69 Z11, all numbers matching, convertible, 300hp/350, manual, factory A/C.

Sorry for the long background, but some of it might matter:

Bought this car 13 years ago from a museum. It allegedly had sat in a barn for years. Driving it home it started to overheat after a while. We stopped and pulled the thermostat and all was good. Drove it that way for several years. When I say "drove it", bear in mind in 13 years I have put less than 1,000 miles on the car. It sits in the garage covered up for months at a time.

Six or so years ago, I decided I wanted to pull the drive train and completely recondition the engine compartment and drive train. I spent thousands of dollars finding the right paints, decals, clips, fasteners, hoses, brackets, new vacuum booster, AC compressor and ll related components - EVERYTHING -anything that didn't look new was either painstakingly reconditioned or replaced with new old stock. Half way through this project, I got a bad diagnosis and was sick and recovering from treatment for a couple of years. So the engine sat on a stand and the radiator was on a shelf.

After recovery, I reconditioned the radiator and painted it with correct radiator paint. I never pulled the cooler for the AC. The engine itself - with no carb, no exhaust manifolds, no brackets or anything - I stripped it down and painted it the correct Chevy orange. I never went into the engine because it ran like a Swiss watch. Documentation showed the engine had been completely rebuilt 9,000 miles prior - which was also over 20 years prior.

Finally finished the project a couple of years ago and it looked show room new. But it wouldn't start!!!! I took it to a local muscle car shop and after a few hours he figured out there was a bad wire under the distributor. Then he calls and says hey, you better come down here and listen to this. I get there, he starts it and it is knocking like crazy. He pulled the pan and it was tearing itself apart. I was stunned because it ran great prior and I never touched the internals. The one thing I did do periodically was pull the spark plugs every so often and spray oil in each cylinder and turn it over with a wrench on the pulley/balancer. I couldn't believe what I was hearing.

So, I paid him $4K to completely rebuild the engine. Boiled, milled, all internals replaced, oil pump, water pump, timing system, heads redone - everything.
Got it back and after driving it about 10 miles to get home, I noticed low oil pressure - like 20lbs It used be at least 60lbs. Drove a bit more and it started overheating. I took it back to him and he was stunned this time to find that the engine was tearing itself apart again. I put less than 50 miles on it and never got close to highway speeds. So he completely rebuilt it again and got after the machine shop to make sure every port, nook and cranny were 100% clean. They insisted that it was sparkling clean the first time and the second time.

So, I've had it back for 2-1/2 years. Probably driven it 100 miles in that time. It has at least 60lbs of oil pressure It's overheating. I tried pulling the thermostat - no difference.

One thing I changed in my work - when I bought it, one of the heater hoses had a "Tee" in and a short 8" piece of hose(also heater hose) connected to a fitting on the right side, center of the block. A shop I had it in soon after I bought pointed it out and said they had never seen that, and they were questioning it. So when I was restoring the compartment, I eliminated it and put a brass plug in the block.

I can putt around town 35mph to 45mph all day long - no overheating. If I get on the highway and maintain 60mph for 10 minutes, the temperature starts to climb. If I keep it up, it approaches the orange zone. I don't let it get there. I can get off the highway and start putting around and the temperature will come right back down again.

It has the clutch fan on it and I assume at slower speeds that is working and the fan is doing its job.

Why is it overheating on the highway?

Is there a process to have the radiator boiled or super cleaned? If it was the radiator- wouldn't it also be a problem at slower speeds?

Could that TEE and side penetration to the block have been for a reason?

Any help or ideas greatly appreciated.

 

Is it normal to paint a radiator? Maybe it was the wrong paint right color and its not letting air flow through it? Thats my best shot in the dark.

 

yeah you can paint a radiator. unless using something really thick to plug the fins of the radiator it shouldn't mess with the airflow

 

I would pull the radiator and have it flow tested at a radiator shop. Sounds like your culprit with the info you gave. (yes it's acceptable to paint your radiator)

 

Thanks all. I will pull the radiator and have it tested.

 

Guys, I found a local shop to do flow testing but I've been too busy to yank the radiator yet. For the record I used special radiator paint of the correct color to paint the radiator. I've also been reading the car could possibly not be overheating at all and I have a goofy gauge acting up. I'm going to install a cheap mechanical gauge today and see what that shows.

 

Guys, sorry for the long gap. Obviously I don't get much time to work on this. I finally swapped in a mechanical gauge and still shows getting hot. I pulled the radiator and the shop says they found significant lime deposits and crud/blockage. Having it re-cored.

I'll install next weekend and swap back in the correct gauge, and see what happens.

 

Are you sure it's really over heating? Where's the sending unit located? When I got the car my sending unit was located in the head and it read hot. I got an infrared temperature gun at Harbor Freight and the temp was fine at the thermostat and radiator but hotter at the head. I moved the sending unit to the intake and it read fine.

 

When you re-install your radiator use distilled water it has all the impurities removed and for cars such as yours that will see little attention for very long periods of time it helps to avoid build up in the radiator...

 

Santa6,

What brand thermostat are you running? Can you post a picture of your coolant hose routing and a picture of the top of the intake manifold?


Tony Huntimer
TestCarDatabase.com