911 Timing Chain Tensioner Malfunction
By Roger W. Chaney
PCA Technical Committee Chairman
This is a frightening experience because some very expensive noises
are heard from the engine when it happens. The sound is a medium
intensity ratchetting which may or may not be noticeable at idle.
The noise increases up to 2500 to 3000 rpm after which it
disappears. There is a danger of slipping valve timing a slight
amount if you run high rpm (over 5000) or load the engine heavily
with this situation. There is no danger of throwing a timing chain
off its sprockets or damaging the engine if you drive conservatively
until you can correct the problem.
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When this first occurred in my 911, everything was again normal
after the engine cooled below 140F oil temperature. All was good for
about 2 weeks; then the noise occurred again. The regional Porsche
service representative was consulted immediately. He informed me
that the probable cause was some kind of “dirt” in the tension
valve. The problem began to occur about once a week, usually on
weekends about 100 miles from home!
This was really beginning to be an annoyance and a
potentially serious problem, so I called the local dealer who
reported he just couldn’t take me under any circumstances
until the next Wednesday! I couldn’t drive the car until then
and the noise would no longer go away when the engine cooled, so
I decided to take personal and immediate action.
The Factory shop repair manual shows all cam drive work done with
the engine removed. How ever, careful examination led me to believe
this was not necessary. I removed the muffler (4 17mm exhaust flange
and 2 15mm hanger strap bolts), the distributor secondary lead to
prevent damage, the oil temperature lead (for the same reason), the
mechanical fuel pumps (2 12mm nuts and 4 flex fuel lines), and the
rear splash and dust cover (10 10mm bolts) by pushing rearward and
down ward. Now both cam housings are fully exposed and can be
pulled by disconnecting an oil supply line (1 12mm and 1 14mm wrench
required) and 10mm cover nuts. You don’t need to worry about the
mechanical pumps on engines after 907001 or the oil lines after
engine 903070. |
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The chain tensioner will now be right in front of you. Notice that I
have been speaking all along of the left tensioner. This is because
the left tensioners have failed more although I know of only 3
tensioner failures on 911 engines including mine. However, there are
undoubtedly more.
Inspect the tensioner to see if it has plenty of oil. You should not
be able to see any part of the tensioner except the upper part of
shaft (see the illustration). If the reservoir is low or empty, you
have found at least the evidence of your problem. You should remove,
clean, and inspect all of the tensioner parts.
To remove, first place a large screwdriver between the idler
sprocket arm and tensioner housing. Carefully pry up to be sure the
chain is tight. Remove the 12mm tensioner retaining nut and slowly
pull the unit out. Keep the plunger from snapping up suddenly as it
clears the sprocket arm. Clean a tensioner by draining all oil
possible
and disassembling as follows: Push the plunger down slowly, and
carefully pry the lock wire ring (figure, item 8) using a small
screwdriver and needle-nose pliers. Allow the piston to slide up and
out of the housing. Hold securely to everything for the inner spring
is very strong! Now turn everything upside-down and allow the
pieces to slide out. You may need to strike the piston against your
hand to dislodge the ball and its cage. Do not try to remove the
ball seat or you will probably damage it. Seats are pressed into the
piston.
Scuff marks on the piston may mean that there is binding in the
cylinder at spring and summer oil temperatures (above 170F). In this
case it is best to replace the entire tensioner assembly (right and
left are identical). If your trouble is evidently “dirt,” clean
all metal parts by flushing in kerosene or other good, unleaded
solvent or by soaking in a “gunk” for about 30 minutes. Wash off
all gunk and reassemble according to the illustration. Dip each
part in engine oil before installation.
The toughest assembly job is replacing the spring ring. Aids you can
use (since most people will not have special tools) are a slightly
used slip-on pencil eraser and a short length of mild steel safety
wire. The eraser fits perfectly over the piston shaft. Press the
piston down until its upper shoulder is just below the snap ring
groove. Hold in this position using safety wire over the eraser and
around the housing. Twist under the housing to secure. Install the
ring using the piston as a guide and 2 small screwdrivers as tools.
Be careful not to slip and score the snap ring, piston, or cylinder bore.
All new and reassembled tensioners must be filled and bled of air.
Remove the wire and eraser and make a bleeder tool from a soft
paperclip, large diameter copper, or brass safety wire by bending as
shown.
Depress the piston and fill the reservoir with engine oil. Allow
piston to rise until it stops. Insert the bleeder tool to depress
the ball valve and work the piston through its full travel slowly
several times. Allow all bubbles to disappear before allowing
piston to rise on each stroke. Keep the reservoir full all the time.
When no more bubbles appear (this may take 5 minutes of bleeding),
install tensioner as far as possible with the bleed tool in place.
Press the piston down to clear the idler sprocket arm, slide in to
its stop, and release. Carefully maneuver the bleed wire out (if it
binds, you may need to remove the tensioner, rebend the bleeder, and
repeat) and close the area in reverse order. Be sure the reservoir
is full.
Reinstall the cam housing cover, with a new gasket if necessary,
making sure the surfaces are clean and use a non-hardening sealing
compound as Willys gasket cement (Kaiser Jeep parts stores), Ford
(Rotunda) gasket cement, or Permatex 3D. Use only a light, even coat
on both gasket faces. Install the gasket, cover, lock nuts and
tighten the nuts evenly and only to a light torque. A 1 finger pull
about 1 in. out on a box wrench is enough as overtorqueing will warp
the cover and cause serious leaks. A correctly installed gasket
can be re-installed several times without replenishing sealer with
no leakage. You may need a little sealer on the fuel pump gaskets as
well.
Clean the fuel line metal sleeves with fine sandpaper to remove
any corrosion, apply a light coat of glycerine outside of each hose,
slide on the sleeves, slide on each hose about 3/8 in.,
and pull on each sleeve as far as it will go.
My problem finally was diagnosed to be a binding piston. A new
tensioner is in and has been working fine for about 2 months.
6/67
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