'65 327/350 overheating at idle

[Don Mancini]

My 1965 327/350 roadster has always overheated at idle. I have done the following in attempt to fix the problem:

1) Installed an aluninum radiator correct for this car.
2) Verified that all shrouding is installed and correct
3) Verified that the correct fan is installed
4) Replaced the original clutch on the fan with a fixed base to increase airflow.
5) Replaced the stock water pump with an Eddelbrock high flow pump.
6) Introduced "Redline" into the cooling system.
7) Tried different temp. thermostats
As a last result, installed an electric fan in front of the radiator.

As a result, I think I own the coolest running corvette while it's moving.
None of the items above changed the overheat at idle problem except the electric fan which prolonged the overheat but it still does overheat.

I have been told that my problem is within the block, the block had already been rebuilt before I purchased the car and I have no history on the type of parts used or work done. Some say the cylynders could have been over bored, others say my car has the lope of a hi perfomance cam which could cause overheat at idle.

I don't know what to do next. Any help would be greatly appreciated

Thanks, Don

[Brian Monticello]

Re: '65 327/350 overheating at idle

Did you verify the temperature reading on the gauge with an IR thermometer? Replacements are notorious for giving high readings - especially above 170 or 180. Is your car actually showing the physical symptoms of an overheated engine (knocking, puking, etc...)?

There have been MANY posts exactly like yours - you may want to check the archives for some great info.

Brian

[G B.]

Re: '65 327/350 overheating at idle

Don, if you will tell me what you mean by overheating in specific terms, I'll try to help. By that I mean: what temperature does your engine reach (measured with an accurate infra-red gun), after what period of time, idling at what rpm, at what ambient temp?

I seriously doubt any heat problem you have is caused by the cam grind or a basic cylinder over-bore. I wish these urban legends would be buried along with the classic "timing chain jump", water "moving too fast" through the radiator, and "factory gelcoat" myths.

[John H.]

Re: '65 327/350 overheating at idle

Don -

Have you verified that it is indeed overheating? Step #1 is to "shoot" the thermostat housing and upper hose with an I.R. gun and compare that reading with the temp gauge at the same time so you know what the gauge is really telling you. Corvette temp gauges are notorious for reading high, especially if the original sending unit has been replaced.

How are you defining "overheating"? What does your gauge read, how long at idle does it take to "overheat", and does it puke coolant from the overflow hose while it's idling? Has your pressure cap been checked vs. its rating?

[Don Mancini]

Re: '65 327/350 overheating at idle

Brian,Jerry and John:

Thanks you for the response. I have not used an IR gun to check the temp, no one has ever suggested that. Does it have to be done while it is overheating? Maybe I need to take it to a local radiator shop and make it overheat.

Overheating at idle on a warm day (above 80 degrees) onlt takes about 10 to 15 minutes for the needle to go into the red and coolent to overflow, below 80 degrees it will still overheat but take longer maybe 20 to 30 minutes.

One day I was on the approach to cross the Golden Gate Bridge and the traffic was very slow (5 to 10 mph) The needle started to rise and just as it was approaching red, I was able to get off the freeway and bring the speed up to approx 30 - 35 mph. Within minutes the gauge was back to normal.

If I'm able to bring the speed up, the overhaeating always corrects itself quickly.

[Joe C.]

Re: '65 327/350 overheating at idle

Don:

When you say "always" overheated, then that is somewhat misleading. How many hours/miles are on the engine rebuild? How long have you owned your car? Is the engine actually boiling over, or do you simply have a high indicated reading on your temp gauge?
The problem could be something as simple as an incorrect engine timing map. It could also be a "tight" engine due to a recent rebuild. There is VERY LITTLE margin for error before overtemp becomes a problem at idle. When you are moving, then the margin increases dramatically.

Here is what I would do:

I. If your car is actually boiling over.

1. Check the rad cap for correct relief pressure.

2. Check your timing map per the specs listed in the Shop Manual

3. Make sure that your heat riser is not frozen shut.

4. If 1, 2 and 3 are OK, suspect a "tight engine" and continue the break in procedure.

II.If your car is not boiling over:

1. Take John H's advice, and use an IR thermometer to verify the accuracy of your temp gauge.

2. If you see a large discrepancy, then consider purchasing another temp sender (there are some very accurate, but not "correct" units available in the aftermarket).

Joe

[Dave Suesz]

Overbore & more...

There seems to be more "meat" on the cylinder wall casting on Chevys than Fords (That lightweight casting technology) but its a two-edged sword. If you bore a smallblock Ford 0.030 over, more heat passes through the thinner cylinder wall, and you are just about guaranteed to need another "row" on the radiator. This affects Chevys less, but the it is still there.

One thing not mentioned as often is distributor advance curve. People rarely have these checked, but since I got some experience with the Sun machine, I have found very few that were even close. Proper curve can change a dog to a demon, without any other tuning change at all. Too much advance at idle compensating for inadequate total advance can cause overheating for sure. The 71 Buick GS had an informal recall for this, just a couple degrees was causing overheat.

[Don Mancini]

Re: '65 327/350 overheating at idle

Hi Joe,

Thanks for your response. I have checked the cap in the past but I will do it again just to be sure.

We had checked the timing a couple times and was within specs both times. The car acrually runs very good.

There is about 1500 miles on the rebuild.

I'm not sure I am familiar with the heat riser. Is it the small stainless tube that goes from the passenger side exhaust manifold to the carb. for operation of the automatic choke. My car does not have that tube. I was told that because I have a Holly carb with an electronic choke, that I do not need that tube.

It is interesting that you ask about this tube and I may not have it on the car. Please tell me more about it

Thanks, Don

[Dave Suesz]

Poor man's temp verification...

Duct tape a digital meat thermometer to the upper radiator hose, in a spot where the fan will not blow on it. Don't tell your wife.

[Brian Monticello]

Re: '65 327/350 overheating at idle

The heat riser valve is a butterfly type valve just below your right exhaust manifold. When closed, it diverts hot exhaust gas to the exhaust cross over passage under your intake manifold. If it gets stuck closed or does not open all the way you have some serious heat to deal with. You can verify that it is opening all the way by making sure the counterweight is "down" all the way. Here what one looks like:

[Les Jacobs]

Re: '65 327/350 overheating at idle

Hi Don I chased a "overheating" problem on a "new" C2 327/340 thru about 300$ worth of new parts (including senders, fan clutch etc) before I checked the temperature. Turns out my instrument gauge was off. (I was convinced it was overheating because it was puking too-which turned out to be that the water level was too high). I used a meat thermometer, then a heat stove thermometer, then I broke down and got an IR thermometer. Funny thing they all pretty much agreed that I wasted my money on "fixes". I would suggest checking the temp first, then decide whether the temp sensing circuit is a problem or not. Good Luck Les

[William V.]

Re: '65 327/350 overheating at idle

Don

I just solved my overheating problem on my 64 365 by purchasing a temp sending unit from letric limited. Before changing the sending unit, the gauge was reading 220 with a 180 deg thurmostat while driving in 6 deg weather or 90 deg summer temps. I knew the engine was not overheating because when I place my hand on the upper hose, it only felt warm. After changing the sending unit the temp is just below 180 with a 20 deg outside tempature. Try the meat thermometer, you may be pleasantly suprised.

The heat riser is a spring controlled butterfly device between the right exhaust maifold output and engine exhaust pipe. When the engine is cold it is closed forcing exhaust gas through the intake manifold to help with engine heat up. When the engine warms up the spring opens the butteryfly allowing the exhaust to flow out the right side. If it is frozen in the closed position, the engine will overheat and loose power. The tube you asked about, supplys hot air through the exhaust manifold to open the choke.

[G B.]

Radiator

By "aluminum radiator correct for this car", do you mean a new DeWitt or Griffin? Used aluminum radiators aren't worth a bullet, and neither are some of the other aftermarket brands.

[Joe C.]

Re: '65 327/350 overheating at idle

Don:

The heat riser has been explained sufficiently in the above posts. Your engine, a 1965 L79, will have the same external configuration as the L76, which is the last SHP solid lifter smallblock produced by Chevrolet until the advent of the "streetable" LT-1. You will certainly have the aforementioned heat riser valve, installed in your pass side manifold (unless you have F.I.).

If you have the correct intake manifold (Joe Lucia will help here), then you WILL have the chrome plated steel heat pipe emanating from the pass side manifold, into the choke housing of your Holley 2818 carb. You do not have an electric choke.

Joe

[Joe C.]

Addendum

You will not have the high capacity oil pan of the L76/L84. This was done as a marketing ploy, to make power steering available with the L79 package. You will have the 4 quart oil pan, rather than the 5+1 pan of the L76/L84. You will not have the high volume oil pump of the L76/L84, and consequently, will have the 60PSI oil pressure gauge, not the 80PSI unit like the L76/L84. You will not have the windage tray, nor the baffled oil pan used with the solid lifter SHP engines. Your timing map is different from the solid lifter SHP engines. "All in" mechanical advance occurs at a higher point than the solids. If you need the exact specs., let me kmow, and I will send them to you.

Joe