vacuum leak or cooling fan

[herr_oberst]

Since day one, my 88 3.2 carrera has a whistle when I reach cruising rpms. I've checked all the vacuum hoses, re torqued all pertinent induction system nuts and bolts, to no avail. Should I keep looking or (hopefully) do these huge fans emit a low whistle at cruising altitude, ie no load?

[burgermeister]

Mine has a very distinct whine between 2400 and 2700 rpm. Not really a whistle, unless it's a really low one - sure does not sound anything like a vacuum leak. Dominates the engine noise at a steady cruise. I've wondered if it's alternator bearings...but the alternator spins freely without noise when I take the belt off. I'm thinking it's some kind of resonance of one of the many sheetmetal pieces - 2600 rpm * 11 (Number of blades on fan?? I am too lazy to go down to the garage and count) / 60 sec/min = around 400 Hz - seems about the frequency I hear.

[der Mond]

I had the manifold off my '87 3.2 to change engine compartment gas lines, and triple-checked all my vacuum and breather hoses upon reassembly. When the local mechanic (non-Porsche) then adjusted my CO (successfully) he said he looked for the vacuum leak and couldn't find it. I said "What vacuum leak?" and he said "the one I hear." The same one you hear. Fan, alternator, whatever, that's just how they sound. Have your CO rechecked anyway if you never have, and drive in peace. Having my CO readjusted gave me a greater kick in the pants feel than anything else I did to the car. Like 15 free horsepower.

[herr_oberst]

Thanks, guys. The input is much appreciated.

Mike

[burgermeister]

der Mond, What is the CO adjustment that gets mentioned all the time? Surely not Carbon Monoxide, which is what it stands for in Chemistry class - I can't imagine a specific adjustment for that unless one was trying to commit suicide with the 911...

Upon closer reconsideration, my fan sounds a lot like a mellow version of gear whine (which it isn't since it sounds the same when revving the engine while parked). Also can't be a sheetmetal resonance since the frequency varies with rpm - a metal piece would only have a single frequency as resonance. So it must be the air space contained by the fan shroud - sort of like an acoustic guitar amplifies or attenuates certain frequencies....

Sind alle Teilhaber an diesem "Faden" Deutsch, oder sind es nur die Usernames??

[Paulporsche]

burgermeister,

CO is carbon monoxide (the same one as in chem class). It's a by product of combustion, and is one of the greenhouse gases. As long as it doesn't get into the cabin, say from huge holes in the heat exchangers, it won't harm you. The problem stems from the vast quantities of it in the atmosphere, which for the last 30 years or so seem to come more from burning fossil fuels, like coal for electrical power, or lesser regulated sources, like airplanes, than from automobiles.

[burgermeister]

Sooo... why would CO be adjustable, and with a screw on the mass airflow meter to boot? Is the screw just a rich/lean offset to the airflow meter (essentially a calibration feature)? Wouldn't the O2 sensor cancel that if it was off very far? And why would adjusting CO increase power output?

I've never heard of a car with adjustable CO before I got my 911...

[der Mond]

I know only a little; here is my Rennlist Post:

At almost 142K miles I removed the FI manifold to replace gas lines. After reinstall with new gaskets and insulators and tightening everything up, my local mechanic found CO at 1.7! He adjusted it to 0.8 and said with the 02 sensor reconnected it read 0.3, which he said was great. He brought the idle back to specs also. This has been the first time I really notice a different sound and feel to the engine. I have no idea when CO had last been adjusted due to no records, but the car has never felt this great since purchase at 118.5K.

...and here is a response by Peter Zimmermann, author of: "The Used 911 Story" (get that book!)

Dennis: Outstanding! I hope that you still have a little time left to drive your car before weather becomes an issue... I'm also glad that you posted this, so many people don't understand how important correctly set CO% really is, and how it can truly wake up a sleepy car. We used to get first time in cars at the shop, and a phone call from the owner shortly after we gave the car back was not unusual. We used to hear comments like, "How did you give my car thirty more horsepower?" Needless to say, we loved those calls because all we did was set everything to factory spec!
Pete