Mack MP8 DEF system not building pressure

[Monkeywithawrench]

2016 Mack GU713 truck with Mack MP8 engine. SCR fault coming on. DEF pump not able to build pressure. 180,000M on the truck.

When this first started a month ago I ran some tests. I have Mack PTT, the licensed version through Mack. The line set coming off the DEF tank going to the pump has push to connect fittings on it. I disconnected the supply line and ran 3/8" clear tube from the header to the pump. Priming the system and connecting the clear tube to the header and the pump inlet, the truck ran fine. Built pressure to 130psi, no problems. Had them run the truck for a week with the clear tube connected to the tank header unit and the pump. No issues. Ordered a new line set, thinking it was the O rings that might be worn out. That was $800. Ran fine until last week.

Truck shows up with SCR fault. Going into Derate. Ran DEF pump test, not able to build pressure. Ordered new Bosch DEF pump from Diesel world or something like that. Mack wanted $2800. New pump for $830, then we overnighted it in........more money. Still not working correctly.

-with 3/8" clear tube connected to header and pump.........not seeing bubbles (air leak) coming from suction side of header.
-When I prime DEF pump, with clear tube........it is able to build pressure.
-Pulled DEF doser nozzle and checked for leaking doser valve-no leaks. Doser nozzle spray pattern correct, doser nozzle putting out proper volume as measured with cup.
-AFTER pump builds pressure-shutting system down, DEF pump does pullback-not able to build pressure again. Pressure builds to 30-40psi, then loses prime........it's seems the pump (the new one.........and the old one) aren't able to draw enough volume to clear the air out. I have run pressure test build up with the return line disconnected. No obstruction in the return line going into the DEF tank header return port. They are new lines, so not obstructed. New pump as of yesterday.....so that should be good.

BOSCH PUMP #21577511

I looked online for 'Theory of operation' for the Bosch DEF pumps but can't find anything. TOTALLY AT A LOSS FOR WHAT IS CAUSING THIS!! FRUSTRATED!!!

Wondering is anyone has run into this before and has some guidance.

Thank you!!

 

[Junkyard]

Is there an electrical issue making it think it should purge like the engine has been shut off?

 

[Monkeywithawrench]

Hey Junkyard............thanks for the reply.

Yah, there is an electrical issue. My brain wasn't firing. i'm an idiot.

The clear poly tube on the inlet of the pump was letting air in. I had a hose clamp on it too. Cut a section off and put it back on. Works fine now. Connected OEM hoses up and ran multiple pressure tests with laptop.......working fine now.

So back story. The DEF pump has coolant lines going to it for heat. Truck would come in with SCR fault. They would park it. I'd get to it the next day........it's cold. Prime pump, run tests.......all good. Ran it with clear tube for a week..........all good. Truck comes in other day SCR fault. They park it..........put poly tube on (clear) not building pressure. Order new pump, put it in...........not building pressure. Finally figured it out today (Saturday) when I have shop to myself, no one to bother me.
One thing I did different.........I put dielectric grease on the nipples for the supply tube.......just in case there might be a tiny leak. But new hose sets, new pump (fitting should be good).......only OLD fitting is in DEF tank header, but no wear marks.
Anyway.........its a win!!

I'll post some pics later tonight......might help someone down the road.

 

[Truck Shop]

Plastic lines with push locks--Freightliner has the issue of those connections leaking
on air compressor coolant lines, same connections used at DEF pump, for DEF &
coolant both will leak.

 

[Monkeywithawrench]

Plastic lines with push locks--Freightliner has the issue of those connections leaking
on air compressor coolant lines, same connections used at DEF pump, for DEF &
coolant both will leak.

Thank you Truck Shop.

Since the line feeding the pump is under suction, I couldn't see any leaks. I was just guessing on the line set.

Thursday I had the truck in the yard, outside the shop door. 3/8" clear poly tube hooked up so I could watch the DEF. Did a pressure build test, and it was 'passed'. Able to build pressure to 130psi. Started the truck and let it run at 900rpm. It ran for 3 hours, then went into SCR derate warning. No DEF pressure.

I think when the pump got hot from the coolant going through it is when it wouldn't work. That's when I pulled the trigger on the new DEF pump.

Hard part is, when you start second guessing yourself on what you've done.

Oh well, it's working now.

A friend of mine has a construction company.......6 Mack dump trucks, 2021, with Detroit engines. Every single one of those have new DEF pumps in them.