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Have a mp8 and no power it has some kinda hastening from 12…

Customer Question
Have a mp8 and...

Have a mp8 and no power it has some kinda hastening from 12 to 14 rpm and fuel pressure is 45 to 50 and on 17 rpm it goes to 60 under Load what could it be

Mechanic's Assistant: What's the make and model of your heavy truck?

It's a Mack

Mechanic's Assistant: What type of fuel do you typically use? When was your last oil change?

Just changed the oil new head new cups injectors like 100k on them

Mechanic's Assistant: Is there anything else the truck Mechanic should know before I connect you? Rest assured that they'll be able to help you.

That's it I think

Submitted: 2 months ago.Category: Medium and Heavy Trucks
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Answered in 4 minutes by:
6/2/2022
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 months ago
Mike Mcmillan
Mike Mcmillan, Shop Foreman/Triage Tech
Category: Medium and Heavy Trucks
Satisfied Customers: 6,444
Experience: Volvo/Mack Master Tech, Certified with Cummins, Eaton, GM, Hino, Detroit Diesel
Verified
Hello and thank you for using Just Answer. My name is ***** ***** I’d like to try and help. I am sorry that you are having issues with your Mack. I know this can be frustrating. I am here to help anyway I can. Please allow me a few moments to look over your question and I will be right back with you. May I please ask with whom I have the pleasure of speaking with today?
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Customer reply replied 2 months ago
Hello This is Frank
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 months ago

Nice to meet you frank. Was the injectors all replaced with the head when that was done?

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Customer reply replied 2 months ago
I did the injector like a year ago
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 months ago

Okay, I am going to list some things to check

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Customer reply replied 2 months ago
Ok
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 months ago
Typically when there is a complaint of what you are experiencing in that rpm range, it is typically either an injector cup leaking, or the needle control valve in an injector(s) is bad. I have seen these cups fail in as little as one week after installation and they will not always cause a hard start complaint. I would run the vgt and egr valve test and make sure they both pass. Then recal the turbo and see if that helps. These will both cause a lug type concern that can replicate this. I would also make sure the trim codes are all set properly from the injectors. This will make a difference when it is loaded. If that is fine, your fuel pressure is still within spec so the volume you are getting will be okay. I would go thru and inspect the overhead adjustment. Make sure the ex valves are checked and the pistons in them removed to make sure the backing caps are not loose. This is a common issue on these. Then if those are all fine, make sure there is not an exhaust restriction and that you do not have a biased egr diff pressure sensor or a plugged boost pressure sensor. If those are fine, I would pull the injectors for cup inspection.
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Customer reply replied 2 months ago
The truck just left the shop this morning and it's deleted and i installed a new actuator new boost and temp sensor only thing I haven't changed is the fuel filter housing but did it 3 year ago, when I dont lock the prime pump I have ni fuel pressure don't know if that's OK
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 months ago

When the primer is not locked down it is possible to cause an aeration issue.

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Customer reply replied 2 months ago
The return it barely has any fuel coming back is that normal because I know my cats and detroit they flow a lot
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 months ago
These are a very minimal return engine. In fact, if you ever feel the fuel tanks in the winter time they are close to ambient temp
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Customer reply replied 2 months ago
sir I notice that and talking about the cups, it had the same hesitation before I did the whole head and we had no cups leaking,...turbo boost is around 25 to 30
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 months ago
What were the ex valves set to and the engine brakes? How many shims had to be swapped out? One other thing to note is the vcb valve for the engine brake. I have had issues with them allowing too much oil pressure thru them as well
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Customer reply replied 2 months ago
I would need to ask the mechanic only thing I know of that the engine brake came on some times without turning the switch
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 months ago
That is usually an indication that the valve is defective
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Customer reply replied 2 months ago
that could cause low power?
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 months ago
it will when it lets too much oil pressure into the rocker shafts.
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Customer reply replied 2 months ago
Alright thanks a lot I'll swapped that one and c what happens
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 months ago
My pleasure. Have a good day
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Customer reply replied 2 months ago
It still wouldn't explain why I would lose power but I'm not familiar with these engines...have a good one
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 months ago
That valve controls the oil pressure to the rocker shaft. The ex valves are piston controlled based on the amount of oil pressure in the shaft. The valve is only supposed to allow 10-14 psi of pressure thru the shaft. If it is any higher, the lash is taken out of the ex valves and will cause a loss of combustion which will equate to low power.
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Customer reply replied 2 months ago
That would make sense c I'm old school I do cat and detroits myself but these kinda engines scare meIf I told u that if a have it idling for 5 min or coming down hill then it makes power maybe for another 5 min and it falls down again
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 months ago

That really does not change anything on repair course for that

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Category: Medium and Heavy Trucks
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