Nightmare with NPR cooling system

Michael Mechanic Texas Posted   Latest  
Resolved
Heavy Duty
2008 Isuzu NPR 5.2L (6 4HK1)
Low Coolant And Eng Shutdown Lites Come On (Eng Does Not Shutdown)

Bubbles in reservoir, tested with block test kit... clean air not combustion. No leaks to be found. Has new thermostats, egr cores, rad cap, water pump, numerous pressure tests, head gasket has been, checked for combustion gases in coolant about 10 times... no; clean air... this eng came as a long block only has about 1k miles on it and have been fighting this issue from day one. Has been to the dealer, they give up released truck... now it's back in my ballcourt. Dealer tech did say with his vacuum machine, to purge air from system, it did not draw down like he was used to seeing. Have had this system stand with pressure on it (20 psi) and no leaks.... Hopefully someone has had this issue and will share repair advise... please! Running short on hair to pull.... the other mechanics won't come within arm's reach anymore.

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Helpful
Nate Owner
California
Nate Default
 

I'm not going to be able to give you any help, but I want to make the situation clearer. 1) Does the truck lose coolant? 2) After it is filled and bled, how long does it take for the warning light to come on again? 3) Has anyone tried using fluorescent dye in the cooling system?

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Nicholas Owner
New York
Nicholas Default
   

You might have air in the egr cooler which sits higher in the system and can trap air. I believe the manual says to use a vacuum coolant filler to suck out all air and suck in fluid. Also did you check to see that the low coolant sensor is working correctly? should measure a certain amount of ohms. theres a small module in the cab that supplies voltage to it. If you unplug the low coolant module…

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Steven Mechanic
Washington
Steven Default
 

Did you verify that the block test kit works by testing the exhaust gasses at the tailpipe? Could have a bad test kit. Also, those test kits only work for leaks that are big enough to let enough exhaust gasses into the cooling system. If it's a minor leak, then the test kit won't pick up on the small amount of gasses. Use a vacuum fill tool to draw a vacuum on the system, then shut it off and…

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Albin Diagnostician
Washington
Albin Default
 

Over the years, I have seen some coolant leaks a little hard to find. About 40 years ago I had a problem like this on a 1965 Peterbilt truck and an old gray haired guy told me to keep an eye on the intake side of the water pump. If perchance the radiator gets a little restricted, at high engine speeds the water pump will pull a slight vacuum on the lower coolant hose. This can suck air into the…

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Matthew Owner/Technician
Massachusetts
Matthew Default
   

If there is CO in the cooling system a bulls eye leak detector would pick it up real quick. maybe someone local has one?

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Christopher Technician
Saskatchewan
Christopher Default
 

Just wondering if your aware there is a special bleed procedure for the coolant system on these engines.. They air lock in the line that comes from the turbo to the cylinder head. This is the only way to bleed these that has worked for me.. Even the bleed ports at the EGR cooler or the thermostat housing will not take this air pocket out.. It can be very frustrating..... The basics of the…

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Christopher Technician
Saskatchewan
Christopher Default
 

Second item to mention, I have had a head gasket that actually drew air in from the outside, which showed itself in a very minor , almost not noticeable external coolant leak... This was leaking on the RH front of the head, behind the A/C bracket.... Has the same symptoms, constant air in coolant system shutting down the engine for low coolant because of the air pocket.

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Robert Owner/Technician
Virginia
Robert Default
 

Put air pressure in glow plug and see what happens same as pressure testing radiator.

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Karl Educator
Nebraska
Karl Default
 

Does this truck have air brakes? Could it be coming from the air compressor?

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Scott Mechanic
Arkansas
Scott Default
 

Had Peterbilt one time with a bad coolant shut off valve that operated on air pressure. Leaking air pressure into coolant.

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Clyde Owner/Technician
South Africa
Clyde Default
   

Air compressor would pressurise the cooling system, does the fault occur at stand still or in operation? loaded unloaded driving on incline or decline, climate conditions? are using pure H20 or Dilullation with coolant? if so what coolant? Radiator cap is it correct cap for radiator or is it a generic cap and does it open at the correct pressure have you tested the cap? heater inside the cab…

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Michael Mechanic
Texas
Michael Resolution
   

Well, Ocham's razor generally proves out... eliminate the possibilities and whatever is left, no matter the improbablity must be the answer! Thanks for all the replys. Desperation bred inspiration I suppose... after reading several comments re: the blue liquid in the block test generally would not turn colors with a small leak... I tried something I never did before. I ran engine up to…

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