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:
>I started up my E28 M5 this morning and it made a horribly loud
>clanking, clunking noise. It went away after about 15 seconds
>so my first thought was that the tensioner had bled down overnight
>and pumped back up. After work, it started up without noise. I
>searched the archives but didn't find any descriptions of pending
>tensioner or rail failure. So what's next?
>1. Could it be something else?
>2. If it is the tensioner bleeding down, how long do I have before
>some kind of catastrophic failure?
>3. My car has 145K . Should I count my blessings, stop driving it
>and fix it?
>4. If I am replacing the tensioner, what are the other natural
>things to replace while the front end of the engine is apart?
>5. Alright, give me the bad news. What are the parts I need, how
>much and what are the best sources?
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
- -- we always leave our cars in gear). The remaining section
discusses Chris Simon's theory of why the tensioner bleeds
down when parking on a hill (I agree with Chris).
<old note to Charles>
Yes, I can't imagine leaving my car out of gear on a hill. Remember
the cam chain tension force is only from the engine turning the cams
over, not from holding back the weight of the car. The car is prevented
from moving by the compression in the cylinders (and gear ratio)
which prevents the crankshaft from turning, which prevents the
gear box and final drive from moving. You could even cut the cam
chain and the car wouldn't roll.
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.
[ Help ]
(143K miles, 26 driver schools in this car -- quiet chain)