'74 Getting Hot

[Lenny K.]

About a month ago the upper hose on my '74 L-82 Auto 350 suddenly gave out...splitting about 6" worth, thankfully about 2 blocks from home....geez...only about 10 years old too....lol.

After replacing the hoses, which I had already set aside for "when I got around to it" and cleaning up the resulting mess, I noticed that the temperature was seemingly running hotter than usual....not overheating per se, but I usually ran about 195-205 with a 190 thermo. I checked the rad cap, etc. and all looked right. Cycled the radiator and ensured that it was full after the mishap. OK....changed the rad cap and thermo to a 160 just to be able to see that it was basically cooling and yes, at idle, it does not go over 160 or so....gauge is fine and shot it with a thermo gun.

I have about a 20yo radiator that looks just fine, the water pump seems to be fine, belts are tight and the viscous fan looks to be just fine. Leaves me with a bit of a mystery as to why the temp seems to be running higher than usual at speed. I live in Phoenix and the ambient air temps are higher right now....we are amidst 100+ temps...but it just seems to be running hotter than in the past....on the freeway speeds it runs upwards of 210-220 at speed.....my dilemma is what could be causing this, or am I being too anal since it is not redlining!?

My thoughts go to:

1. Time to flush/replace the radiator...a bit rusty perhaps, but seems to be flowing inside with the cap off and engine running.
2. Advance is set too high
3. Internal "problem" in the engine
4. ????

Does anyone have any clues as to what else I might look at.....any advice appreciated....especially from anyone else with a '74 auto. My RPMs at 70mph seem a bit high...about 3,500...could the T400 not really be going into a high gear!? Looking for suggestions and thanks for any comments and observations.....

[Timothy B.]

Re: '74 Getting Hot

Lenny,

I would start be installing the correct 190 thermostat back into the engine. The radiator is full when the cold coolant level is approx 1" below the top. I am not sure what the owners mannual says about the 1974 but that's what is in the 1967 service manual.

If all the radiator seals are in place and you are running 3500rpm in 100* weather it's probably fine at 220*. With a 50% mix of antifreeze and a good 15lb radiator cap the engine should be fine up to 240-250*.

You may also want to have that cap checked to make sure it holds it's stamped pressure.

[Edward J.]

Re: '74 Getting Hot

Lenny, Alot of time a plugged radiator can cause a slow temp. rise while driving at hwy. speeds,scale builds up internally in the core and limits flow through the radiator core which causes a slow temp increase and sometimes if bad enough can cause overheat. If the radiator is of older vintage its possiable that the problem.

Another check you may what to look at is the radiator cooling fins, they can plug up with debis limiting air flow. If the car has A/C check condensor fins as well , and check betwen the radiator and condensor for leaves and debis. make sure the sealng strips around the rad. are make sure there still in place.

[Lenny K.]

Re: '74 Getting Hot

Thanks for the suggestions.....I am going to put the 190 degree back in but wanted to just diagnose with the 160 so I could see what was hoppening. The radiator is probably 20+ years old....hesitant to get it flushed, but may do so. This whole concern was just that I noticed that before the hose went, things just were cooler.....maybe just a vivid imagination on my part, but anyway like I said, it is not OVERHEATING per se, just running hotter than it did....

Any further suggestions and observations are appreciated....but the obvious things look fine.....

[James S.]

Re: '74 Getting Hot

I had similar problems with a total resto on the engine, radiator, etc. on a 60 Vette I installed a high flow thermostat and that really helps the flow of fluid back to the radiator. And as posted check the cap to ensure pressure is correct.

[Michael W.]

Re: '74 Getting Hot

The radiator is full when the cold coolant level is approx 1" below the top. I am not sure what the owners mannual says about the 1974 but that's what is in the 1967 service manual.

'73 and up Corvettes have external non-pressurized coolant recovery tanks.

As such, the radiator is designed to be run 100% full at all times.

[Paul L.]

Re: '74 Getting Hot

I had to replace the fuel pump on my 1974 with A/C a couple of days ago owing to a faulty diaphragm (you will find this job described in the dictionary under cruel and unusual punishment). Today I managed a 30-mile test run including highway speeds. Everything is A-OK!

The engine is an L-48 and I too have the TH400. At 70mph the tach was reading 2,800rpm. The L-82 may have different gearing to account for your 3,500rpm but it does seem a little high. Perhaps someone with a C3 L-82 automatic can make a more informed comment.

*Edit* - Ambient temp is 75*F and the car runs at 180*F. As per Mike's comment I do have the chin spoiler and appropriate rad seals. I did NOT have the A/C on.

(thumbnail)

[Michael W.]

Re: '74 Getting Hot

Lenny-

Is the chin spoiler attached and in good condition? All fan shroud seals present?

[Martin N.]

Re: '74 Getting Hot

LENNY-
Is your car A/C equipped? If so also make sure that the condensor is clear of debris to allow air flow to reach the radiator cooling fins. Also make sure you "burp" your baby to get all the air out of your liquid cooling system. That's bit me on a car or two in the past.

Marty

[Kenneth T.]

Re: '74 Getting Hot

Lenny,

Ed has a good point about the old radiator having some clogged tubes. You mentioned you have a therm gun, take the car out and get her warmed up real good and use the gun to check for hot / cold areas on the core to see if it is in full use, you may be surprised.

Ken

[Lenny K.]

Re: '74 Getting Hot

Just got a sec but thanks for the observations and suggestions. Originally had air but working parts removed last engine rebuild....smile. Have the nose piece and shrouding in place, no visible damage to core/fins....will shoot it after getting it hot so see if clogging might be an issue....LK

[John H.]

Re: '74 Getting Hot

My RPMs at 70mph seem a bit high...about 3,500...could the T400 not really be going into a high gear!?

Lenny -

IF your speedo and tach are accurate, 3500 rpm at 70 mph calculates to a 4.11 axle.

It's been my experience that 90% of cooling problems are related either to the radiator or to the airflow through it. Highway speed shouldn't be an airflow issue unless the condenser and/or radiator are clogged with bugs, dirt, etc., or the lower air dam is missing; C3's are "bottom-breathers", and if the original air dam is missing, you WILL have highway-speed cooling problems.

That leaves the radiator as the culprit.

[Rich C.]

Re: '74 Getting Hot

Is the car basically stock? The 3500 RPM'S @ 70 mph sounds pretty high!

[Jon D.]

Re: '74 Getting Hot

With the engine at operating temp and thermostat open snap the top and bottom radiator tank with an infared temp gun. Snap the top tank by the inlet hose and the bottom by the outlet to the water pump. If the differential is more than 12 to 15 degrees the core is restricted internally impeding flow. Also check everything that has been mentioned so far.

On the 74 there was a 4 inch piece of hose attached to the nipple (inside the coolant recovery tank) on the bottom of the cap for the coolant recovery tank to draw coolant back to the radiator. Check to make sure it has not come off and laying in the bottom of the tank under cloudy coolant. This was very common allowing the system to suck air.

[John H.]

Re: '74 Getting Hot

With the engine at operating temp and thermostat open snap the top and bottom radiator tank with an infared temp gun. Snap the top tank by the inlet hose and the bottom by the outlet to the water pump. If the differential is more than 12 to 15 degrees the core is restricted internally impeding flow. Also check everything that has been mentioned so far.

Jon -

Could you expand on that or clarify it? In my 50 years of working on cooling systems, I expect to see the coolant temperature exiting the radiator at least 20* cooler than the temp at the radiator inlet, preferably 30* cooler for a properly-functioning system. Increased flow restriction in the core will result in a smaller temperature differential between the inlet and outlet.