I'm having cooling problems on my 69. I have a 69 427 w/ A/C cooling set up. (Smaller, skinny brass expasion tank. There isn't a filler neck on the radiator) When I drive coolant seems to warm up normally, (with full coolant) the temp stays at about 210 mark. However, when I shut down, coolant begins boiling off and coming out of the overflow in the expansion tank neck. Regardless of how much coolant is in the system, it seems to overflow until there is too little coolant to cool the system. I suspected that the cap didn't keep sufficient pressure on the system to keep the coolant from boiling without the water pump pressurizing the system so I replaced it. Got the same results.
Question is do you guys think I have a cooling efficiency problem or a system pressure problem. In other words, the cooling efficiency is such that I have to have too much water in the system to to keep it cool, so it continually overflows, or it overflows because of some sort of pressure problem?

 

I would start by doing a proper deep flush and going from there.

Check this thread.

http://forums.corvetteforum.com/show...php?t=1582996&

 

When the engine (and system) are fully warm, the expansion tank should be about 1/2 full, at least that's how mine runs. My tank is about 6" in diameter and aluminum. I'm running the factory L-88 radiator. And down the highway at 55 or over, my temp will climb. I'm not getting enough air forced thru the radiator. Car is just too slick without a chin spoiler. I don't have a/c either..

Maybe you can figure out how to get more air thru the radiator. Are all the seals in good shape around the core support? That will make a difference.

 

Thanks for the advice. The deep flush is in progress, the coolant looks good, but there was some rust in the radiator stop ****, so this is a good first step. Little intimidated by the post on AS100 and trying to figure that out, but thanks for including it. Think I'll do some good back flushing through the block/ radiator/ heater to ensure it's all clean.

The radiator seals are a bit ratty looking. Additionally, it looks like someone ground down the ends of the cooling fan blades at some point. There's probably 1.5 inches of clearance between the ends of the fan blades and the radiator shroud.

Any takers on the system pressure discussion? I'm confused as to why the temp appears normal when the car is running, but as soon as the water pump stops, it immediately starts boiling over and dumping water overboard.

 

www.dewitts.com
take a look at his web .... on the top of his page click on downloads, then on the left click on cooling.pdf... some very good info.

 

This is perfectly normal and to be expected. First, it's not the water pump that pressurizes the system, it's the expansion of the coolant when heated.

The engine core is always hotter than the coolant temperature when it's running. The coolant absorbs some of the heat to be later 'dumped' to atmosphere via the radiator. The net coolant temperature we hope to see on the gauge is the modulation set point of the thermostat.

As long as the radiator has sufficient heat exchanging capabilities and enough air to dump the heat, all is well.

When the engine is shutdown, the flow is stopped and the coolant still in the block becomes hotter than normal running state due to soak back effect. If the resultant extra pressure in the cooling system exceeds the rating on the cap, or if the system is overfilled (there's not enough airspace in the system to allow compression) the cap will allow the excess pressure to vent overboard.

If your car is running at 210 all the time, you've got serious problems. How old is the radiator?

 

The system should not boil over unless something is worng. If it maintains a good temp while running and boils over after shut wown the first thing I would suspect is the cap, put a new one on there, they are cheap. The Pressure cap is there to keep pressure on the system to keep it from boiling over. Water boils at 212, your normal operating temp is 210, that is with coolant flow. When you shut down, the coolant in the block will go well above 212, but will not boil if the cap holds pressure, boiling further expands the coolant. Look it up but I think its a 16 or 17 lb cap. ANyways, a new one should solve the problem

 

Great redirect to the John Hinckley article. I actually had printed that one out months ago, but rereading it really helped.

Radiator is original (I suspect) and deep down, I suspect that a new one is the route I'll have to go eventually, although I dread that....

First step is to head to NAPA for a new cap/ thermostat. After that, more aggressive flushing, next radiator out for new seals/ mount refurb and more flushing and cleaning. If all that fails, I suppose I'm in to a replacement radiator. I guess cheapo's like me who hate bubba and hate spending money are in the wrong business...
Thanks for the advice and wish me luck!
cheers
pj

 

If the cap replacement doesn't do the trick the second thing I'd look for is a blown head gasket. A small leak would give the symptoms you're describing where combustion gasses leak into the water jacket and over pressurize the system.

 

SHouldn't the clearance at the fan/shroud be tighter?

 

Hi,

I also have a 69 vette with a bb 427. No A/C.

When I bought it the previous owner was complaining about a similar problem than yours. The temp gauge was on 210 when driving around 50 or above but when the engine was running slow or shut down the temp was climbing in the red zone and cooling fluid was boiling and escaping by the expansion tank cap.

It came from several things: dirty circuit + air pocket in the circuit + tired thermostat.
We drained the circuit, flushed it, put a new thermostat, refilled the circuit paying attention to remove any air pocket.

It fixed everything. I can drive in the traffic without having to always keep a eye on the temp gauge and no droplets nor steam anymore when I shut down the engine.

 

Great news. Thanks for the reply. Can you explain a little more about clearing the air pocket? Just open the system at various points to ensure there is no air trapped? thanks again!

 

Yes indeed,

Plus, the most difficult hose to get completely filled up is the big one that is connected to the front side of the air intake.

We disconnected that hose with the neck from the air intake, kept it vertical, filled it then we used a simple metal plate to seal it, all by hand. We got as close as possible to the manifold and removed the metal plate as quick as possible pushing the fitting against the seal on the manifold. We almost did not loose any coolant. But it took us several attemps!

If after all this does not work for you, you can also try to put a bigger water pump. The coolant will flow quicker in the circuit so it will keep a lower temp giving you a bigger differential temp between the coolant temp and the engine block temp. This greater dif temp will give you more room when shutting down the engine and could allow to stay below the boiling temp of the coolant. But not sure it is that good for the engine as you will run below 210.

 

First of all, I agree with what Mike Ward said, even tho I don't understand half of it. When I was a kid driving poorly maintained old cars I noticed on several occasions that a radiator hose would blow AFTER I shut the engine off - so I figured out that the hot engine superheated the coolant once it stopped circulating through the radiator. You should also keep in mind that a superheating engine block can also melt your wiring AFTER you shut the engine off. We highly recommend battery shutoff devices to avoid having your vette catch on fire after you shut it off and walk away from it!

I have a 68 bigblock, but I do have a radiator cap. I was having similar puking coolant problems, especially after shutdown, and it turned out I was simply overfilling the radiator. When you have a rad cap you are supposed to only fill up with coolant to within 3" of the filler neck - three inches is a lot of room. The remainder is air, so I am not 100% clear on this airpocket talk. I don't know how you are supposed to determine the fill level in a system with no rad cap, but I would consult the service manual.

I went through all the simple easy stuff, drain and flush, drain and flush, new high flow thermostat (Robert Shaw or Mr Gasket) sealed off the radiator (used pipe insulation) but the big surprise was how much cooler the car ran after I installed ceramic coated headers.


Here is a hint, but this is the 68 service manual, not 69, emphasis added by me

 

good call "ROBERT SHAW or MR. GASKET thermostat" and maybe try more timing.... Lars or Barry and others on timing....i'm running 50deg. total timing on a mild L36.... 90+ air temp running 180 thermostat and the gauge says 195..... 70 mph... (new DeWitts aluminum radiator and tank)

good luck!!!

 

BB fans are supposed to be very close to the shroud. Less than 1/2 inch clearance is usual.

 

First... Do you have the correct pressure cap on the coolant system?
Secondly... Is the cap working properly?
Third....What thermostat do you have in the system?

The 69 vette had a 180 degree stat in it origianally. Your car shouldn't run at 210..ever...with a 180 stat in it. Also, if your pressure cap is the right one and is working correctly, your system shouldn't be boiling. Even after you shut the engine off and the latent heat in the block raises the temp some, it shouldn't be going over 230 degrees (and with the right cap it shouldn't boil at that temp.).

You could have a lot of built-up lime/scale in your radiator and block. Get a bottle of radiator CLEANER (not radiator flush) and follow the directions [to the letter]. Afterwards, put in a 180 deg. stat and a new radiator cap. I think your overheating will be fixed.

 

I also have a 69 L36 but no air. When my engine started doing this it was after I was messing around with the engine timing. I think engine timing is critical with these big blocks. Your fan being 1.5 inches away from the shroud does not sound right either. I think mine is about 1/2 an inch away. My engine bay is all factory correct. As previously stated make sure all your radiator/fan shroud seals are good and you still have that little black spoiler mounted under the car. I use a 160 raditor stat. Good Luck