Hello everyone,

Andy here and my dad and i have restored a 68' convertible with a 350 stroked to a 383(non matching numbers). Got a couple questions and thought maybe i could get some feedback. Here are the details to engine and cooling:

My dad (who has been a mechanic for the past 30+ years) had 400 crank with new cam and new lifters installed in the engine. The pistons size was also increased and hes got a set of hedman shorty headers with a magnaflow exhaust. I dont really know the exacty geometry of the engine modifications but thats just the basics of it. He also swapped out the radiator with a bigger 27" one and also attached a two speed electric fan on there.

The Problem:

Engine is running hot during highway driving speeds. The weird thing is that it runs just fine for city driving. On the highway it will make it ways to the 250 degree but once kicked into neutral it drops back down in about a minute or two.

What we have tried:

-Tried higher octane gas
-swapped out the carborator
-put in a new balancer
-ran without thermostat
-50/50 mix
-ran the fan direct to the battery so its always on
-There was a valve on one of the heads that was causing to much pressure build up in the engine which lead to the gaskets blowing out and causing oil spill. That was replaced with a functioning one but now the intake is cutting through onto the gasket and still losing some oil.

Section II/ crate engine/solution

Any solutions?

What my dad is thinking is that maybe the engine is just bad. He was mentioning how its not working smooth and the balance was off. Could that be the problem?

Thinking about getting the crate engine. Iv heard and searched several things about maybe companies. I am intregued most by the gm zz4 and zz383. Iv also heard about year one and their rebuild engine. Any other suggestions or good words iv missed? Anything worth looking for?

Thanks to all for reading and for any input.

Andy

 

This is a fairly common probelm with Vettes. Make sure you have all the foam seals around the radiator. Make sure they are in good shape and all there. If you are letting air around the radiator this will cause it to run hot. The way the Vette radiator is positioned it is crucial to get all the air through it rather than around it.

 

This is always the first suggestion for posts like this, so I'll be the one to make it.

Check the bottom radiator hose and make sure that there is a spring inside it. If not, the hose could be collapsing at highway speeds.

 

I was just getting on forums today to post this exact problem. I thought I was the only one with this issue. Isn't it usually highway speeds that these cars cool just fine and it is idle traffic that they have problems with? From what I understand there is usually plenty of cool fresh air at fast speeds but as slow speeds the hot air moves around to the front of the radiator again. I am not trying to hijack your thread here but I will post my set-up to see what our similarities are so we can find the common denominator here.

Vitals:
Engine: GM Performance ZZ502 (this)
Year: 75 coupe (this)
Radiator: DeWitts Aluminum with dual Spal fans (this)
Trans: Automatic

Symptoms:
Slightly overheats at idle when parked.
Really overheats as I drive at highway speeds.

So far...
Ran without thermostat. Also boiled thermostat.
I run Dexcool.

Mvisible, how aggressive is your cam? Does a very aggressive cam make an engine run hotter? I think I'm having trouble with getting the hot air out of the engine compartment.

 

Gordonm, this would be the case if it overheated in idle traffic at no speed. Not the case when driving down the highway. In fact I've been informed that some vehicles could run without a fan at highway speeds because of the natural air flow through the radiator.

 

This is very common. Also make sure it has the lower trim that directs the air up into the radiator.

 

Doesnt matter to me about the hijack thread we are just trying tp figure out similar issues! Ours is a manual 4 speed and yes it only overheats on the highway and very little at city. Im not sure how aggressive the cam is but mine is not a DeWitts radiator like yours. Mine is just an off the shelf kinda deal i have no idea where my dad got it from but he has taken it to a radiator shop and they checked it out and it was working fine, it is a 4 core radiator.

Iv also been searching the boards trying to find similar issues with 68's and have found a couple interesting things about the front spoiler but most have been 70-80's models. Does that front air dam or spoiler apply to my 68?

 

I was looking at some pictures of our car and compared it to some pictures i found on the internet and i noticed this little piece that i highlighted.

highlighted picture: http://img28.imageshack.us/i/69247384.jpg/ http://img130.imageshack.us/img130/6057/97176970.jpg


our car: http://img339.imageshack.us/i/img00068201002271344.jpg/


Could this be our problem?

 

I think there is a good chance. In fact, after reading you previous post I did a little more researching and noticed that others have had their overheating problems fixed with this simple dam. I believe they were standard on all of our cars.

So within 10 mins of reading your post I had one on order. It was about 39.00 for air dam and 7.99 for mounting hardware. Shipping was more due to the oversize. I really wanted to get 2 day air but shipping would have been $57.

So now I have one on the way. I'm thinking I will use synthetic oil too. I'm told that helps a little.

I don't know what my timing is but I believe this may have an impact as well. Does anyone want to chime in on this?

Your car looks very sharp/clean.


~Brad

 

Have to have all the foam seals in place and the front spoiler.

 

No not exactly. You would think so but at highway speeds the air is not going through the radiator, it is blowing by where the seals should be. The motor is runnig hotter due to more rpms on the highway. At idle the motor needs less cooling due to less rpms. There have been so many posts about this topic, and the majority of them are bad seals or seals not even there. The Vette is unique in its cooling capabilities or lack of it. We are not blessed with a vertical radiator and have to rely on all the air being pushed through it. With the seals gone it goes right on by the radiator.

 

Where is your front air dam! You need to get one on there. You are not pushing the air up into the radiator.

 

Where can i get one from? nevermind my dad went and already picked it up! i will update the thread to see if the dam actually makes a difference.

 

Got mine and the hardware from www.ecklers.com. Total with extra shipping charges was $67.96.

 

My engine heats up at idle also. I have no seals as I didnt put them back when I installed my aluminum radiator. I guess I should put them back

 

You might also want to check and verify your ignition timing and vacuum advance - retarded timing and missing/inoperable vacuum advance are probably the biggest contributors to hot running. Make sure you have 36 degrees total timing and that the vacuum advance is pulling in an additional 10-16 degrees at cruise for a total combined timing at cruise in the 46-52 degree range.

Lars

 

Pardon my ignorance, but he's running electric fans. Wouldn't that imply the cowl for the old crank driven fan is gone and hence there's nothing to seal?

He does need that dam though.

 

Have you confirmed the accuracy of your gauge with an IR gun? I have seen alot of parts thrown at overheating problems that were nothing more than a faulty send unit or inaccurate gauge.

Generally, highway overheating points to the radiator. A nice Dewitts and all the proper seals in place will help.

Overheating while idling points to fans. I agree with Lars on making sure the timing is set properly. Good luck

 

Unless you want the fans running all the time. At highway speeds you still want the seals there so all the air is going through the radiator. When the fans kick on yes the seals are not really needed because the fans are drawing all the air through the radiator.

 

So regardless if the air dam works or not we should put all the sealing back on with the electric fan just like as if manual fan was still on there?