Overheating when shut off

[Charles F.]

Here's the story: 68 L36, 4 speed with A/C. When I got the car two years ago, it ran about 190 degrees, but when as soon as shut off, the temp soared and the car puked coolant from the overflow hose. Tried a Dewitt radiator: car ran 170 degrees but still puked coolant when off. Posted the problem here, but no definitive answer.

Fast forward ahead 16 months (now): just got the car back from a pro frame-off restoration. Engine bored and stroked to 496, cam changed and now dyno at 475 HP and 593 torque on pump gas. Still puking coolant when off, but running at 190 degrees. Changed radiator cap to 24 pound cap (from 15) and the problem went away .

Had found a porosity at the intake manifold which was epoxied and the air through the manifold stopped.

Engine was built at Precision Automotive in Connecticut (reputable place). Block was inspected etc and all seemed well (no water jacket blockages). Timing is spot on and engine was dyno tuned. No vacuum issues known. Stock carb.

Being told that this built-up engine puts out more heat...doesn't make much sense to me as the problem existed with the engine in stock form. Most everything was either rebuilt or replaced.

Concerned that the increased pressure needed to solve the puking when turned off will stress other parts of the system not designed for this pressure (i.e., hoses).

Although the car no longer pukes with the racing radiator cap, I would still like to know the potential causes of the problem. Any thoughts ?

Thanks

Chuck Faillace

[Duke W.]

Re: Overheating when shut off

...'68 emission controlled engine - probably has ported vacuum advance, Change it to full time and replace the VAC with an Echlin VC-1765 or equivalent as long as the engine pulls at least 14" at idle with about 25 deg. total idle timing. If less than 14" you need a VC-1810.

The problem will probably go away, and you can reinstall the 15 psi cap.

Duke

[Stewart A.]

Re: Overheating when shut off

Duke he mentioned that the car has been bored. Could the sleeves be getting rather thin and on shut down heat sink be effecting the temp.

[Patrick H.]

Re: Overheating when shut off

Too much coolant in the system.
It'll puke until it finds its correct level, and then stop.
This is a relatively frequent occurrence.
Most people think the car overheats, so they add more coolant when the system cools down and perpetuate the problem.
Just leave it alone.

[Michael W.]

Re: Overheating when shut off

Just leave it alone.

And stop looking at the temp gauge.

All engines will 'appear' to overheat after shutoff due to heat soakback from the block into the coolant. Perfectly normal.

[Charles F.]

Re: Overheating when shut off

Definitely not too much coolant in the sytem...been very careful about that. When cold, the coolant level is at least four inches from the top of the radiator.

The issue is not the coolant temperature (not worried about what the temp gauge shows) but the fact that there was a half gallon of coolant flooding my driveway when I shut the car off. I could hear the system boiling: gurgling noises, then steam, then the coolant puke.

Don't think it has anything to do with the rebuild because it was acting exactly the same way before the rebuild.

Duke, thanks for the information. Will pass it to the rebuilder this weekend and see what he says. I don't know the current specs in this regard.

Chuck

[Jake D.]

Re: Overheating when shut off

You could install a coolant overflow recovery system. If it bothers you that it is not factory, take it off when you have it judged. With the kit you also get a new radiator cap.

[Bill M.]

Re: Overheating when shut off

A friend of mine got to drive a '68 427/400 automatic with A/C for a week in the fall of '67. This car was brand new. It puked coolant out of the overflow every time it was shut down...

[Patrick H.]

Re: Overheating when shut off

Definitely not too much coolant in the sytem...been very careful about that. When cold, the coolant level is at least four inches from the top of the radiator.

The issue is not the coolant temperature (not worried about what the temp gauge shows) but the fact that there was a half gallon of coolant flooding my driveway when I shut the car off. I could hear the system boiling: gurgling noises, then steam, then the coolant puke.

Chuck

...and those are the exact symptoms of too much coolant.
I thought the same thing too as I had the coolant level right at "the mark" on the radiator. When I finally left it alone and let it find its own level, it stopped puking.

But, you're welcome to chase this for as long as you'd like.

[Duke W.]

Re: Overheating when shut off

Duke he mentioned that the car has been bored. Could the sleeves be getting rather thin and on shut down heat sink be effecting the temp.

A lot of the mass inside the engine is hotter than the coolant, and after shutdown this heat migrates to the coolant and increases coolant temperature.

Heat always flows from high to low temperature. Ported vacuum advance retards the timing to well below optimum at idle and low speed, so a lot of the internal mass of the engine, especially in the head is hotter than it would be if the total idle timing was in the ideal range of mid-twenties to low thirties deg. BTDC, so not only does the engine run hot at idle and low speed, but the coolant will heat up a lot more after shutdown.

This was a problem with early emission controlled engines, and that's why GM eventually added coolant recovery systems to virtually all configurations, and it's fairly easy and unobtrusive to add one to an engine that was not originally so equipped.

"Thin cylinder walls" theoretically should retain less heat and transfer heat more readily to the cooling system. Overboring is often blamed as the cause of overheating, either during running or after shutdown, but in most cases I believe its due to issues with the spark advance map. especially when the cam is changed from OE.

Typical "engine builders" are just clueless about this issue. They are basically mechanics, not engineers, and they don't understand engineering issues.

Duke

[Stewart A.]

Re: Overheating when shut off

Duke in laymans terms could you explain to me the spark problem causing the overheating If it's not to much trouble. It's over my head. I have 2 Hot Rods that over heat and would like to follow this path to see if the that's why it could be getting to hot after shut down.
Stewy

[Duke W.]

Re: Overheating when shut off

Send me an email (NOT a PM) and I'll send you a paper that should answer your questions.

[Paul H.]

Re: Overheating when shut off

Too much coolant in the system.
It'll puke until it finds its correct level, and then stop.
This is a relatively frequent occurrence.
Most people think the car overheats, so they add more coolant when the system cools down and perpetuate the problem.
Just leave it alone.

I agree. Can't rule out other causes as Duke mentioned but overfill is the most common reason for puking and overheating is mistakenly blamed.

[Duke W.]

Re: Overheating when shut off

I sent out a couple of the vacuum advance spec papers by Lars Grimsrud that also has a section authored by me, but let me try to summarize the issue.

One has to think in terms of the spark advance map. Engines are not one-dimensional. There is much more to them than peak power at high revs, which is how most guys think. An engine has to idle and operate at part load over a wide range of engine speeds. Because the throttle is an analog device there are virtually an infinite number of speed-load conditions, but we can limit the discussion to three conditions - idle, cruise, and WOT.

The optimium spark advance is a function of speed and load, which can be represented by engine speed and manifold vacuum, and the following general rules apply.

1. Rich mixtures increase flame propagation speed, lean mixtures decrease flame propagation speed.

2. Low density mixtures (such as part load when the engine is throttled) decrease flame propagation speed. High density mixtures (WOT) increase flame propagation speed.

2. Exhaust gas dilution decreases flame propagation speed

3. An engine may not tolerate the ideal spark advance for some operating conditions due to detonation.

The First Law of Thermodynamics - "energy is conserved" - requires that we account for all the fuel's energy. It either goes to producing energy at the crankshart, goes out the exhaust, or is dissipated in the cooling system.

The optimum spark advance may be determined experimentally by testing the engine at many speed/load conditons to find the spark advance value that maximizes power under any condition (without detonation), including part throttle/load, and this will also correspond to lowest EGT and minimum BSFC.

At idle we look for the highest idle speed/lowest EGT.

Using a SBC as a typical example the optimum spark advance ranges are as follows:

1. Idle - low 20s to low 30s deg.
2. Cruise - 40 to 54 deg.
3. WOT - 36 - 40 deg.

Such a spark advance map can be developed using a combination of centrifugal and vacuum advance.

At idle the required advance can usually be placed in the proper range with the sum of initial timing and full vacuum advance. Engines with high overlap cams have a lot of exhaust gas dilution at idle so a they require something in the upper range.

Cruise is a rather broad condition - depends on gearing, etc. The 365/375 HP engines can tolerate a very fast centrifugal curve due to the late closing inlet valve that results in relatively low DCR relative to the SCR and the high overlap creates a lot of exhaust gas dilution at idle and low revs/load. The 24 degrees of centrifugal is all in at 2350 RPM, so at most cruise speeds above 2350 the total cruise timing is initial, plus full centrifugal, plus full vacuum, say 14 + 24 + 16 = 54 deg. At idle the 14 initial plus full vacuum yields 30 and the total WOT timing above 2350 is 38.

A 327/300 needs less total idle timing and cannot tolerate as quick a centrifugal curve due to detonation. The '66-'67 300 HP engines have 30 degrees of centrifugal, but it's not all in until 5100. With 8 initial, and a correct 16 degrees VAC, total idle timing is 24, total cruise timing at about 2500 should be at least 40 and peak WOT timing is 38.

Retarding the idle timing from the optimum range via ported vacuum advance or no vacuum advance increases EGT, which increases the tempeature of the cast iron mass, which transfers more heat to the cooling system. This is why ported advance often results in high coolant temperature at idle and low speed driving, and when the engine is shut down the "hot soak" can heat up the coolant, which increases cooling system pressure and can open the pressure relief valve and expel coolant.

Most "engine builders", back yard hot-rodders, etc. do not understand the above concepts, especially the need/effect of a properly set-up vacuum advance. As a result, they either don't install a VAC, don't connect it, or select one that is not suitable for the engine's manifold vacuum characterisitcs.

For all OE Corvette engines (that are either OE full time vacuum advance or converted from ported vacuum advance) there is one of three currently/recently available VACs that provide a maximum of 16 deg. advance at 15", 12", or 8" manifold vacuum. One of these will meet the needs of all these OE engine configurations.

If a non-OE cam is installed, the VAC should be chosen using the "Two-Inch Rule", that is discussed in the paper, and it will likely be one of the three.

Duke