Heres a report I wrote for my comp one class on the 6.9/7.3 IDIs. Thought you guys might like to read it.
Well here she is in all her glory.
International Harvester/Ford 6.9 and 7.3 IDI Diesels
John Boggs
Baker College of Owosso
In 1978, International Harvester and Ford Motor Company reached an
agreement for the research, design and manufacture of a powerhouse diesel engine
which would ultimately launch a never-ending revolution. After countless hours of
excruciating testing, the 6.9L Indirect Injected (IDI), naturally aspirated (NA), V8 diesel,
inhabited the engine bay of the Ford ¾-ton and 1-ton heavy-duty trucks. The 6.9L IDI
produced 170 horsepower and 310 foot pounds of torque. In 1988, the 6.9L IDI was
replaced with the 7.3L IDI. The 7.3L IDI produced 185 horsepower and 360 foot pounds
of torque. Both engines pale in comparison to present day diesels, but started a lifelong
heavy-duty truck feud between Dodge Chrysler, General Motors Company, and Ford
Motor Company. The 6.9/7.3L IDI diesels were proven to be reliable, solid engines and
were used in a wide range of applications, but as a good as a setup as they were, they
weren’t without a few common problems which of course meant they went through a
few changes during their production run.
The 6.9s were blessed with a cast iron block and four bolt main bearings. This
made for a solid bottom end and base to build off of. It also got a forged steel
crankshaft, plenty heavy enough to handle the forces developed in such an engine. To
aid in cold starting, International Harvester Company designed the engine to include
glow plugs. Glow plugs are a threaded plug that screw into the cylinder head of an
engine and use electricity to create heat in the pre-combustion chamber to help raise
the temperatures inside the cylinders to the 500 degrees Fahrenheit necessary for diesel
fuel to auto ignite. The glow plugs used were Motorcraft/Beru ZD1As. These are non
temperature limiting single coil glow plugs designed to run off 6 volts. The way that IH
designed the glow plug relay, the 12 volts that started at the battery was stepped
down enough that by the time the juice made it to the glow plugs it didn’t burn them
up. Between the 20.7:1 (1983) 21.5:1 (1984+) compression of the 6.9, and the glow
plug system, they are fairly easy to start in the cold when the starting system is in good
shape. For those extremely cold nights, a block heater was also included.
Of course, just like any other engine, the 6.9 had a few common problems.
The most common being the return lines giving up the ghost and leaking air leading to
hard start situations. International Harvester Company designed the return line system
with plastic caps on top of each injector with O-rings sealing the cap to the top of the
injector, and rubber lines running from each of the caps. After several heating/cooling
cycles, the O rings would become hard, and if disturbed would leak air into the return
line system. The reason that this leads to a hard start is because the 6.9 depends on
vacuum to hold prime. The 6.9 has a mechanical fuel pump that is operated off of a
lobe on the camshaft, so of course fuel is only pumped when the engine is turning.
Owners would crank on the starter longer than normal without doing anything accept
pumping fuel. Once the air was finally purged from the system, the engine would fire
right up no problems. The problem is, after a while, all that cranking takes its toll on the
starting system. A worn out starter won’t spin the engine fast enough to create the
heat necessary to ignite the fuel, leading to a hard start. If the first problem is not
corrected in a timely fashion, it creates other problems as well and leads to quite an
upset owner. A common modification is to add an electric fuel pump, which doesn’t fix
the problems, it just covers them up. It also, if not properly done, creates the risk of
rupturing the diaphragm in the mechanical pump and filling the crankcase with diesel
fuel.
Another common problem that affected both engines was the Crankcase
Depression Regulator getting plugged up and causing excessive oil usage. At high rpms,
a valve in the CDR is supposed to close to keep the engine from burning its own oil, but
when the CDR isn’t cleaned regularly, it won’t close and the engine ends up burning oil.
The CDR also is supposed to redirect vapors from in the crankcase into the engine to be
burned. Some people abandon the CDR altogether and install a road draft tube instead,
similar to what is found on most semis. The road draft tube works just fine, but some
don’t like it because it tends to make the underside of the vehicle oily.
The early 6.9s blocks were weak in the area around the block heater and
prone to cracking. If the block heater wasn’t used, this was not much of an issue, but
if it was used, you ran the risk of the block cracking and possibly being out an engine.
If the block cracked, your options were to try to have it welded, or scrap the thing and
get another engine. The issue was quickly corrected by Ford.
The 7.3 had an issue that mainly affected it, but on occasion could affect the
6.9 or for that matter any high compression engine. When they decided to bore out the
6.9 to 7.3 liters, it made the cylinder walls dangerously thin. This made the 7.3s much
more susceptible to cavitation. Cavitation happens when bubbles form on the cylinder
walls and eat away at the metal that forms the cylinder. If this is allowed to go on long
enough, the bubbles will finally eat a hole right through the cylinder wall. Then your
options are to scrap the engine, or try to sleeve the offending hole. The unfortunate
thing is that due to the already thin cylinder walls, your rebuild options are limited.
Once cavitation happens, there’s nothing that can be done to reverse the affects,
however cavitation can be stopped and prevented. The use of supplemental coolant
additives or SCA’s is necessary to prevent cavitation. What the SCA’s do is line the
outside of the cylinder wall and form a sacrificial protective layer that the bubbles eat at
instead of the cylinder walls. It’s recommended that you check the SCA levels in your
coolant every six months to ensure maximum protection from cavitation.
The 6.9s fuel system is a relatively simple one, and is stone age compared to
the diesels of today. A Standadyne DB2 rotary injection pump is the heart of the
system. This pump only develops injection pressures of about 3200 psi, nothing
compared to the newer engines. The injection pump is quite a complicated piece of
engineering, and a whole essay in itself could be written on it. Basically, pressure is
built up inside the pump, and as it turns, ports are uncovered which the pressurized fuel
is pushed through and is fired through the injector. The pump controls, injection
pressure, timing of the injection event, metering of the fuel injected, and firing the
injectors in the correct order. Inside the pump, there is an Allen head screw that
adjusts the amount of fuel that is delivered to the engine. This screw is useful for fine
tuning the engine, and helping to adjust the boost if turbocharged. The pumps timing
also plays a key part in tuning the engine for power and fuel economy. Underneath the
timing cover there is a mess of gears with timing marks. Once all the timing marks are
aligned, the pump can be adjusted finer by turning the pump to the drivers or
passengers side. The factory provided two timing marks on the pump housing to get the
timing within a ballpark range. It makes it so the engine can be run and driven enough
to get the dynamic timing set.
The engine was used in many different applications. It powered Ford pickups from 1983
up till 1994.5. It was also used in medium duty trucks and busses. The IDI was great
engine for lots of different things do to the versatility of it, and the reliability of it. The
engine only requires one wire to run, and has a reversible oil pan which allows it to
adapt to different configurations. The lack of computers, and a very basic fuel system
will allow it to run on an array of fuels. It can run on standard number two diesel fuel,
biodiesel, automatic transmission fluid, vegetable oil, engine oil, and gear oil among other
things.
In 1994 Ford decided to turbo charge their 7.3 engines. These engines stayed
the same as their naturally aspirated brethren, with a few exceptions. The turbo
engines had bigger wristpins, and had the return lines re-routed away from the turbo.
Also the glow plug controller was relocated to the passenger side valve cover instead of
the back of the engine where it used to live. Ford decided to go with an ATS turbo that
added only about 5 horsepower. Many believe that the engines output figures were
fudged so as not to better the new Powerstroke that came out the following year.
Obviously no one would want to buy the newer engine if the old engine was more
powerful. The smashed downpipe and 2.25 inch outlet flange of the turbo helped to limit
its output. Almost immediately after ford started selling the new turbocharged IDIs, ATS
came out with an upgraded downpipe and outlet flange that greatly reduced exhaust
restriction and boosted power quite a bit. Since 1983 aftermarket turbos had been
available for the IDI from banks, hypermax, ATS, and the little known ray-jay. 7.3s are
only good for about 15 psi of boost in stock form, 6.9s not much more than 10. If the
boost is allowed to go any higher, the chances of a blow head gasket increase
exponentially. Most people overcome this issue by installing head studs instead of the
stock head bolts. After that, holding intake gaskets becomes a problem.
Aside from turbo charging, there are many other modifications that IDI owners
have come up with to get the most power out of their engines. One would be the soup
bowl mod. Underneath the air cleaner, there’s a big bowl shaped thing that is believed
to hinder air flow into the intake. Cut that bowl off and you have the soup bowl mod.
In addition to the soup bowl mod, a lot of people build homemade ram air intakes. There
are also performance injectors and injection pumps available from DPS, and Moose
products. Moose products builds the moose fuel systems that deliver unbelievable
amounts of power and have a great spray pattern. Moose fuel systems are generally
only recommended for use with turbo charged engines, but they do make a nice rebuilt
pump for the person that chooses to remain naturally aspirated. They are a favorite
among people building their IDIs for performance, and for your average Joe looking for
quality fuel system parts. There’s also an online source that will regrind your camshaft
for you that’s supposed to increase torque and horsepower.
All in all, the 6.9L and 7.3L IDI’s were reliable power plants that put out a
respectable amount of power and were good for many years of service. They were used
in many different applications, and only had a few common problems all of which were
cheap to fix. Today the old IDI’s are almost forgotten, except for the few owners and
enthusiasts that still keep them alive. Just remember, every time you see a commercial
on the television for the new Powerstroke, or Cummins, or Duramax you can thank the
old IDIs for starting the wars between the big three companies that allow the consumers
to reap the rewards that take the form of the high horsepower diesels of today.
References
7.3L IDI Diesel Pre-Powerstroke Ford Diesel Power.(n.d.) Retrieved from
7.3L IDI Diesel
How the Stanadyne DB2 Fuel Injection Pump Works. (n.d.) Retrieved from
Oliver Diesel - Engines & Outdoors
Bomacz, R.T. (2007, March). International 6.9L Engine - Diesel Tech
The IDI Father Of The Power Stroke. Diesel Power
Bennet, S. (2011) Heavy duty truck systems (5th ed.) Delmar, Cengage Learning
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