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buzz/hum Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Fri, 10 Aug 2001 08:39:03
In Reply to: "Spark Knock", Derek, Fri, 10 Aug 2001 01:04:20 Members do not see ads below this line. - Help Keep This Site Online - Signup |
That buzz/hum you hear from the back of the car is the fuel pump. It's buried in the fuel tank under the trunk. You'll notice it's louder if you ever drive around with the rear seats down. Perfectly normal.
The turbo system in your car has APC - Automatic Performance Control. There is a knock detector in the engine, and if it detects knock, it reduces turbo boost automatically. Engines are more prone to knock under high boost. The APC can dial the turbo back from Full Boost (needle at the top of yellow/beginning of red) all the way down to base boost (needle mid yellow).
If you can get full boost from your car, then there isn't any knock. Find a nice long uphill, and hit it in 3rd gear at 45, and floor it. You should see full boost.
You should never hear knock, because the APC will change boost to prevent it. Because of this, you can use any octane fuel between 87 and 93 (or higher) - US numbers. The octane rating of fuel indicates how resistant it is to knock, the higher the number, the less likely to knock. Just about any Saab turbo engine will provide full boost at 93 octane. Because every engine is a little different, and those differences increase with age, some engines need better gas to prevent knock at full boost, but others don't. My '88 9000T gets full boost on 87 octane. But the exact same engine in my old 900T needed 91 to get full boost. Engine to engine differences. If you run lower octane and you don't get full boost, you won't harm the engine, but you won't get as much performance, either.
As to the crackling noise, my guess would be some mount is slightly loose - not a lot, but enough to shift just a little and the metal on metal sound makes a crackling noise. If you're worried, have your mechanic tighten up the steering rack mounting bolts. No, the mount isn't about to fly out of the car - but the rack can loosen up just a little, and those small shifts under load can make some interesting popping sounds.
Yes, alignment can affect braking, but not a lot unless the alignment is way out of whack. As Bill J says, what are the symptoms?
posted by 140.157....
, Fri, 10 Aug 2001 08:39:03 <-- Viewing This Message
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