How do you diagnose a liner or piston rap from a Land Rover
engine?
These are the most serious noises, because they call for
complete engine teardown and rebuild.
Liner noises are often confused with valve train rap, and considerable
time may be spent chasing a false trail.
Piston and liner noises sound similar but the setup and
circumstances may differ. Here are some
videos where you can see and hear the difference.
Piston noises tend to come on gradually, with no noise at
first and increasing noise as the engine warms up. One good clue to a piston noise is that they
tend to worsen under throttle, and go away if the plug wire is removed from the
affected cylinder.
The following video shows the piston movement and associated
noise in a dismantled engine:
Piston rap happens when the pistons are too loose in the
bore. This was particularly a problem
for the 2003-4 Discovery engines. The
pistons are usually tapered with more wear at the bottom. This taper causes a poor combustion gas seal at the piston rings, so motors with sloppy pistons may use oil, or have dirty oil.
Those things aside, piston rap is not fatal to the engine. A truck with loose pistons may run for years,
making gradually more noise. We fix the
problem with new flanged liners and new pistons. The liners are honed to the exact size of the
pistons and the rebuilt engine typically runs more smoothly and silently than
it did when new.
Liner noises are different in that they often come on
suddenly like flicking as switch, as the engine gets warm. The engine will be silent, and then,
suddenly, it raps so loud you think it’s coming apart. Once rapping the liner noise is not affected
by pulling a plug wire. The noise may go
away under throttle on the road, as combustion pressure jams the liner in place.
In a cold engine the liner is held tight by metal contraction. As the motor heats up the aluminum block expands more than the steel liner, and at some temp the liner comes loose. That is when the rap starts. That's why it has a sudden onset, and that sets it apart from gradually increasing piston noise.
In a cold engine the liner is held tight by metal contraction. As the motor heats up the aluminum block expands more than the steel liner, and at some temp the liner comes loose. That is when the rap starts. That's why it has a sudden onset, and that sets it apart from gradually increasing piston noise.
The video below shows (in the center of the frame) the liner
moving up and down as the crankshaft is rocked back and forth in this 2003 Land
Rover Discovery 4.6 engine. The noise in the video is what you hear when the engine is running, just louder.
Liners move when they were installed incorrectly. In the manufacture of the motor the liner was
supposed to be pressed into the cylinder bore until it seated against the step
cast into the block. Then the liner would be cut off flush and
countersunk. Liners that are properly
installed CANNOT move, because they are constrained at the bottom by the block
casting and at the top by the head and head gasket.
After the liner moves for a period of time it will destroy
the head gasket fire ring, and the engine will fail. Until then, the truck will run fine.
Check out the photos of an original liner, and a flanged
liner. We fix this problem by fitting new liners with flanges at the top, so
they cannot move up and down. The liner
is clamped between the block and the head gasket and cannot move. The fit at the bottom – where the original errors
occurred – is no longer important.
 |
| The original Land Rover V8 liner, the one that breaks loose and moves (c) JE Robison Service |
 |
| A flanged liner, which cannot move (C) JE Robison Service |
Here is a closeup of an engine block, counterbored for flanged liners. If Land Rover had taken this step to build a better quality engine, these failures would not be happening today. Carmakers make all sorts of tradeoffs to same money, and this one is not working out too well
 |
| Land Rover block, ready to fit flanged or top hat liner (c) JE Robison Service |
In addition, the flanged liners seal against combustion
gases getting into the cooling system through cracks in the block casting
behind the liner. This is another and
more common cause of Land Rover engine failure.
As of this writing – Spring 2016 – the cost to bring a truck
into our shop, rebuild the motor to address failures like this and repair the
other issues common to a worn out motor costs $11-15,000. When motors are rebuilt we can upgrade from
4.0 to 4.6 and we can balance and blueprint for greater smoothness. J E Robison
has been doing these rebuilds and upgrades longer than any other shop in the
USA, and we’ve seen a very good service record on the rebuilt motors.
The change to flange liners addresses a fundamental weakness in the engine, and renders them much less susceptible to future failure. To read more about liner issues and what we do to rebuild and improve these motors I suggest this essay from 2012
The change to flange liners addresses a fundamental weakness in the engine, and renders them much less susceptible to future failure. To read more about liner issues and what we do to rebuild and improve these motors I suggest this essay from 2012
(c) 2016 John Elder Robison
John Elder Robison is the general manager of J E Robison Service Company, celebrating 30 years of independent Land Rover, Rolls-Royce and Bentley restoration and repair in Springfield, Massachusetts. John is a longtime technical consultant to the Land Rover, Rolls-Royce and Bentley clubs, and he’s owned and restored many fine British motorcars. Find him online at www.robisonservice.com or in the real world at 413-785-1665
Reading this article will make you smarter, especially when it comes to car stuff. So it's good for you. But don't take that too far - printing and eating it will probably make you sick.
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2 comments:
Great information, very informative. Thanks for the post, love your blog.
Matt
nice post
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