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Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 8327
Experience:  I am ASE Master Truck Certified.
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I have a 2005 gmc c7500 with a cat motor (c7)? It down rates

Customer Question

I have a 2005 gmc c7500 with a cat motor (c7)? It down rates at about 60mph for about 5-10 seconds with dash lights dropping speed until about 40mph and then runs fine. What's the deal with that?
Submitted: 4 months ago.
Category: Medium and Heavy Trucks
Expert:  Cat Man replied 4 months ago.
Thanks for asking your question. I will try and help you out.
Does the yellow check engine light come on when this happens?
Customer: replied 4 months ago.
there's two that flash on
Expert:  Cat Man replied 4 months ago.
Thanks for the information. There are many things that can cause your problem. Most often I see an injection actuation pressure problem or a vehicle speed signal problem.
Since the engine light comes on. Please try and pull the codes by using the cruise control switches. If you have never done this. Here is how.
Use the following procedure to retrieve the flash codes from the check engine light:
Turn the ignition key switch to the On position/ Engine OFF.
Turn the cruise control ON/OFF switch to the OFF position.
Move the SET/RESUME switch to either position. Hold that position until the yellow lamp begins to flash.
The yellow lamp will flash in order to indicate a two digit flash code and the SET/RESUME switch may be released. The sequence of flashes represents the system diagnostic message. Count the first sequence of flashes in order to determine the first digit of the flash code. After a two second pause, the second sequence of flashes will identify the second digit of the flash code.
Any additional flash codes will follow after a pause. These codes will be displayed in the same manner. Flash Code 55 indicates that No Detected Faults have occurred since the ignition key switch has been turned to the ON position.Let me know what two digit code you get and I can tell you what the problem is.
Customer: replied 4 months ago.
Hey I don't have cruise control in this truck, is there another method?
Expert:  Cat Man replied 4 months ago.
I see. Unfortunately when there is no cruise control, there is no other way to pull codes. I am sorry.
The only other option is to have someone connect a service tool like Cat ET software to the engine and read the codes. I wish there was another way.
There are many codes that can cause your problem. I would hate to guess. I suspect an injection actuation pressure system problems. I see this a lot on these engines. Injection actuation pressure problems are caused by faulty heui pumps, faulty IAP sensors, or leaking injectors.
Please reply back if you need more help or have more questions. I am happy to assist.
Expert:  Cat Man replied 4 months 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.

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