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1973 Chevrolet Corvette - Shark Bites

'68-'82 Corvette Tech

Chris Petris Nov 1, 2007

Only the Strong Survive
We own a '73 454 T-top coupe with 84,000 miles on it. We just had the A/C repaired. It was not that cold after repairing it. Then we had the radiator replaced. Our mechanic put in a heavy-duty one. The car now is overheating when the A/C is on and our mechanic can't figure it out.
Linda Stygar, Florida

In defense of your mechanic, Corvette overheating issues are difficult to diagnose and in a class all by themselves. Every other GM product with a 454 had a large engine compartment that let the hot air out. Your '73 has limited air flow, keeping an already hot situation even hotter.

To start with, have a combustion gas test performed to make sure the head gaskets aren't leaking combustion gas into the cooling system. A combustion leak tester kit is used, and this kit comes with a ball, tubes, test fluid, aspirator bulb, and engine adapter (cone-shaped device you place in your radiator filler cap). If there's exhaust in your coolant, the test fluid changes color. If the liquid changes color, the combustion gases are entering the cooling system, and no radiator or fan will cool it properly. Major work is required, but it's best to know the status of the head gaskets up front.

Now a performance check of the engine is required to make sure it's running well with correct fuel mixture and ignition timing. Late ignition timing will create extreme engine and exhaust manifold heat, making the engine compartment toasty. With today's fuel, we usually have the timing set late to prevent spark knock with all that iron used for the block and heads. Another reason to set the timing later than the factory specs is starting issues, bad battery cables, weak batteries, and worn starters make for hard hot starting. We recommend fuel additives to boost octane in the big-block Corvettes, even with the '73's relatively low-compression engine. This allows proper ignition timing dependant on engine horsepower configuration. You need to make sure the engine is not running lean on fuel from a dirty carburetor or clogged fuel system

Once the engine performance is corrected (if necessary), the cooling system itself can be checked. You mentioned a large radiator was installed, and since the problem is still there that may have made the situation worse. Air must flow through the radiator core to cool the coolant. Stepping up to a 11/42-inch-tube, four-core radiator from a three-core was the norm for many years. The additional air resistance has been found to negate the benefits of the additional row of tubes because air doesn't flow as well through the bigger radiator. Today's radiators are mostly aluminum and use large 1-inch tubes for more surface area. The aluminum has better heat convection to also add cooling, although the initial cost is greater. They also last longer. I can understand that you may want to keep the original look of the cooper/brass OE radiator, but in this case you may want to step up to the aluminum radiator.

Another area that is often missed when diagnosing cooling system woes is the direction of air flow, or at least containing the air and funneling it through the radiator. The radiator and shroud originally had seals around the core support and across the top of the core support to ensure any air passing through the front of the car was directed through the radiator. This is especially important at speeds above 40-45 mph. Below that speed, the cooling fan has to maintain air flow. This is where the fan shroud seals come in to play. If they're missing, the fan will draw air from around the radiator, not through it.

The fan clutch regulates how much fan lock-up is occurring to limit engine drag at highway speeds. There is no need or limited need for the cooling fan after 45 mph is reached unless you have airflow or engine performance issues. The original fan clutch is a thermostatically controlled unit that should have maximum fan speeds at low engine speeds above 180 degrees. When the engine is cold, the fan should have very little drag. As engine temperature increases so should the drag. When the engine is fully warmed up, the fan should stop almost immediately when the engine is shut off. If the fan keeps spinning, the clutch is bad. Another test is when the engine is hot, shut the engine down, grab the fan, and try to turn it. You should feel noticeable resistance, if not, the clutch is bad.

In addition, there are other factors that may affect cooling: water pump impellers can deteriorate; a wrong pulley size can affect low-speed cooling; improper antifreeze/water mix can increase temperatures. We use the 50/50 rule for mixture for freeze protection in northern climates. In the southern states, you only need to worry about worst case scenario. The 60-percent water/40-percent antifreeze is adequate freeze protection and will help dissipate more heat. In your area (Florida), 70-percent water and 30-percent antifreeze would be fine and is the most dilution you should go with because other engine protection additives would be too low.

As you can see there are many variables and possibilities to consider when cooling issues arise. Hopefully, this will get you cool again. the A/C won't be cool until the engine it relies on has good air flow through the A/C condenser in front of the radiator. Additionally, a hot engine compartment makes it difficult to cool the passenger compartment.

I have owned a '75 coupe for the past six years. it has all the options offered in 1975; it's one of 144 with the Z07 option. Recently two issues have developed: 1) The driver-side headlight assembly won't open; the passenger side is ok, so where do I begin to fix this? 2) The passenger-side power window won't go down. I found if I push down on the window while activating the switch, it will usually go down. It does work in the up position.
Larry, Two Rivers

The headlight issue has a few possibilities, both vacuum leak related. Either the actuator relay valve has an internal leak, or the headlamp actuator has an external leak. With one headlamp working, it tells us that the rest of the system is working properly and the problem is in one area. The headlamp actuator relay valves are mounted on the underside of the headlamp reinforcement between the headlamps at the front edge of the hood opening, with one valve operating one headlamp independently. You can remove the vacuum hoses from the relay valve in question for diagnosis. The yellow-striped center hose is the vacuum supply. The upper red-striped hose is headlamp close, while the lower green-striped hose is headlamp open. A quick check of actuator operation is easy. Use a short piece of quarter-inch tubing to connect the yellow center hose to the lower green hose. The headlamp should open. Then connect the upper red hose to the yellow hose, and the lamp should now close. If the headlamp now opens, the relay valve is the offending part. If not, the actuator rod seal is most likely torn.

Actuator rod seals require actuator removal to replace them correctly. In a pinch, the seals can be replaced with the actuator in the car, but the dust seal can't be installed properly. The task isn't too difficult. The balance springs are the most difficult to remove. Make sure you have a good hold on them when popping them on and off. Once the balance springs are off, the pivot pin can be removed that connects the actuator rod end to the assembly. Then the four half-inch nuts can be removed to remove the actuator. The rod-end seals and dust boots are available from Corvette Central in kit form (actuator seal kit PN 443012, $24.95), which consists of the seal, dust boot, and retaining ring.

As far as the window is concerned, the window regulator teeth are worn at the top end, which explains why you can get it started and it goes down. When the window is going up, momentum takes over and it raises itself fully. You have a few choices here. Replace the regulator completely or use the repair kit that has the gear and pin necessary to repair the original. It's not too tough to do with the right tools (vise, big hammer, or hydraulic press). There's also a gear repair that goes over the original gear. Either way the regulator must come out for the repairs.

Should I or Shouldn't I?
I own a '91 C4 and do not have a lot of experience concerning C3 models. My wife and I found a '71 Corvette we are interested in purchasing, but I have a few questions I'm hoping you can answer. The Corvette is a low-option car that has a 350/275hp engine with automatic transmission. It has air conditioning and four-wheel disc brakes with no power assist. The engine compartment has been detailed and is very presentable. The car was originally silver, but has been painted red. The paint is above average with no cracks or cob webbing. There is a subtle appearance of possible body repair or a manufacturing seam running from fender to fender behind the pop-up headlights. Is this normal? the fiber optics work, but you need to be in total darkness to see they are illuminated. I do not understand this feature. One last question: the tach and speedometer work, but the speedometer needle dances a bit. any major concerns?
No Name Given, via e-mail

All sharks have a metal reinforcement running across the area between the headlights and front of the hood that the headlight assemblies mount to. Early sharks used aluminum rivets and bonding adhesive, but later on the rivets were not used. Thus, bonding adhesive was solely relied on to keep the reinforcement in place. The '72 has the rivets, and by now, most of them have had enough corrosion to make the rivets grow, causing dimples in two rows: one close to the headlights and the other at the front of the hood. If you see a seam instead of the dimples from the rivets, work has been done on the rivets or the nose may have had a repair piece patched in, possibly from a collision.

The front surround is one complete piece that goes from the windshield pillars forward to the bumper. The side fenders are bonded to the upper surround about 2 inches down along the side fender crown. You can see the seam on the fenders of original cars as shrinkage takes place. The reason for the info: In some cases, pieces are replaced instead of the recommended complete upper surround panel. Replacing the entire upper surround is recommended because the same shrinkage eventually occurs when repair pieces are bonded together. Look inside the fenders and the surrounding area of the suspect seam for fiberglass strips that are used to bond the pieces together. You may have to feel the area near the headlights for the strip or globs of adhesive. Those areas are hard to access and can tell a story. Sometimes the bodyman can't get a grinder in to clean up the area, and most people don't know where to look. There should be strips of fiberglass about 2-inches wide behind the seam for the upper surround mentioned earlier running from the door area to the front of the fender. Any other strips would indicate repair work. This is also the case at the rear. The strips would be in the same area from the rear of the door to the bumper.

The fiberoptics system was way ahead of its time. The purpose of the option was to inform you of failed bulbs, and during night driving, it works well. With the top down or in full sunlight, it's hard to see them even when everything is clean and working properly. The fiberoptic strands connect to each exterior light with a lens and rubber mounting grommet. Many times when paint or bodywork is done, the wiring and fiberoptics harnesses are left unprotected, and the ends of the fiberoptic stranded cables and lenses are covered with primers and paint. The fact the car had recent paint would be a good reason why the lights don't have a strong light. The fiberoptic stranded cables are delicate. Bending them sharply breaks the strands, so over the years the cables get damaged and this limits light travel to the console-mounted indicators. My '69 has a few working well, and in the near future, I'll take each grommet out and clean and polish the lens to see if that fixes the issue with the ones that aren't working quite as well.

The speedo probably needs to be serviced, or it could be as simple as a dry speedo cable or worn driven gear causing the bounce. My concern would be the odometer. While you're out test driving the car, check the odometer for movement. It's common to find the speedo working and the odometer not working. The odometer has the extra load of the trip meter. The added load eventually wears out the jackshaft that runs the odometer and trip meter, while the speedo keeps working. The bouncing needle would indicate possible odometer failure or imminent failure.

Any steel bumper Corvette in decent shape is worth $20,000 considering the cost of parts and labor today. With the car being an A/C coupe, it helps, but the convertibles are more sought after and, accordingly, the price goes up with the convertibles. You can expect to spend a minimum $1,000 and up to $5,000 on repairs and enhancements. My theory is good solid frame, minimal crash damage, and all other obstacles can be overcome. My only real concern would be the seam near the headlamps. Is it from crash damage? The damage repairs usually take six months to a year to show up as unexplained seams from panel repairs. If I felt comfortable with the frame and body condition, I would offer $2-3,000 less than the asking price because of the obvious repairs necessary, including the gauges and possible unforeseen items that may crop up later.

Shark Questions?
Shark Bites
9036 Brittany Way
Tampa, FL 33619
While mail cannot be answered personally, letters and responses will be published as space permits.



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