Jack Olson's post is pretty much on the money.
I bought a '91 C4 with about 18K on it two months ago. I love it, but the 964 series is NOT for the faint of heart or the budget oriented.
I think the biggest problem is probably the Freudenberg dual mass flywheels, simply because I've read several posts about previous owners needing them replaced and the repair isn't cheap. In my opinion, even if the Freudenberg seems good and you need a clutch job, that is the time to put the a new flywheel in. I wouldn't go out of my way and do it just because there is a risk, I would only replace it if it failed or I needed a new clutch. The worst case scenario that I've come across occurs when someone has a clutch job done, and then 2000 miles later the Freudenberg fails, requiring all the work to be redone. The typical fix is either a switch to a LUC or a switch to a single mass (usually light weight) unit with a chip that can accomadate the change properly. The more reliable LUCs became standard sometime in 1992. On the other hand, I've also read posts about people using the Freudenbergs well into VERY high mileage. Roland Kunz (an expert on p-cars) has stated that the Freudenberg failures are likely caused by racing and/or very hard driving. In my opinion, it's sort of a crap shoot with the Freudenberg...it may very well already be replaced on a car you're looking at. Checking records and serial numbers on these cars is VERY, VERY important. KNOW WHAT YOU'RE BUYING AND GETTING INTO!!
As far as the cylinder leakage problem goes, it was a problem but how severe a problem is difficult to determine. I used Bruce Anderson's book as a guide and was able to purchase a car with the cylinder head leakage problems addressed at the factory (my car was built in early Feb. of 1991, and it was roughly 150 production units from the engine modifications...so mine was one of the early ones, and there are no leaks). When I was looking at 964s, leakage (if any) was usually non-existant or limited. If I found a "leaker" it was often due to seals leaking on cars approaching or exceeding 100K miles. This is not uncommon on any car. I've looked at a TON of 964s and leakage was prevalent only on the higher mileage models. Either Porsche fixed these under warranty or the problem is overblown. I can't honestly tell you how serious the problems really are. The fix, if you have this problem, is almost a complete overhaul ($$$$$$$$$$$) unless you can get Porsche to assist you (not really likely almost 10 years after the cars inception).
The distributor is a dual distributor that is belt driven. Porsche originally had a problem with the belt driving the secondary distributor breaking. Symptoms can be as mild as a loss of power to extreme detonation that can, in the right (or perhaps a better word would be wrong) conditions, blow a hole in the top of the pistons. Most people seem to suffer either a slight loss of power or some knocking, and I've never personally seen or heard of someone blowing holes in their pistons, but there is no doubt in my mind that this has happened. It isn't uncommon for the dual distributor to fail and the owner to be unaware of it. The "fix" is to install the vent kit (Ozone created by arcing in the distributor chamber is believed to cause the belts to fail prematurely) that costs about $15.00. Any decent P-car mechanic could install it in less than 1/2 hour. It's an easy job. If the belt fails, you need to buy a remanufactured unit or have it repaired.
Is the 964 a worse car than any of it's Porsche forebearers? I kind of doubt it. The SC's have problems with studs, chain tensioners, poor AC/ventilation, pop off valves, not to mention the normal problems caused by age. The newer Carreras can have valve guide problems, and if they have a 915 transaxle may need some costly repairs. Every car made always has it's own set of problems in my opinion.
Is the 964 (or for that matter any 911) worse than any of the Japanese cars? I doubt that too. I bought an '85 MR2 that turned into the biggest money pit I've ever owned several years ago. I ended up doing a top end overhau at 90K, the AC failed, the electrical system needed to be overhauled. The MR2 was supposedly a "cheap" car...until you end up paying the prices Toyota wants for replacement parts...damn near ANY old Porsche is cheap by comparison! When I bought my Porsche I traded in a Dodge Stealth RT/TT (Dodge's relabelling of a Mitsubishi 300GT VR4). If you want to talk about unreliable and costly, look at anywhere from $4000-$8000 for a transmission repair (swap out only, no parts available...even for oil leaks), engines that like to spin their bearings at about 80K-100K miles which often costs $3000, and since the car is impossible to work on you get the added feature of paying premiums for labor on a car that's depreciating so rapidly in value you'ld be better off parting it out than repairing it.
My take is that ALL cars have their own set of risks. The 964 WILL be more costly to work on (more difficult, 12 sparks instead of 6, parts prices are higher), but in my opinion the added power and performance, not to mention creature comforts (like AC that actually DOES something) is worth the money.
As far as 964s go, the later the model year, the better. Call up several independent p-car mechanics and get their impressions. I did that and my results were really sort of inconclusive, probably based on the experience of the mechanics.
Hope this helps,
Bill Wagner
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