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Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 8419
Experience:  I am ASE Master Truck Certified.
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C7 acert new Hui new act pres.sensor still have low RPM miss

Customer Question

C7 acert new Hui new act pres.sensor still have low RPM miss .sounds like 1 cylinder only .codes 18-63-39
JA: We'll do all we can to help. Tell me a bit more about what's going on so the Heavy Equipment Mechanic can help you best.
Customer: Starts after reaching full temperature and gets worse the more you run it
JA: Is there anything else important you think the Heavy Equipment Mechanic should know?
Customer: No just low on power ,truck 2 weeks ago would cruise at 85 mph fully loaded pass everything but fuel station now want pass a school bus
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the Heavy Equipment Mechanic about your situation and then connect you two.
Submitted: 5 months ago.
Category: Medium and Heavy Trucks
Expert:  Cat Man replied 5 months ago.
Thanks for asking your question. I will try and help you out.
Does the yellow check engine light come on when the engine is running?
Also. Has the fuel pressure been checked?
Customer: replied 5 months ago.
No cel after new Hui and. Act pres.sensor changed both after engine started running very bad had codes 18,63 39,now just low RPM MIS after reaching operating temperature
Customer: replied 5 months ago.
What should fuel pressure be?
Expert:  Cat Man replied 5 months ago.
Thanks for the information.
If there is no light. I would first suspect a fuel pressure problem. I see this a lot on these engines.
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.
You could also have a fuel return line restriction. This too will cause low power or a miss.Please reply back if you need more help or have more questions. I am happy to assist.
Customer: replied 5 months ago.
Transfer pump came with the NEW HUI
Expert:  Cat Man replied 5 months ago.
Thanks for the information.
Yes most often they do. But I would first verify the fuel pressure. If it's low I would suspect the regulator. I replace these all the time for fuel pressure problems.
Please reply back if you need more help or have more questions. I am happy to assist.
Customer: replied 5 months ago.
what should fuel pressure be at idle and high idle
Expert:  Cat Man replied 5 months ago.
The fuel pressure should be around 65 at idle. 85 at high idle.
Customer: replied 5 months ago.
10-4 thanks, ***** ***** be back to you in 30 mins or less.
Expert:  Cat Man replied 5 months ago.
You are very welcome my friend. I am happy I could help.
Customer: replied 5 months ago.
Fuel pressure specs are great 70 to about 83 .I have been a heavy Equipment mech.my entire life. I have built most every type engine and power shift trans there are, and pride myself on being a trouble -shooter but this makes no sense to me with the codes present -18-63 -39 with new Hui- actuation sensor
And correct fuel pressure HELP HELP HELP!!!!!!!
Customer: replied 5 months ago.
Any ideas? ??
Expert:  Cat Man replied 5 months ago.
If the codes are active. All that's left to cause them is an injector leaking oil.
If the codes are not active and the fuel pressure is good. I must suspect a faulty injector. To confirm you would need to have a cylinder cutout test performed on the engine.
Please reply back if you need more help or have more questions. I am happy to assist.
Expert:  Cat Man replied 5 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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