Technical Chevy 327 overheating mystery

Hello, this is my first post here....I hope I am in the right forum. I am working on my 1965 Excalibur, which is from their first year of production and has Studebaker frame and suspension with Corvette engine and transmission. I have been restoring it since 1985 and now it is complete except for (sigh) an overheating issue. It gets up to about 180 degrees and stays there for awhile, then creeps up more to 210, 215, 220, 225..........
The engine is a factory Chevy L76 327/365 hp with 11:1 compression and Holley 4150 (2818). I had it rebuilt to a short block by a local engine builder in 1997, then it sat in my garage on an engine stand until 2016, when I was able to get back on it and finish my restoration. It started right off running hot and I have been trying to chase down the reason for a couple of years now....for about 1000 miles.
I suspect rust in the cooling passages of the engine, since on first inspection the coolant was quite rusty and I have found rust particles (chunks) in the radiator top tank. I have flushed it twice and had it done a third time. Also a Thermocure treatment. Without going into a lot of detail I have done the following:
- checked for exhaust leak into the radiator (none)
- pressure tested radiator (good)
- new thermostat, radiator cap
- compression check (good)
- cylinder leak down test (good)
- verified valve lash (John Hinckley method for 30-30 cam)
- retorqued intake manifold
- installed aux electric radiator fan
- verified lack of (or minimal) detonation
- rebuilt carb
- checked for vacuum leak with propane
- phenolic spacer under carb
- cleaned rust chunks out of radiator with magnets
- installed Tefba coolant filter
- replaced original points distributor with Pertronix Ignitor III
- checked for spark jumping between #5 and #7 spark plug wires
- retarded timing to 10 degrees initial, 31 degrees final (no vacuum) 48 total (with vacuum)
- IR thermometer check of radiator..... uniformly 185 degrees [edit 203 top / 192 bott]
- IR thermometer check on sides of block (near spark plugs) after engine is heated up shows #5 at 245 degrees and all others at about 195 degrees

So, this last item looks very promising to me as a clue. At 245 degrees, it is probably boiling the coolant in that one spot. I searched the internet and could not find anything about "#5 cylinder running hot". I am visualizing maybe a small cooling passage near #5 that is clogged with rust.....or how about a wrong or poorly placed head gasket that is blocking a cooling passage?
Any ideas?
Thanks,
Gary

 

Did you plug all the water inlets and outlets over the time the engine sat? I am thinking mice may have made a home in there during that time.

 

How much of an over bore was done on the motor?

 

I wish I had thought of that at the time, but I didn't. It was up off the floor, on an engine stand, and covered with an old blanket. If I had just plugged those holes...….

 

I think your best move after everything that you have done is to pull the heads and get a visual on what’s going on...you may have to pressurize the block with an air gun to see if any big chunks fly out...


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There was no overbore (that I am aware of), and we re-used the same pistons with chrome moly rings. It only had 45000 miles on it (the failure was a broken ring).

 

Around here it is mud dauber wasp they get into places mice wont ever try.

 

I see what you are saying. So I guess I would take the intake manifold off and the heads and thermostat and water pump. Then to pressurize the block, squirt compressed air into the cooling passages down around the cylinders and watch the debris come out where the water pump was?

 

Good point...….where I am (Washington) we have yellow jackets that find a place to hibernate every winter.

 

I'm not sure, but you may be able to remove the frost plug and see if there is anything in that area.

 

That's a good thought....may reveal the problem without going too "deep"

 

My experience with SBC's is that the #5 cyl. runs warmer. What about the water pump? Is it stock or high flow/volume, and does it have the bypass in the pump mounting on the passenger side or is it blocked? If it's blocked it balances the flow to both banks, but you need a bypass out the front of the intake manifold to the top of the pump.

Also with 11:1 compression, I wouldn't retard the timing. It will make it run warmer. I like the Flamethrower III, but I usually give it about 15* initial and limit the mechanical to about 15*-20*, and limit the vacuum advance to about 8*-10*.

Adjust the initial so it starts without kick-back and have the mechanical all in at about 2500-3000 rpm or so. When you floor it, the vacuum advance goes away and retards the timing accordingly. Have fun!

 

So it is actually pushing fluid out of the radiator or catch tank when it gets hot, or are you just seeing the temp on the gauge? and does this occur while sitting still or driving? and if driving it is worse in traffic or at highway speeds? these are all clues that can point you to the cause. Also is it electric gauge or mechanical and where is the sender placed on the motor?

 

I'd have to ask if it has the correct water pump for the belt rotation on the engine.
That is if you put a new pump on it during the redo. There are "short" Corvette pumps that are reverse rotation if you just happened to be looking for a short Corvette pump when you were pump hunting.
You confirmed that you are checking it with a heat gun so I won't go into the deal of some electric temp gauges not reading right if it happens to have an electric gauge. My OT truck was reading ok on the gauge but running hot and I found out that it had the wrong sending unit. the reverse could happen.
Lower hose collapsing? or kinked because it doesn't fit as well as it really could? Some of those hoses have to have a spring in them to keep them from collapsing but you don't get a new spring with them now in most cases.
You aren't the first one who fought this battle by a long shot so take it one step at a time and sort it out.

 

If there's rust in the engine, there is certainly rust in the radiator.
Clean the radiator by pulling the top tank and poke through with an Auto Trans dipstick

Make sure that all air going through the grille, goes through the radiator.[seal the edges]

The more horsepower = more heat.
Now I am going to suggest taking the hood off, and try it out.
The heat has to dissipate somewhere ,the air might be getting trapped in the small engine bay [and also restricting air coming through the radiator]

 

This^^^^^^
Yes, it always struck me odd that so many articles in magazines over the years promoted all kinds of ways to improve system cooling and completely skipped over the simple concept of allowing hot air to escape from the engine compartment, I finally realized the reason was there was no product to promote and that it was their advertisers that dictated how those articles were formatted.

 

Yep, people don't realize that the engine is actually Air Cooled [but with a "water heat transfer" system]
Modern racing cars use tiny radiators but pump out 700+ hp. [F1 radiators have about 8"x 8" frontal area x2]

Look at the grille opening of an early F5000 , with a tiny opening low to the ground. This opening was ducted into a larger area where the Radiator was perpendicular to air flow.
Then after the radiator the ducted area reduced again in size ,to exit into the main air stream.

It acted the opposite to a venturi by slowing down the air speed as it passed through the radiator.
The exit for air flowing out is larger than the grille opening

 

Rust in a fresh short block would be a concern of mine...I would pull the heads and have a look.....

 

Take your freeze plugs out and use a hanger wire to clean out the crud and flush the holes with a hose . Install new plugs .

 

I seem to find that most slow overheat conditions are radiator related. Either clogged internally or airflow problem. Is the radiator special to that car or did they source the radiator from another car? Check the radiator with your heat gun for warm and cold spots. Your hand will work if you can get to it. You you find that is the problem you have some options. If it is sourced from another car replace it, recore it, or have it rodded out.

 

It's amazing how much crud can be removed from the block by rigging up a small diameter silicon hose to to your garden hose to insert into the block drains on each side just above the pan rail. It is possible to work the hose around inside which removes even more. But I found it wouldn't simply flush though. It became a matter of inserting for 10 to 20 seconds and then removing and monitoring the output - this would get dirtier at the end of each flush drain cycle. I did this probably 100 times on each side and had a river of rusty stuff as a result. I did the usual flushes beforehand but this resulted in the most muck output, quite scary really!

After everything you've done already this might just make the difference and has the advantage of requiring minimal disassembly!

Embarrassingly mine all became necessary after about 15 years of cooling system neglect but I never had any cooling issues. I've cycled the engine a few times now and the coolant is clean. I hope i caught it before it was too late!

Chris

 

Try to find somebody with a back flushing tool. We used an old one on my dad's 54 Buick that would overheat and vaporlock. You hook your air line and garden hose to it and then clamp the end to your radiator hose. Open the valve on the backflusher and watch the magic happen. Do this to both the radiator and the engine, and I would recommend doing it outside if you don't want a mess to clean up in your garage. It cured my dad's overheating issue and I have used it on other cars that I have had the radiator out of just as a precaution.

 

If the engine block is rusty inside putting a clean radiator on the vehicle without solving the source of the problem will just plug-up the clean radiator and heater core.


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My car has the original water pump and I am not sure if Chevy made it high volume for these engines. I wasn't aware of the bypass, but mine has a U shaped hose coming out the top of the pump and going into the front of the intake manifold (driver side). On the passenger side, there is a valve/hose that goes from the pump to the heater core, then returns to the intake manifold (pass side). Photo attached.

When I plotted my mechanical advance curve for the Pertronix, I came up with 33* all in at 2350 rpm. The GM spec for my distributor is 24* at 2350, so I will get some new springs and try to adjust that down. The vacuum advance of the Pertronix was just about 16*, which is like the GM spec of 16.5* (at the crankshaft). I understood from reading somewhere that the vacuum advance was higher on this engine because the manifold vacuum is lower on this engine due to the high lift cam(?) and it needs more advance to maintain idle speed.

Thanks for your comments

 

Yes, coolant gets pushed into the catch tank, whether moving or sitting still, then*****s back into the engine when it cools off. I added my catch tank during the restoration...…...the original car had nothing, just a drain tube from the filler neck. The gauge is electric....a new SW and I got a new sender at the same time.

It seems to be worse at freeway speeds I remember one test drive after reducing the exhaust back pressure when it was staying at 180* (on the gauge) on the freeway until I stepped on it just to get up to 6000 rpm, then it quickly went up to 220*. I drove this car in the Arizona heat for years (before my rebuild) with no overheating issues.

 

We used to take the thermostat out and replace it with a restrictor plate...they came in a kit with different size holes drilled in them and each hole size represented a different temperature but it maintained the back-pressure in the cooling system.


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The pump is original. I bought my new SW gauge and a sending unit at the same time from Summit Racing and I have calibrated it in heated water comparing the gauge reading with a thermometer in the water. I found it okay up to 210*, except it reads 10* low below 180*. I think I remember checking for springs in the hoses....I will double check.

Thank you for your comments

 

Two things. I don't see a fan shroud, and is vacuum advance connected to manifold or ported vacuum source.

 

The radiator had a fresh rebuild and rodding out, then stored away until ready to install. I am certain there are rust particles in there because I have fished many out of the top tank with a string of small magnets, then installed a filter in the top hose to catch any more.

My car's engine bay is not much wider than the engine and they used the compartment as a "shroud" by blocking off the bottom with an aluminum panel to keep air from*****ing up from the road and direct it through the radiator. I wish there was an actual shroud, but there is really not room to fit one. There seems to be plenty of ventilation for hot air through louvers cut in the side panels and top of the hood.

 

I hear you...…..I know there is rust