Troubleshooting: 5 Common Cat C9 Engine Problems

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The Caterpillar, or Cat C9 engine, was used in many manufacturers coaches from 2004 to 2010, when it was discontinued for road use. Because it is a diesel engine, it would have been difficult to engineer the engine to perform at more stringent environmental restrictions to make it run cleaner.

That being said, it is still in use for boating applications where higher torque performance is a priority, as in towing or use as generators in Tugboats, although it is no longer being made.

It is a 6-cylinder engine, with maximum horsepower coming in at about 335. However, what makes it a great coach engine are the high torque figures, which reach up to 1150 ft. lb’s. For moving even the largest coaches, the Cat C9 has plenty of mojo for the job.

It is a robust and reliable engine, but as of this writing, they are over 10 years old in the RV field, and with all motor vehicle engines, the older it gets, the more prone it will be towards breakdowns.

However, because a version of them is still in common use for marine applications, repair parts will still be relatively easy to come by if you do need your engine serviced.

5 Common C9 Problems

As with all internal combustion engines, no matter if they run on gasoline or diesel fuel, problems are going to crop up. 3 of these issues are going to be fuel injector related, although they are all separate and unique problems unto themselves.

With that, here are some of the more common issues you may expect to experience if you own a Cat C9.

Fuel Injector Leaks

Diesel engines have very high compression ratios in their cylinders, which is one of the biggest reasons they have such high torque numbers. But this pressure also puts a lot of strain on the fuel injectors, or perhaps it is better to say the fuel injector seals.

Unlike auto fuel injectors, which are generally threaded into place, Cat C9 injectors are screwed down into place and held there with a cover. If this isn’t done, the high pressure will literally blow out of their mounting hole, so they need to be securely mounted.

However, over time, the seals used on the injectors will begin to harden, and eventually the pressure of the engine will create gaps. This will, inevitably, cause the injectors to leak. In general, the more mileage on the engine, the greater the chance that the seals will go bad and need to be replaced.

If you begin to smell a strong diesel fuel smell around the valve cover, and you notice a drop in fuel mileage, the chances are good that the injector seals are leaking.

Fuel Injectors Becoming Plugged Up

Fuel injector nozzles have needle sized holes that inject fuel into the cylinder.

Although there are filters to filter out any dirt or grit in the fuel line, perhaps because of old fuel, or for whatever reason, C9 injectors are prone to plugging up. Most injectors are routinely replaced before the 100,000 mile mark due to plugging up. If you notice a loss of power, engine rpm’s, or miles per gallon, chances are your injectors are plugged.

There are some injector cleaners on the market that may or may not help. If you are beginning to experience power loss, it’s a far less expensive prospect to invest in a can of injector cleaner and see if that makes any difference, before taking your engine in to get serviced.

HEUI Injectors

This type of unit uses pressurized oil to fire the fuel injectors, and it is as complicated as it sounds. If the engine is extremely well maintained, meaning all filters cleaned or changed regularly, engine oil changed religiously, you may get a lifetime of service from your HEUI pump, as it is called.

More often than not, dust and dirt get into the system and damage the intricate internal working parts. Engine performance will slowly degrade the more gunked up the HEUI pump becomes.

Temperature Sensor

All modern engines have sensors that keep the engine’s computer in the know about how it is running. Many sensors won’t do much except to flash a light and alert the driver of a potential issue, which can then be taken care of. No harm, no foul. The C9 temperature sensor works a bit differently.

When it goes out, it will always register that the engine is cold. The computer knows that a cold engine is more prone to wear and failure if it is running at a higher RPM, so in the case of a C9 with a temperature sensor failure, the computer won’t let the driver accelerate to a speed much beyond a certain low RPM, even if the engine has achieved optimum running temperature.

You may think there is something terminally wrong with your coach in this instance, but it is just a temperature sensor that is no longer functioning properly. If you have mechanical DIY experience, you may be able to change this sensor yourself.

Engine Oil and Filter

The Cat C9 has been shown to be very sensitive to dirty oil. The cleaner the oil, the better it will run. Worse is if the filter gets clogged and begins to bypass dirty oil back into the engine, you could cause some serious damage.

Not only will you begin to lose performance, but reliability and longevity will also be cut short. The simplest way to fix this is to use a high grade filter and synthetic oil. Make sure to change the oil out at the required intervals, and use the best synthetic oil you can find.

Final Thoughts

The Cat C9 engine has been praised for going the long haul while being relatively free of problems. That doesn’t mean problems don’t exist, as is detailed in the post above. But by using fresh fuel, keeping the filters cleaned and using a high-grade synthetic oil, doing that will go a long way to keeping your Cat C9 on the road for hundreds of thousands of miles.

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