Pre-Purchase Inspection

Since C4 production stopped in 1996, any purchase made will be of a used automobile whose condition should always be assumed to be suspect.

Because of the mystique of the Corvette© in general and the drop dead good looks of the C4 in particular, it is easy to get caught up in the moment and find yourself the owner of a fiberglass basket case.

Although it isn't possible to list everything you should check there are some general guidelines that you should follow whenever you plan on purchasing one of these wonderful automobiles.

Records

The very best defense against buying a carload of trouble is to ask for the service records.  If the automobile has had only one or two owners, you may be able to get the entire history in this manner however, even if there has been a whole host of owners, the last owner should have kept the receipts from the present service history and if he or she cannot or will not produce them, it is a red flag.

All GM dealers conduct their business via computers and if you approach a dealer to do a pre-purchase inspection of the automobile, you may be able to persuade them to print out any records related to the VIN number.  Although these records are purged from time to time, it is worth asking.

Finally, if the selling individual will disclose the shop or dealer which has been maintaining the automobile while they have owned it,  you can approach them with your questions should the seller be unable or unwilling to produce records or receipts which clearly identify the automobile in question.

If there are no records and no leads on who has been maintaining the automobile, you will either have to thoroughly check everything on the vehicle or walk away from the potential purchase. 

Often, since the seller will seldom pick up any costs associated with a pre-purchase inspection, walking away may be the best idea in these cases.

Still, lets be optimistic and assume you have found what appears to be a low to moderate mileage C4 that appears to be in good shape.  What to look for in that situation is the question.

Appearance

Every dealer, every person selling an automobile, everyone who hasn't spent the last 100 years off the planet knows it is the appearance of an automobile that first gets a potential buyer's attention and if the visual pleasure is great enough, the buyer can be rendered unconscious to the defects present in the automobile.

This is particularly true in the case of the Corvette.

What to look for

Normally, a savvy seller will have the automobile "detailed" prior to placing it on the market.

A detail shop will steam clean the engine compartment, apply a top coat of Armor All® or another of the wax based interior products to the dash,  seats (if leather or vinyl) and all plastic trim.

The shop will thoroughly wash and  wax the exterior with a high gloss wax and apply Armor All to the tires.

Recently, Blue Coral® and Dupli-Color® have teamed up to produce Dupli-Color Color Wax which hides swirl marks and minor scratches in the finish and detailers are using that product as well to make the appearance as pleasing as possible.

A prospective buyer should look at the exterior finish in bright sunlight.  Although the vehicle has been detailed, you will be able to pick up subtle clues that there are scratches and paint problems hidden under the wax if you examine the surface carefully.

Another trick is to apply Armor All to the actual paint. This technique hides problems with chipped clear coat and will make the front of the C4 in particular---where stone chips take a heavy toll---look better than it actually is.

You will also want to look carefully at all the weather strip material on the car.  It is almost a certainty that some of it will be cracked but if all the strips are in bad shape, it will cost you several hundred dollars to replace everything even if you do the work yourself.

Inside the automobile

Look for areas of wear in the carpet.

A common trick is to apply flat finish matching paint (particularly if the carpet is black) to areas where the carpet has been been worn through to the car's body.  Unless you look closely, you will think the carpet is in great shape until you purchase the vehicle and later notice you were tricked.

By the same token, if you observe new floor mats, be sure to look under them for signs of wear.

In the rear cargo compartment, look for cracked or damaged trim, broken or damaged top hold down hardware and scratched or otherwise damaged plastic seat backs.

Finding these situations and pointing them out to the seller along with protestations that you will have to have the car restored will sometimes result in a lower price.

Damage History

Many Corvettes have some damage history.  Most is very minor but occasionally you run into some major problems usually caused by an accident.   Before you spend your money, take the time to make sure you are not buying something that until recently was in a body shop or worse a junk yard.

Fiberglass Repairs

Unless a fiberglass panel has been actually replaced, it is relatively easy to determine if repairs have been made.

On the side of the panel away from the side normally viewed, look for a built up area where the resin and fiber have been applied to repair the damage.

Look at the underside of fenders, the inside of doors and inside the engine compartment.

In the rear, take off the license plate and using a mirror and a flashlight, look at the rear of the automobile for damage.

When a fiberglass repair is made,  the area is reinforced with fiberglass material and often stiffeners or doublers are installed to make the repaired area strong.   You should never see bulges or areas where the underside of the fiberglass is raged appearing and thicker than normal unless a repair has been made.

Symmetrical built up areas are acceptable if they are obviously part of the fiberglass structure formed when it was originally laid up.  These areas will be smooth and very commercial in appearance.  There will be nothing ragged about them nor will they appear to have occurred recently.

Frame Repairs

Frame repairs normally show heat discoloration or slight wrinkles where the metal has been reformed.

While the car is on a lift, look closely at all of the frame structure and supports for any sign of heat treatment or wrinkling.

While you might tolerate a little fiberglass repair you should never buy a Corvette that shows frame damage.  Unless the repair is perfect, uneven tire wear, poor cornering, pulling to one side and even life threatening situations are all possibilities.  It isn't worth it.

Visual Inspection

You will want to climb into the cockpit and take the automobile for a spin right away but wait a bit on that.  If there are serious problems present, why bother?

First go to where the car is located, do not allow the car to be brought to you.  The reason:  you want to see how the engine starts when it is cold.

After you have performed the visual checks of the appearance items, you will want to take a look at a few areas.  Here are some things to check:

• Check the tires for remaining tread and any signs of sidewall damage or plugs and for any uneven wear..
• Note how easy the clamshell hood is to operate.  It should release and lift without binding.
• Feel the engine's valve covers to make sure the engine really is cold.
• Look for any missing fasteners or loose cables anywhere in the engine compartment.
• Look closely for any signs of oil leakage.  Take a mirror and check the rear of the intake manifold as this is an occasional source of leaks but difficult to see from above.
• Inspect the plug wires for signs of cracking or damage.
• Look at all the many hoses for cracking or leakage.
• Look at the serpentine belt for signs of fraying.
• Check the battery terminals for corrosion.
• Remove the air filter cover and check the air filter condition.
• Check the entire front end again for signs of physical damage and repairs.
• Check the coolant tank to see that it has sufficient coolant according to the marking.
• Pull the dipstick and check the oil.  If it has had a brand new oil change, be especially vigilant.  Also, be careful if the oil viscosity feels "stiff".  It is a common practice to put oil additives in the crankcase to hide excessive oil usage.
• Press the hatch release on the edge of the driver's door and the hatch should release on a hatchback model.  Reseat the hatch and perform the same test using the yellow button in the glove box.  If you hear a solenoid sound but the hatch does not release, there may be misalignment of the retainer.  No activity indicates a blown fuse of bad release solenoid.
• Check the security system.  Lower the power windows, exit the car locking them with the power locking switch as you do so.  With the door lock key at the ready, reach into the vehicle and open the door.  The alarm should sound (horn should pulse) and this will continue until you unlock the door using the key.

Replace the air filter cover and ask the owner/dealer to start the engine while you watch.

• Note if the engine starts immediately (Within 2-3 seconds of cranking when cold).
• There should be no "knocking" sound for more than a second after start up.
• Note if the owner/dealer depresses the accelerator at all (Look at the throttle body where the throttle cable attaches to it for any sign of movement when the starter is engaged).

If the engine is slow to start or the owner/dealer "helps" it, there may be hidden engine problems. A knocking sound that continues for more than just a second indicates rod bearing/lower end problems and is potentially very serious and expensive to repair.

When the engine starts, ask the owner/dealer to immediately turn it off and reposition yourself to the rear of the vehicle.  Wait one minute and then ask the owner/dealer to restart the engine while you watch for any sign of smoke from the exhaust. (If it is a cool or cold day, do not confuse condensation with oil smoke.  Condensation will be odorless, oil smoke will not).

Oil smoke normally indicates problems with either the piston rings, the valve guides or both.

Allow the engine to idle for a full three minutes after which time it should be running smoothly unless the ambient temperature is very cold in which case you may have to wait as long as five to seven minutes.

When the engine has warmed up, pull the PCV valve (the hose on the left side of the engine going into the valve cover) and make sure very little oil smoke comes out the hole where the PCV valve formerly was located.  A very small amount is OK but none if what you want and any more than a tiny amount indicates cylinder/valve guide problems.   

After the engine idle is as smooth as you feel it will get, place your hand near one of the two exhaust pipe outlets and feel for a smooth, almost continuous pressure.  You are doing an elementary test for a weak cylinder. and if you can detect instances where the pressure varies here, there is at least one cylinder with very low compression indeed.

Again ask the owner/dealer to shut off the engine and then immediately restart it.

It should start immediately and there should be no sign of oil smoke at the instant it "catches" or again, there may be piston ring/valve guide problems present.

Additional Mechanical Issues

Leaving the engine run, exchange places with the owner/dealer and perform the following checks from the cockpit:

NOTE:  All references to the LCD, Digital Dash or Electronic Instrument Panel, apply only to the 1984-1989 models of the C4 Corvette.  1990 and later have a traditional instrument gauges plus a LCD speedometer display.

• Check the power windows, power door locks, power seats (if present) and power rear view mirrors for proper operation.
• Examine the Electronic Instrument Panel.  The LCD display should have the same amount of light present all across the display.  (i.e. no part of the display is darker than any other).  If not the case, one or more of the four very expensive bulbs in the instrument panel have burned out.  With the labor involved, replacing even one bulb is  $100 minimum and all four will cost over twice that amount).
• Observe that there is a trip odometer mileage reading, remaining range and average MPG reading available for display on the Electronic Instrument Display.  If not, there are problems with the unit and a potential repair bill of several hundred dollars.  (You must verify this later to be certain as will be explained below).
• Turn on the parking lights as and operate the headlamp switch.  Check that both headlights rotate to the up position.  Leave the lights on for now.
• Block the photocell on the digital dash (it's the small, clear window above the speedometer at about 11 o'clock) and rotate the dimmer knob on the headlamp switch.  The instrument panel should dim.  If not, that's another big repair bill to make it work.
• Exit the vehicle and have the owner/dealer operate the bright/dim headlamp control as well as the turn signals.  Do all the external lamps work (including the cornering lights when the turn signal lights are flashing)?
• Return to the vehicle and turn on the air conditioner and radio.  Place the headlamps on high beam. Switch the coolant temp/volts switch to volts and observe the reading.  A reading of less than 11.5 volts with this load indicates potential alternator/battery problems.
• Turn off headlamps and parking lamps.  Adjust the air conditioner and verify that it produces cold air.  If the controller is the electronic variety, set the temperature to the lowest setting (press the cool button until the display will not indicate any lower) and observe that the A/C fan goes into it's highest setting.  Then press the warm button until the fan speed drops back.  Observe the indicated reference temperature and determine if you agree that the setting approximately matches the actual ambient temperature in the vehicle.  If so, the controller and A/C appear to be OK.
• Turn the radio off.  Check that the antenna retracts.  Turn it back on and verify the antenna fully extends and that the radio works on both the AM and FM band plus make certain that all speakers work.  You should be able to place the volume control to 1/2 to 3/4 fully clockwise and the speakers should not distort the sound.
• The engine should be at or near operating temperature:  Check the idle oil pressure and temperature plus the coolant temperature.  (If they are not at operating temperature yet, check them later but do make sure the sensors are giving you an output.  Note that you will get a red warning light if the oil pressure is low.  It should never be low except at the instant of startup and then only for a second or so).
• Exit the vehicle and (if an automatic) pull the transmission dip stick.  Make certain the transmission fluid is at a proper level.  If the car is a manual transmission, have the owner/dealer slowly engage the clutch while you listen for any chattering sound or observe any tendency for the car to lurch as the clutch is slowly engaged which may indicate a slipping clutch.

Drivability Issues

Obey all traffic laws and do not endanger yourself, anyone else or any property as you perform the following tests:

• Return to the drivers seat and fasten the seat belt.  Make sure your passenger does the same. Note that there was a service bulletin on the seat belts for the slackeners.  If the owner/dealer doesn't know if the SB was done, a Chevrolet dealer should be able to find out from the VIN number.   In any event, make sure you can obtain slack on both shoulder belts.
• With your foot securely on the brake, place the transmission in a forward gear.  If a manual transmission, slowly engage the clutch and observe for a "chattering" feeling.  If automatic, observe for delayed response to being placed in gear or unusual noises as the transmission engages.
• Check the transmission in reverse for the same symptoms.
• Drive the vehicle and observe for unusual sounds or lack of response.  (You will hear the ABS system cycle and see a yellow caution light flash on 1987 and later models when you first move the car.  This is the normal testing of the system by one of the computers.   If you don't see this happen on 1987 and later models, there is something wrong with the ABS system).
• Look for erratic shifting in an automatic transmission by trying different acceleration regimes.  The shifts should be swift and sure no matter what the load on the transmission (meaning no matter if you "get on it" or "baby it", the transmission should perform properly.
• Look for synchronizer problems in a manual transmission.  The shift points should go smoothly with no need to "double clutch" or "coach" the transmission into changing gears.  There should be no problems with either up shifting or downshifting and the shifter throws should be short and positive.
• On a flat road, check that the vehicle rolls straight and true without pulling to either side. (Note that the road must not have a "crown" and must be flat in all other respects or this test cannot be conducted).
• In an area where it is safe to do so and the road is flat, accelerate to 30 MPH and apply the brakes as in a panic stop. The car should stop straight ahead without pulling to one side.
• Accelerate again to approximately 25 MPH and make a gentle stop listening for the shrill sound that indicates that the wear limits on the brake pads has been exceeded.
• In an area where it is safe to perform the test, accelerate to 20-25 MPH and rapidly move the steering wheel to the right and left approximately 15 degrees.  The car should respond instantly and there should be no "slop" in the steering linkage. (When you move the wheel even a tiny bit, the car should respond).
• In an area where it is safe to do so,  accelerate to 40 MPH and move the steering wheel approximately 30 degrees first to the right and then to the left.  There should be no sway at all.   The car should track as if it were on rails. you should hear no snapping noises or other unusual sounds.  Suspect suspension problems if the car sways or if you hear unusual noises.
• In an area where it is safe to do so and while traveling at 30 MPH or greater, engage the cruise control by momentarily pressing the "set" button on the top of the turn signal control.   The computer should capture and maintain the speed you are traveling at the moment of engaging the system.  Press the set switch again for an instant and the speed should increase one (1) MPH.  Press the coast button for an instant (it is located at the end of the turn signal arm) and the speed should decrease precisely one (1) MPH.   If the system works in this manner, it is working properly.
• Press the clear buttons on the trip odometer and average mileage (MPG) displays and observe that new calculations begin.  To verify that this system is working properly, when you next shut down the engine, make a note of the readings.  If they have disappeared when you then restart the car, the Electronic Instrument Panel definitely has problems and the repair will be expensive.

Final Checks

Drive the vehicle to a service station with a lift and once the car is elevated on the lift, inspect the underside as follows:

• Inspect the underside for any signs of engine oil or transmission fluid leakage.  Even a drop is cause for alarm depending on where it is coming from. Check the rear axle for the same thing.
• Inspect the suspension for broken or damaged components.  (Also inspect the shock absorbers for any sign of oil leakage.  They should not have oil present on their exterior).
• Inspect the exhaust system for any signs of damage, broken or loose hangers or corrosion.  Look carefully at the exhaust manifolds for any signs of cracking or damage as well as the pipes, cat converter and mufflers. These are all expensive items to replace.
• On a high mileage car, look carefully for any signs of rust or corrosion on the frame or supporting structure.
• Again search for any sign of collision damage, looking at the frame in particular for any signs of straightening or heating.
• Examine the steering arm rubber boots for signs of damage.
• Check the spare tire and jack.
• Examine the tires for signs of uneven wear which would indicate alignment problems.
• Finally, lower the car from the lift and using the procedures explained in the ECM/PCM codes section of this web site, determine if any trouble codes are present in the ECM/PCM's memory.

Summary

If the vehicle passes these checks, depending on age you may wish to have a cylinder compression test performed.  If you are suspicious by nature, you may also wish to make sure that the serial numbers match and that the car has not been modified for speed contests in any manner.  These checks are best made by a professional repair shop or GM dealer.

A car that passes the tests above would appear to be in reasonable mechanical condition but the one thing you always have to remember is "If the car is perfect, why is the owner getting rid of it or why did the previous owner trade it in to the dealer?"

In other words, you can almost certainly expect to spend money on a used car when you purchase it!