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Ron Stygar Carl Buckland Dale Beuning Forums Help
From digest.v9.n1189 Fri Aug 13 06:01:44 1999 From: "Pete Read" <pete.read_at_boeing.com> Date: Thu, 12 Aug 1999 07:51:10 -0700 Subject: Re: <ALL><E28 M5> Cam Chain Rattling Problem at Start-up
Danial Ma writes:
>1. Could it be something else? Danial, While this is a specific question about an M5, I believe most of my advice applies to all BMWs with timing chains (versus belts). I've talked with several people about cam chains in general and M5/M6s specifically because it's such a big concern for a car with a $10K head. Cam chain guide rails wear, but weak cam chain tensioners seem to be a more critical problem because they allow slack in the chain, causing chain whip, which can break guide rails or let timing jump. Cam Chain Noises Any time a cam chain makes noise at start-up, don't rev the engine, let it idle until it quiets down. Rattling noises mean the cam chain is not tensioned properly. Revving the engine with a "loose" chain can cause it to jump timing, causing considerable damage to the engine. If your car is parked on level ground and the cam chain makes noise at start-up on a regular basis (like once a week), it's probably time to replace the cam chain tensioner because it's bleeding down (not holding pressure). The tensioner assembly (cylinder and piston) screws in at the right front side of the engine. M5/M6 Tensioner Assembly 11 31 1 307 782 $81.95 If it makes noise once in a great while, don't worry about it. Parking on a Hill Parking on a hill can cause a similar problem if the engine is turning the reverse direction (e.g. pointed up hill in a forward gear). The latest issue of the Roundel mentioned this but didn't offer much of an explanation or the obvious solution of picking the proper gear to prevent the problem. When parking on a hill, select a forward or reverse gear that corresponds to the direction the car would move if it was left to roll, i.e. reverse when facing uphill and first gear when facing downhill. This keeps the tension on the correct side of the cam chain (engine rotating in normal direction). Detailed Explanation of Parking on a Hill Problem For convenience, I'm copying part of my reply to Charles Karnati when were discussing cam chains privately last year. The first paragraph addresses people who say never leave a car in gear on a hill for fear of stretching the cam chain (Charles and I disagree
<old note to Charles> I don't have a cam chain rattling problem, but I thought about what Chris said, and decided that his explanation makes sense. It may be easier to picture this is if you compare a cam chain to a bicycle chain (multi-speed with tensioner/derailleur on the bottom). The top of the bicycle chain is taut from the drive force of the front chainwheel (pedals) driving the gear on the rear wheel. The bottom of the chain is relatively slack, so that's where the tensioner/derailleur is located. The cam chain works the same way. The crankshaft sprocket (like the chainwheel/pedals) drives the camshaft sprockets. The direction of rotation (clockwise when standing in front of the car) makes the driver's side taut while the passenger side is relatively loose. So, like the bicycle chain, the cam chain tensioner is located on the loose passenger side of the engine. Now, if the car is parked on a hill, nose up, the car will try to roll backwards. If the transmission is in any of the forward gears the engine will try to rotate the reverse direction, making the passenger/tensioner side of the chain taut. This could push the tensioner back, making the chain rattle some on start-up. I don't think the rattling is a big deal, but here's an easy way to prevent the problem. If the car is pointed downhill, put the transmission in first. If it's pointed uphill, put it in reverse. This will keep the engine rotating in the normal direction with tension on the correct, driver's side of the chain, reducing rattling on start-up. Of course, if the emergency brake is applied first and it's real strong, none of this should make a difference. <end old note to Charles> Other Ominous Noises Another ominous noise heard while idling is gear backlash in the transmission, especially when the car is hot (clutch out, neutral, input shaft is spinning) Confirm it's gear backlash by pushing in the clutch -- the noise should stop and you should stop worrying.
Pete Read |
