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Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 8412
Experience:  I am ASE Master Truck Certified.
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Cat C7 330, 6 speed Allison. Developed a intermittent

Customer Question

Cat C7 330, 6 speed Allison. Developed a intermittent hesitation below 2200 rpm. Power comes in and out (more out than in) Beyond 2200 rpm full power is available until the next up-shift and then the cycle repeats. Power feels better when slightly backed off of WOT but still hesitates. Seems to idle rough as well. What would cause this?
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Cat Man replied 1 year ago.

Thanks for asking your question and requesting me. I will try and help you out.

Does the yellow check engine light come on when this happens? Or no light just low power and hesitation.

Customer: replied 1 year ago.
No light.
Expert:  Cat Man replied 1 year ago.

Thanks for the information.

Since the check engine light does not come on, I would suspect 1 of 2 things are causing your problem.

The first is low fuel pressure. If there is a low fuel pressure problem, the check engine light will not come on and the engine will have low power. Your engine should maintain a fuel pressure between 65 and 85 psi. There is a plug beside of the secondary fuel filter that is used to check the pressure. If it is low, I would first try the fuel filters if you haven’t already. If it is still low, you may have a failed fuel pressure regulator, which is located on the back of the cylinder head. I have replaced a ton of these for low fuel pressure. It is kinda a known problem. You could also have a weak fuel transfer pump or restricted hand primer pump, if your engine has one.

Second would be worn injectors. This has been known to happen overtime. If the fuel pressure turns out to be OK. I would suspect the injectors. I have seen this a lot.

Please reply back if you need more help or have more questions. I am happy to assist.

Customer: replied 1 year ago.
Would you suggest anything else as a potential cause? I haven't checked the fuel pressure yet but will however the fuel regulator, and fuel transfer pump were both replaced within the past year and the fuel filter was just changed.
Expert:  Cat Man replied 1 year ago.

The only other possibility would be a sensor reading wrong. Like a boost pressure sensor or intake air temp sensor. They have been known to read wrong and cause the engine to have low power. If the sensors are reading correctly. I must suspect low fuel pressure or the injectors.

Please reply back if you need more help or have more questions. I am happy to assist.

Expert:  Cat Man replied 1 year ago.

Hey. I just wanted to see how things were going. Please let me know if you need more help. I am happy to assist. If my information was helpful, please leave a good or higher rating so that I receive credit. Thanks.

Customer: replied 1 year ago.
Replaced the intake air temp sensor, no change. Pick up the required equipment today to test the fuel pressure. Will do tonight. Also picked up a boost pressure sensor as a possibility. Will update you once I know something more.
Expert:  Cat Man replied 1 year ago.

Ok sounds good. Thanks for the update.