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Cat Man
Cat Man, Caterpillar Engine Expert
Category: Heavy Equipment
Satisfied Customers: 8327
Experience:  23 Years Caterpillar Engine experience.
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I have a GMC 7500 with cat C7 250hp 860 tq. I bhave had oil

Resolved Question:

I have a GMC 7500 with cat C7 250hp 860 tq. I bhave had oil "O" rings at the front and rear of the cyl.head oil gallery replaced ,the front twice ,the gasket between the black ond oil cooler twice ,the oil pan , and now I have more oil leaks. Why are these motors so leak prone ?
Submitted: 6 years ago.
Category: Heavy Equipment
Expert:  Cat Man replied 6 years ago.

Hi. Thanks for using JA! I will try and help you out.

I have seen oil cooler and oil pans leak, thats for sure. But not the Orings in the head so much. Have you had other mechanical problems with your engine? What kind of oil pressure does your engine normally have?

Customer: replied 6 years ago.
The oil pressure is normal-- but the oil pressure in the gallery through the head is part of the higb pressure "HUIE" system. It feeds the injectors and is pressurized by the "high pressure pump" so---- what would the normal oil presssure have to do with that ?
Expert:  Cat Man replied 6 years ago.

Thanks for the information.

I was interested in what your oil pressure was in case is was higher than it should be. This would not have an effect on the cylinder head but it would on the oil cooler. If the relief valve in the oil pump is stuck closed, the system will have higher than normal oil pressure causing the oil cool gasket to fail prematurely.

The Injection Actuation Pressure system for the fuel system is constantly changing from 3000 to 870 psi. The O rings on these plugs endure alot of stress while dealing with this pressure. Not to mention the temperature of the engine going from 215 to ambient. Over time, the O rings begin to leak. Caterpillar has change the material content for these several times. The problem always gets better but eventually the oil will begin to seep out once again. The most important thing about these O rings to make sure to lubricate the O rings upon installation. When they are put on dry, they may get cut when the plugs are torqued.


Please let me know if you have any more questions.

Customer: replied 6 years ago.
My initial question was why this engine was so prone to oil leaks . My previous truck with a 3126 had none of the constant and ever changing oil leak problems. I wanted to know if oil leaks were problems that were common and ongoing with the redesign of the 3126 into the C7. I have had many independent mechanics tell me that these C7 are 50% dry and 50% constant leakers. Is there a reason for this ?
Expert:  Cat Man replied 6 years ago.

The C7 fuel system was redesigned from the one used on 3126B engines. The 3126B engine used a HI300 design. Due to new emission laws, the HI300 was not able to meet the demands needed for the new laws. So the HI300B was born. This fuel system has 4 injection modes compared to the 2 the HI300 had. The injection actuation pressures also run higher than the HI 300 did.

I believe the new fuel system design used on C7 engines is the reason for the oil leaks.


Please let me know if you have any more questions.


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