1968 overheating problems at idol

[Bill912]

My 1968 Z28 has the original 302 in it. The engine has been completely rebuilt and all the cooling systems have been replaced. It has the large 23 inch radiator. When I stop at a red light for any length of time it overheats. As soon as I start moving it will cool back down. Does anyone have any ideas that could help me ?

Thank you,
Bill912

[vtfb68]

Welcome Bill,
 seems like a air flow issue, be sure it has the correct shroud and fan in the correct factory location. rebuilt to original build specs ?
  Good luck,
     VT

[firstgenaddict]

Check your fan clutch, when it gets warm it should be more difficult to turn, if not then you are not moving the correct volume of air across the radiator, which would explain why when moving the temp drops back --- airflow.   

[Bill912]

Hi , thanks for responding. Here is some more information that may be helpful to you. This is my second clutch fan. It has the correct pulley. It has a GM shroud that is also correct. It is the correct distance between the fan and radiator. I have a correct 5 bladed fan as far as I am aware. I'm wondering if you know if they made a fan with more blades on it. A new big block and Z28 radiator.

[z28z11]

Bill,

Need some additional details -

Thermostat, age and temp ?

Coolant type and mix ?

Rebuilt water pump ? Correct number ? I'd look at impeller type and size if you can.

How are you gauging water temp ? Mechanical or electric ?

Get an inexpensive infared thermometer and check the temps across the radiator, engine block, etc. Gauges/senders can be inaccurate as all get out (recent problem with my son's Jeep with high temp at idle, hooked a Bosch mechanical gauge up to find out the factory gauge with a new sender was 80-90 degrees off. Infared showed 140 degrees across the entire radiator surface, gauge showed 225-260 degrees).

You can always run waterless coolant, get a higher flow water pump if everything checks out.

Regards,
Steve

[TRLAND]

It hasn't been mentioned yet but if your cooling system is all in good shape and you are still overheating maybe it's related to timing.  From JohnZ's Timing 101 paper: "Anyone with a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling".  Are you using ported or manifold vacuum?

[ds1]

I too am thinking timing.  Not enough will cause it to heat up quickly

[mikefam]

How do you define overheating? Do you have a temp gauge? Is it loosing coolant? It may be as simple as a bad radiator cap.

Mike.

[Bill912]

I have a temperature gauge and have changed that 2 times. I am not losing no water. I will try your suggestion with the radiator cap.

Thank you,

[KurtS]

If your parts are new and correct, then make sure it's a real problem (via infraread) and work on the timing.

You can always run waterless coolant, get a higher flow water pump if everything checks out.

Why? There's nothing wrong with the original design. High flow water pump or oil pump is not needed or even desired.

[lynnbilodeau]

Slight variation in temp is normal.  Do you have an actual temp gauge with numbers or the factory C and H where we don't really know how cold C is or how hot H is?

If it is only rising slightly, and you are looking at a factory gauge, I don't know that you have an issue.   Only way to tell is put an accurate gauge on it.

If it is indeed running TOO hot at idle only, then it is clearly an air flow issue.  You said the fan blade was "a correct 5 bladed fan as far as I am aware".   Check the diameter and make certain it is the correct one.  There shouldn't be much of a gap between the outer edge of the blade and the inside edge of the shroud.   My 68 El Camino has an average of about 1/2 inch gap.   Fan blades should be approx 1/2 in and 1/2 out of the shroud.

You said you were on your second fan clutch.   Are you sure it is correct?  At idle, with the hood up, it should be pulling a lot of air at operating temp ("a lot" is only an estimate..... don't know how to tell you to gauge this, but if you have ever "felt" the air flow when one is working correctly, you know what I mean).  Also (and this sounds really stupid, but I have ACTUALLY SEEN this one time on a car that came in the shop with a complaint of overheating at idle) check to make sure there isn't an obstruction of sorts on the radiator.   The car we had come in had picked up a piece of cardboard about the size of a cereal box and it had lodged perfectly against the radiator and just sat there.    Pulled it off and the car was fixed.  Not likely, but don't overlook the easiest and cheapest fix.

In answer to your question, yes they made 7 blade fans.   All the 69 performance models have a 7 blade fan.  Not sure if it bolts to the 68 clutch.   Someone here probably knows.   68 and 69 had different bolt patterns on the WP hub, as well as long pump in 69 and short pump in 68.

Highly unlikely you have any issue other than air flow given what you described.

[Larry]

What fuel are your using? In my 1968 Z/28, which is all stock, I use 100 octane Turbo Blue which burns a lot cooler than pump gas. I've found that most unleaded fuels make a car with high compression run hotter. Remember in 1968 all gas was leaded which burned cooler then the unleaded that we use now. You might give that a try.

[bcmiller]

As others have said "hot" is a relative term.  Is this a fairly new rebuild of the shortblock?  If so, it may run warmer than you would expect.

But honestly, if you are not loosing coolant, it is probably not a big issue.