©Kondinin Group
from the workshop:
Reproduction in whole
or part is not permitted
without permission.
machinery maintenance
FreecalL 1800 677 761
Josh
Giumelli
kondinin group
1
Chasing faults in
diesel engines
A little diesel engine know-how can go a long way to keeping farm trucks
and tractors motoring, not to mention the money saved from avoiding the
need for a professional mechanic.
The colour of the exhaust smoke is a fair indication
of a tired engine. Most older diesel engines will
smoke heavily under start-up and acceleration.
Blue smoke while running indicates burning oil,
most likely from worn rings, cylinder liners or worn
or damaged pistons. Heavy black smoke indicates
a lack of oxygen in the cylinders for complete
combustion of the fuel. The first item to check in
this instance is that the air filter is not clogged.
Another cause could be a faulty turbocharger.
If a modern diesel engine is not running as
well as it should, the usual course of action
is to have a service agent plug in a computer
to diagnose the fault.
On farms across the country, thousands
of older diesel trucks and tractors are still
earning their keep as reliable workhorses.
But when anything goes wrong, a little
know-how and a methodical approach
usually can isolate the problem.
4
A lot can be learnt about the engine by
starting it cold. Diesel engines struggle to
start on poorly atomised fuel, evident
through large amounts of white smoke from
the exhaust as the engine is turned over.
Worn or faulty injectors or injection pump
are the most likely culprits. On engines
fitted with glow plugs, difficult starting is
often due to the failure of a plug (see step
six). A lack of compression also will make a
cold engine difficult to start.
6
2
A misfire on a particular cylinder will lead to rough
running and a periodic puff of white smoke from
the exhaust pipe. With the fuel stop engaged to
prevent the engine from starting, crank the engine
with the starter motor. If it appears to race every
couple of revolutions, it is likely one cylinder is low
on compression.
If glow plugs are suspected of being the cause of
difficult starting, remove and inspect the filaments.
Worn engines equipped with glow plugs for starting
will often start and run more easily when compression
is lower than engines without glow plugs.
7
5
Oil mist from the crankcase breather or dipstick tube
(inset) indicates excessive crankcase pressurisation.
This is caused by worn or damaged pistons, rings
or liners. A worn engine in this condition acts
like an air compressor and the excess air escaping
the crankcase leads to an oily film over the entire
engine and bonnet.
3
It is difficult to chase faults on a filthy engine, not
to mention it being dirty work. A quick degreasing
and pressure-washing will help locate any potential
leaks and keep the workshop tidy.
Check under the radiator cap for oily residue, which
indicates a blown head gasket. If a combustion
gas analyser is available it can be used to detect
the presence of combustion gas in the coolant (see
Farming Ahead, No. 179). A milky grey emulsion on
the dipstick indicates water in the engine oil from
either a blown head gasket or leaking liner seals.
On diesel engines, glow plugs are either connected
electrically in parallel or in series in a daisy-chain
fashion. If one plug fails in a series-connected
layout, then none of the glow plugs will operate.
If one plug fails in a parallel arrangement, the rest
of the plugs will still work and the engine still should
be relatively easy to start. Parallel-connected glow
plugs will be rated at a higher voltage (near 12
volts) than series plugs. The series plugs pictured
are only rated at 0.9v.
Farming Ahead May 2008 No. 196 www.farmingahead.com.au
45
from the workshop:
machinery maintenance
8
A test lead can be made to check the function of
the glow plug across a battery — it should glow
red–orange as shown. Be careful with low-voltage
plugs — only touch them onto a battery terminal
momentarily or they will blow. Alternatively,
test the plugs one at a time by connecting to
the glow plug wires with the plug removed from
the engine.
11
14
The diesel compression test kit contains several
adapters for fitting to the cylinder, either through
the glow plug hole or injector hole. The gauge unit
will handle up to 70 bar, far in excess of a petrol
engine compression tester (see Farming Ahead,
No. 123).
Remove the injector of the misfiring cylinder.
If the injector is tight, use a pinch bar as pictured
to apply pressure.
9
15
12
Install the adapter into the glow plug hole.
Depending on the style of tester used, a valve with
quick-attach coupling might need to be screwed
into the adapter (pictured).
16
To trace a misfiring cylinder, first check for fuel leaks
in the injection system with the engine running.
The engine must be clean for this operation. Never
‘feel’ for a leak; fuel injection systems operate at
several thousand pounds per square inch and fuel
can be injected easily into a hand. It is for this
reason that fingers should never be placed over an
injector nozzle when it is removed from an engine
and actuated.
10
Re-attach the injector to the fuel line and crank
the engine using the starter with the throttle open.
With a sheet of paper, inspect the spray pattern
(it should form a cone-shaped spray). Never place
a finger over the nozzle. If the injector is faulty
remove all injectors for overhaul and resetting by
an injector service firm.
13
Run the engine and progressively crack open the
injector fuel lines until fuel flows from the union.
This usually takes only one-quarter of a turn.
On a cylinder firing correctly, the exhaust note will
change and the engine might labour when the
union is cracked open. On a non-firing cylinder,
there will be little difference. This method is similar
to progressively removing spark plug leads from
petrol engines to test for a misfiring cylinder.
If poor compression appears to be at fault, a
diesel compression test kit can help determine
the fault without stripping down the engine.
The unit pictured was bought new from eBay
for $65.
46 Farming Ahead May 2008 No. 196 www.farmingahead.com.au
Attach the compression tester as shown. Ensure the
fuel shut–off is activated to prevent injection during
testing. If the engine is fitted with a throttle valve
in the air intake (such as a Fordson Power Major),
ensure it is in the wide-open position.
17
Crank the engine with the starter motor and
note the compression reading rising in a jerky
fashion on the gauge. After about 6–8 revolutions
of the engine it will level out at a final reading.
With the reading noted, release the compression
in the tester by pushing in the button below the
gauge. Repeat the procedure for other cylinders.
from the workshop:
machinery maintenance
18
19
20
Compare the readings taken from all cylinders.
If the manufacturer’s specified compression
An oil can is the handiest method for adding
pressure is available, this will make a valuable
the oil (plain engine oil) but first test the amount
comparison. For a healthy engine, compression
the can pumps from pressing the lever once.
readings should not vary by 10–15 per cent. If in
Add no more than one tablespoon and repeat
doubt, re-test any low cylinders to confirm low
the test on offending cylinders. If the compression
readings. If two adjacent cylinders have low
readings rise significantly, it indicates worn piston
readings, it is likely the head gasket is leaking
rings are at fault. If there is little or no increase,
between them, allowing mixing of gas between
it indicates leaky valves or a blown head gasket.
the cylinders. If readings are low, carry out a wet
Take care as the head gasket can sometimes blow
compression test. This is where a small amount
through to an oil gallery such as a pushrod hole.
of oil is added to the cylinder. This is risky due
There will be no evidence of combustion products
to the high compression ratios found in diesel
1engines,
1 4 2 which
4 _ 1means
3 0 xthat
1 if8 too
8 .much
p doil
f is added,
P a g e in the
1 coolant
1 4 / and
4 / no0 coolant
8 , in
4 the
: 2oil6but P
M
gassing
of the sump will occur.
the engine could lock hydraulically.
Go to the top of the
class with MERLO.
Before removing the head to rectify any possible
valve problems, check the valves are not riding
on cylinders with low compression. This means
there should be clearance between the valve stem
and the rocker arm when the valve is shut, as
shown by the feeler gauge (pictured) If not, the
valve could be held open slightly, resulting in a
loss of compression.
Contact Josh Giumelli
[email protected]
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47