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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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MY 99 C12 AT ROAD SPEED RUNS GREAT THEN OUT OF NO WHERE IT

Customer Question

MY 99 C12 AT ROAD SPEED RUNS GREAT THEN OUT OF NO WHERE IT LOSSES POWER AND STARTS MISSING ,NO ENGINE LIGHT ,PULL OF ROAD SHUT ENGINE DOWN REBOOT ECM AND CAN GO WITH NOW PROBLEM AGAIN FOR ANYWERE FROM 1/2 HR TO 2 HRS BEFORE HAPPENS AGAIN ,HAVE CHECKED ALL WIRING GROUNDS CANT FIND ANYTHING COULD THERE BE A FAULTY SENSOR ?K-MOTION JOHN
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Cat Man replied 1 year ago.

Hey john. Thanks for asking your question. I will try and help you out.

So the yellow check engine light does not come on when this happens?

Has the fuel pressure been checked while driving?

Customer: replied 1 year ago.
no yellow check engine light just happens all of a sudden while driving starts shaking like u joint go losses power ,wont excellerate,no fuel pressure issue , just have to reboot ecm and its back to normal, just turn key off for 30 seconds restart.and truck runs normal for 10 to 50 miles before might do it again was going to try cam sensor next.
Expert:  Cat Man replied 1 year ago.

Thanks for the information.

There are several things I have seen cause low power like this.

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 75 and 90 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 underneath the bottom of the filter housing. 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.

If the fuel pressure is good. I would next suspect the ECM or the power going to it. Since you disconnect the ECM and it runs normal. You could have a faulty ECM or a power supply problem. I have seen ecms get hot and cause the engine to run poorly. Unfortunately the only way to verify is to try a known good ECM from another truck.

You can try a cam sensor but to be honest I have not seen a cam sensor make an engine do this before. Sorry.

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.

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