I've had an '84 targa for a little over a month (first Porsche). The car is in good shape and runs well. No leaks or smoke, etc. The 915 is a tad balky but I understand this is common. Soon after purchase I noticed a hollow rotating sound that seemed to come from the rear as the car moved. It grew to a distinct tap. I took it to my mechanic who found that the clutch adjustment assembly was touching the heat exchanger. He "radiused" the exchanger to provide clearance, which took care of the tapping sound. He also found that two inner CV joint flange bolts had sheared off; he extracted the two and replaced all six, and also checked the other side and found that they were okay. Since that time (three weeks ago), the tapping has not returned but the initial hollow rotating sound persists. Any ideas? Am I simply unaccustomed to the sound of an older Porsche as it moves? Also there is a slight ringing sound from underneath (approximately midship), on occasion as the car idles or comes to a stop. Any thoughts on that. I very much appreciate your attention and any advice you can give.
Moss,
On the "hollow" sound, do you hear it while accelerating, does it go away or get louder or softer on decel? Do you only hear it in certain gears, or in all gears? Does it get louder or stronger as the speed of the car increases? Last if you hear the noise while driving down the road and put the car in neutral does the noise continue?
On the clutch assist "omega" spring that your mechanic "clearanced" to the exhaust, have your mechanic disassemble the clutch linkage and check two items. 1) check the post the spring attaches to on the clutch arm, this has two needle bearings supporting it and they may be worn. 2) have him check the fixed post that mounts to the differential housing cover and has a hardened pivot pin pressed into the mount. If either of these is worn it can cause the issue you're seeing with the spring hitting the heat exchanger. You shouldn't have to clearance or grind the housing.
Marc
Marc Bixen
RedLineTechnik.com
Red Line Service
Marc:
Thanks for your reply and thoughtful questions. Can you let me know the significance of each?
As for answers, the hollow rotating sound is virtually the same whether the car is under acceleration or the opposite. It does not grow louder at increasing speed, and it continues when the clutch is depressed.
I thought it would be easier to hear when the car is buttoned up but no, it's especially clear when the targa top is off.
I can't make sense of it. It's annoying. And I'm reluctant to put the car in the shop again.
Sincerely,
mg
Moss,
The first question, louder or softer on or off the gas has to do with the pinion shaft bearing and whether it is loose in the case. The individual gear question has to do with whether you have an issue with the actual gear teeth being worn or pitted, or a problem with the needle bearings that the free gear rotates on. Many times these problems will also be associated with a metallic look or a metal flake look to the gear oil when you drain it. To diagnose issues with gear teeth, dog teeth and syncro/brake bands we use a high tech bore scope and visually inspect the inner workings of the gearbox. The pinion and needle bearings require disassembly to inspect.
Noise at road speed can be differential bearings or wheel bearings which can be diagnosed using a stethoscope or a long screwdriver, tip on the suspect part, handle to your ear.
You may also be hearing a noise in either your Alternator bearings (place the stethoscope on the fan housing) or the A/C compressor clutch bearings. If you suspect this you can remove the belt and run the car without it. If attempting this on the alternator, don't run the car more than a minute or so. Remember this is an air cooled engine and the alternator belt runs the cooling fan also. If you run it without the alt. belt your dash lights will remain on.
Marc
Marc Bixen
RedLineTechnik.com
Red Line Service
