I have a 1999 coupe and love it along with my 78. Totally different experiences driving the 2 cars. To me the C-5 is a rocket ship and handles exemplary. 2 weeks ago I drove on the NCM Motor Sports Track and had one heck of a time doing so in my 99.
Really my only advise is don't necessarily buy the first one that comes along as, there are a lot of low millage well taken care of C-5s and a lot that
weren't taken care of, as you well know. Look for service records if available.
If he is not in a hurry and does not limit himself to only cars in his area I feel confident that he can find someone's C-5 that has been well cared for and could be a single owner car. That is what I would look for.
Age is still the biggest problem with all the systems. A garage kept car will always be best.
As with any car that is older be sure that the weather stripping is sound, as it is getting harder to get and more expensive. The repop stuff is cheaper but never fits exact and not as soft. One area of wear is on the removable top towards the rear, where it meets the upper body. I have replaced mine on the driver side with repop because I could no longer find NOS and was not entirely happy with the results.
Listen for creaks and moans as this is a common problem with the C-5 as they get older. If it does do this when the suspension moves it
can be a relatively easy fix, as is likely caused by the rubber retainers that support the torsion bars front and rear as the bars move.
Be sure to check the torsion bar links, I just had mine replaced at 52,000 miles, as the nylon seals failed to protect the bearings. This was 100% age related. You can look under rear t5o check, but will need a lift to check the front.
Be sure to check all the electrical systems for function, as any repairs here can be costly and look in the computer display for any error codes located in the speedo/tach array.
Lastly, don't buy a C-5 owned or driven by anyone under 35
, as it has likely had a hard life! As you know run-flats are expensive to replace if threadbare.
I am sure most of this you know, but I hope it helps your friend in some way,
John
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John D. Grant "Corvette; Cure for the common mid-life crisis!"