with all this rusty silt that could have plugged some of my cores .

 

Yeah that's possible. There was a recent thread about coolant, distilled, tap, de-ionized etc. It generated quite a bit of discussion and the general consensus at the end was that there isn't much difference. However, who knows what is in your well water, it may have some particular something in it that your radiator didn't like.

 

Inspect the lower rad hose. If it collapses at highway speeds that is considered a restriction. Does it have the spring in it? Does it feel soft?

I like the 50/50 premixed coolant. No guesswork just pour it in, Tap water can contain excessive amounts of minerals = not good for the system.

Distilled water does not have the mineral load which makes it ok to mix with straight coolant.

 

Just wondering if that can physically happen if the system is pressurized to 15+ PSI.

 

I have often wondered the same thing. However, GM didn’t spend a penny more than needed to build these cars. If the spring was unnecessary, GM wouldn’t have put it there.

 

Any chance the spoiler under the radiator support is missing or damaged? The spoiler is there to creat a low pressure area behind the radiator, which help force airflow thru the radiator.

 

Matt my spoiler ,and fan shroud are all in good shape .My lower hose has no signs of collapsing. im pretty sure this was a case of clogged lower half of radiator.Thjs does make a case for the larger tubes in the radiator with less chance of clogging with finer debris.

 

If you look around, I believe I remember seeing articles that show 2 large rows perform better than 4 small tubes in aluminum radiators. Could be a flashback of a hallucination, too...

 

The water pump will cause a ▲P (differential pressure) between the in & outlets regardless of system static pressure. A weak or soft hose will collapse in the right conditions.
Hose technology is supposed to have evolved to eliminate the need for the anti-collapse spring. Emphasis on the word supposed. (think chineasium).

 

if it has a clutch fan still I would bet its starting to go bad. If it is not spinning free at highway speeds it will can cause the air moving thru Rad to slow and not remove enough heat from Rad.
next is T-stat is not opening fully at operating temp.
Put a timing light on it and check the vac advance operation also.

 

I understand about the delta P but I question if it is enough to collapse the hose. Seems it would take negative pressure to make the hose collapse. And if it is a weak hose, will it collapse enough to restrict coolant flow? I have never seen any data that supports this claim.

 

Sorry guys but I guess it's time to revisit this thread as my overheating seems to rearing it's ugly head again . I thought by doing all the flushing and cleaning I was good to go the temps seemed to be doing good although it was September and I had only taken the car out a few times . Ffw to now( 90 and humid here in the northeast )I went to get gas and give the car a good exercise before the upcoming car show and my temp was right back in the 230 range . I got home and felt the rad and was cool as can be on the lower 1/3 of it .yeah I know points to a clogged lower portion of the radiator . I drained all the coolant and tried flushing with cool water and the radiator is nice and clean and I can see water flowing through all the tubes nicely. I have checked all the usual suspects again thermostat, clutch fan belt tightness . Vacuum advance and all seems normal . So my question now is . Is it possible the impeller in my water pump is slipping under load ants giving me the circulation I need ? When the car is up to temp with the cap off I can clearly see antifreeze flowing through the radiator, but is it enough ? As I said earlier in this thread I never changed anything with this motor it just seemed to start happening .

 

What gauge are you measuring with? The factory piece is more of a suggestion than accurate reading. Could be a bad sending unit, too.

 

Overheating on the highway is usually radiator related. If the bottom 1/3 of the radiator is cool, that 1/3 of the radiator is blocked and isn't dissipating heat. Flushing won't clean blocked radiator tubes, rodding them will. Before rodding the tubes I'd try a treatment or two with CRC ThermoCure but if the results are the lower 1/3 is still cool the radiator needs to be rodded or replaced.

 

I have auto meter gauges and have checked it with my ir gun and they match pretty closely I’ve always shut it off before it starts boiling over although it’s percolating pretty heavily .

 

Why not just replace the radiator or have it refurbished?

 

even though I can see water flowing through the tubes ? This summit 4 core radiator only has about 2000 miles on it along with the rebuilt 455 . It all just looks too clean to be blocked . I’m not saying it isn’t possible but was wondering if a water pump flow issue could act this way

 

I may have to go this route if nothing else works

 

Unless the impeller has corroded off the water pump shaft (which, while not impossible, is certainly improbable), there is no way a pump can cause overheating.

 

I doubt anything can be seen flowing through the lower 1/3 tubes and even if it is flowing through those tubes it still isn't dissipating heat in 1/3 of the radiator. It is easy to understand being baffled by a newer radiator having a blockage but the symptom and test strongly indicate that it does. If you have a radiator shop local perhaps they could remove a tank to inspect the core.

As Joe stated water pumps are rarely the reason for overheating, IIRC I've seen one with rotted impeller fins cause an overheat. The countless others made noise, were loose and wobbly or they leaked.

Delaying fixing this could cause engine and transmission problems.

 

Ok judging from the responses it’s highly unlikely it’s a water pump issue . So I’ll go back to the radiator. Too late to replace it before the Adirondack nationals so I’ll run it as is and will report back with my results.

 

I didnt see you answer if you had a spring in the lower hose or not?

 

no I don’t have a spring in the lower hose but see no signs of it collapsing while running it at high rpm sitting in the driveway . I may put one in while I have it apart though just to eliminate one more possible issue

 

Give it a try, it cant hurt

 

The spring isn't the problem. I don't have a spring in any of my hoses, and none of my vehicles overheat. The cooling system is pressurized at 16 psi above atmospheric.

 

what i meant by seeing the flow in the lower tubes was when i fill up the rad with the hose i can see water coming through the tubes on the opposite side as it fills. at any rate it got another flush and new coolant. the good thing is i can drive it all day as long as im not cruising on the highway at 3500 rpms

 

Understood but at the end of the day the cold spot is a place that doesn't get rid of heat. A CRC ThermoCure treatment would be worth a try. Flushing with water forever won't clear blocked tubes.

 

^^^x2

 

Because the problem just started after per your report two years of cool operation, I too would suspect the aluminum radiator assuming nothing else has changed or has been changed. Aluminum radiators have caused problems in the past due to galvanic action caused by a reaction of differing metals and coolant perhaps but not specifically rated for the aluminum in the system. I have heard and read but not from authoritative sources the 'rated for all' parts house coolants actually are not rated for all, but are satisfactory for cars at the end of their useful life to allow the cooling system to soldier on a while longer. Few people actually add the tablets GM used to specify in coolant changes for aluminum engines and cooling system parts, to prolong the useful life of the metals involved.

So if it were me I would take that drive on the freeway and get to a speed where the overheating starts, then carefully move the shifter to neutral and coast a few hundred feet, then back to drive position increasing engine RPM and see if the engine temp takes a abrupt drop. This can indicate coolant moving too quickly through the system or as a distant second the possibility of any radiator being compromised. Then I would mix a 50-50 blend of muriatic acid and any water, fill the removed radiator with this solution out on the ground, with perhaps a couple of joined hoses to prevent leakage, then maybe let it sit for maybe 30min or a hour. Then drain it, and if you see schmutz or scale or vomit or really dark water I too would suspect closed or clogged pipes regardless of how old your radiator is. Rinse and flush all associated parts very well before reinstallation.

I have been amazed in the past how well new aluminum radiators cool engines even bored way out, but there is a trade off in short life due to corrosion as discussed and cracking and leaks from work hardening of the aluminum material. Not all aluminum is the same, and some of the radiators out there might as well be made of pot metal and low alloy, soft as they prove to be in use. Good luck

 

Just tossing out some information: car radiators do not work on the evaporative cooling principle, so the humidity level is totally irrelevant to the system’s operation.

 

I’m using factory copper/brass in my big block ‘66’s.

I’m no radiator guru, but can run in traffic without stressing out these days. I appreciate that low speed is different from high speed overheating, but here are some things that helped my engines cool down:

1) Rear fan shroud that covers and seals (or is pretty tight) to the ENTIRE back of the radiator. This ensures that whatever air the fan is pulling is running across the radiator cores. In ‘66 Olds only made one shroud which covered the Cutlass radiators completely, but left the big cars about 30% uncovered. They cheaped out, in my view, but it’s solveable with a couple of extra shroud chunks from later Olds…

2) Radiator rubber curtains on the front of the radiator which prevent incoming air from flowing around the radiator and force it through the core. In my ‘66’s, there were 2: Left, Right and Lower. Just for good measure I added some closed cell foam folded over between the top of the radiator core and the cover to prevent air from flowing over the top of the radiator.

3) Heat-operated fan clutch & 7 blade GM factory fan. Hayden has a bunch of models which can fit old Olds, see my post on clutch fans from 2016 or so. I think I’m using a 2747 in one of my cars now. Any maybe 2705 in the other. I can’t recall off the top of my head.

4) My heater cores are known-good. I.e., I’ve replaced them both in the past 5 years or so. The soft brass (or now cheapie aluminum) is not that sturdy so sooner or later your passenger’s feet get a glycol bath. I look at that as an opportunity to be sure that there’s no blockage in the heater core…

5) If you have A/C, make sure your water valve is operating as designed. It’s another potential blockage.

6) I’m not a great believer in secret sauce. However I can report that my temperatures dropped 10 degrees or so when I switched from 50/50 premix using tap water to this combination: 70% distilled water; 30% antifreeze; 2 bottles of water wetter/photoflow per car.

7) For decades I ran 160°F thermostats & have recently switched to 180°F. My understanding is that Olds big blocks were designed around a 180° running temperature. I’ve had to get used to running a little warmer, but my temps stay <195 most of the time, expect on 95°F days in traffic.

I hope these tips help a bit. Best of luck. I hope it’s just a plugged radiator.
Chris