648H Overheating

Lockwood · April 24, 2018, 08:31:38 PM

G,day all just looking for some help in trying to sort coolant temp fluctuations on JD648H .Temp gauge goes from about ½to ¾ on flat ground but will drop back to half on heavy uphill snig Only thing I've noticed it appears to have very little circulation in header tank.Any advice will be greatly appreciated

mike_belben · April 24, 2018, 10:40:25 PM

Get a cheap IR temp gun and first verify that the gauge is being honest. Look at the temp difference going into and coming out of the radiator to be sure its shedding heat properly. Put your stat in a pot on the stove and verify its opening at the set temp. If not, replace.

6bt cummins thermostats swing like that constantly until the whole system is up to temp. Some people drill a bleed hole to stabilize it.

How rusty does the coolant look?

Lockwood · April 25, 2018, 04:34:39 AM

Thanks for your advice Mike ,have just changed coolant a week ago with JohnDeere coolant

mike_belben · April 25, 2018, 09:57:18 AM

I bet that was spendy.

Any evidence things were sludged up in there?

Is the radiator covered in oil and dust?

Does it have a fan clutch and is it working?

barbender · April 25, 2018, 11:02:56 AM

Simple things first, like Mike is suggesting. Our worst phantom overheating problems have been caused by cracked heads, that was accompanied by fluctuations in power output as well.

Gary_C · April 25, 2018, 11:29:35 AM

If you just changed the coolant, you may have created an air lock in the system. Sometimes the airlock may work itself out but sometimes you may have to crack a hose to make sure the system is full of coolant.

Some engines have small hoses to relieve the airlocks but I don't know if that engine has one.

nativewolf · April 25, 2018, 12:29:59 PM

Also, have you tried replacing thermostats? We have a nice cummins engine that turned out to have overheating due to 2 bad thermostats, replaced those and solved everything, easy fix and not too costly.

mike_belben · April 25, 2018, 03:24:03 PM

Just fyi for anyone reading, a cracked head or blown headgasket will pretty much always put combustion or atleast compression pressure into the radiator top hose.

If its ballooned up and swelling like it wants to pop the clamp, or the cap vent is always venting despite normal temp, thats compression in the coolant. HG or cracked head/block/liner

Fluid mixing may or may not come with it so dont go by that.

kiko · April 25, 2018, 11:29:24 PM

Check the water pump , it has plastic fins that will wear down and also spin off the shaft. It can be easily removed but even if good you will need a new gasket. I would also make a belt wiring diagram before removing the belt.

kiko · April 25, 2018, 11:33:16 PM

Also on a side note, this is an egr engine so the egr cooler also has to considered in an engine overheat issue especially with coolant loss.

Lockwood · April 26, 2018, 04:01:20 AM

Thanks for all the info guys ,and possible causes back to work today and being cold turned heater on in cab and temp stayed around 177f all day but as soon as I turn heater off up she rises any thoughts on this. Cheers

mike_belben · April 26, 2018, 09:55:47 AM

Your cooling is being achieved by heat radiated off the engine to the air via fan flow, and the heater core, not the radiator.

If the cooling system was working as designed the heater loop would have neglible effect on overall temp.

brianJ · April 26, 2018, 10:44:20 AM

@ Mike DO you know how open the thermostats should be when boiling on the stove? I am thinking open all the way? I am having a similar problem with my Mack CHN. Only seems to overheat under load. Tuesday when I tried I was fine until I got to a steep hill. Then straight into the red. Luckily I was out in a field.

mike_belben · April 26, 2018, 01:17:10 PM

when testing its important to know if you are testing a spec for initial crack temp or a full open temp. The hotter you get the further they open, probably to a stopper or coil bind. Full boil full open.

a troubleshooter must understand the fine details of an entire system and then look for forks in the road that get finer and finer until theres is no fork left and you arrive at the root issue. So ill dump my best full picture here and hope it serves others in the coming years. This is gonna take a while.

*Full load produces full cylinder pressure.
The greater the pressure difference between a combustion space and the water jacket surrounding it, the more combustion gasses per unit of time will transfer from high pressure to low pressure, through any available path that we obviously dont want.

Therefore a leaky head gasket will naturally blow out a radiator much faster at full load. But with gas transfer you would not "catch it" easily because as soon as you come back to idle the crack stops leaking enough gas to witness, it has been vented with little evidence. Most old gaskets are gonna leak a little and the cap will burp the accumulated air without enough violence to spit coolant with it so we are none the wiser. When a HG really goes at full load there is too much air volume and coolant is blasting out. It explodes things with a serious boom and massive coolant cloud. Overheated coolant cannot get hot enough fast enough to explode metal parts and hoses. That is the work of direct gas pressure leakage that the little cap vent could not expel fast enough. Theres a hissing air sound as it inflates until kaboom. Theres 2000+ psi in there so its serious energy.

Now to thermal transfer.

*Full load also produces max temp of gasses within the combustion space. Gas pressure and temp are inseparably correlated in direct proportion to each other.

The greater the temp difference from inside to outside the bore, the faster the rate of exchange from the gas to the metal to the liquid. It is a much slower process and often the temp is highest or climbing after youve crested the hilltop. Normal engine cooling has a lag time. this is a critical sign to look for in troubleshooting overheat issues.
"Is it the HG or the waterpump?" The symptons can very easily mask the true cause and make one unsuccessfully throw parts at it.

A radiator can handle lots of over temp, but not much overpressure at all, thats why it is vented somewhere between 3 and 20 psi depending on the cap's piston diameter and spring pressure. If you are losing volumes of coolant that correlate to the time spent standing on the loud pedal, it is likely gas pressure pushing it out, especially if overheating temps are not achieved at bottom of radiator. When gas leakage pushes the coolant out, the temp rise begins by less coolant molecules to share the thermal load. The fluid loss happens immediately. The temp rise in the fluid takes some time.

High exhaust gas temp (EGT) can be the cause of much overheating. It occurs when there is not enough boost to bring the injected fuel volume in contact with enough air molecules. I know we think of turbos as the cause of heat but its false. Injectors and their timing are the heat source. Turbos are like a cold air gun. If not enough cold air and turbulence break up the liquid fuel shot in the bore, it will burn slow, late and black in the exhaust. Coolant surrounds the exhaust ports but high EGT centralizes a bunch more temp in a smaller region so the coolant around the exhaust will boil and create gas in the system that needs venting.

If you are tasked with some serious overheat troubleshooting youre gonna need some tools. IR Temp gun is paramount. Dash gauges are almost irrelevant. Next is a way to see gas bypassing the cap under load in real time. Duct tape a clear soda bottle hung off the sidemirror half full of water. Secure some tubing into it from the overflow nipple on the radiator. Beat the bag out of this truck and safely watch the bottle for gas and liquid coloring. If you get bubbles almost immediately with every romp you have a crack or gasket/liner/oring leak. (Bad rings, cracked pistons, shot valve guides etc leak pressure out the crankcase breather)

With gas leakage eliminated we move to coolant side. Is the radiator same temp top to bottom? Is it clogged inside or outside or lacking airflow? Is the fan working? Is there a missing louver that allows an air bypass? Thats major. Is the fan shroud smashed? Fan on backwards? Wrong fan with wrong attack angle for push or pull style using current rotation? Is the heater core same temp as rest of system? Water pump belt slipping? Belt routed backward? Impeller spinning on fan shaft or busted? Wrong pump? Coolant empty? Sludged up? Airbound? Forgotten rag, tape or cap sucked up into inlet from last service? Collapsed internal hose? Hose bent too sharp? Tstat stuck or installed backwards (produces no heat on a dt466A/B/C)

My mellon is squeezed, if that dont fix it, its above my pay grade!

KyLogger · April 26, 2018, 10:25:41 PM

Mike, that is the best explanation of overheating (complete with all them big words) that I have ever heard! This needs to be a sticky!

Tom

mike_belben · April 27, 2018, 12:59:44 AM

you guys who are going to work every day and getting taxed up the wazoo (as i have been since 13yrs old) have been putting some food on our table while we recover from a series of bad landings. I figure i owe something back to all of you and endeavor to repay. If i cant pay you in cash, i'll pay you in savings and reduced downtime. I consider this my day job for now.

Thanks for going to work every day guys. My wife, kids and I sincerely appreciate it.

brianJ · May 19, 2018, 05:30:53 PM

The follow up. Replaced the radiator cap. With the cooling system operating at proper pressure the thermostats can open all the way and gives me all the cooling I need.

I could kick myself for not replacing the temp sensor, thermostats and cap all at once the first time thru. They are all cheap enough. Wonder how much life I took out of my engine from dithering around.

luvmexfood · May 19, 2018, 06:39:15 PM

While this may not apply it's good info to know. I once had an Oldsmobile car that began to run hot. Replaced the thermostat with no help. Noticed the radiator had a pretty good bow on the bottom from hitting something. Possibly from hitting a dog one morning that ran right out in front of me. Replaced radiator and again no changes.

Then on a whim one day I noticed that the lower louver that directed air up to the radiator also had a bow. Looked natural it was so perfect a bow. Hooked up a cable puller to it and a fence post and straightened. Fixed the problem. Sometimes it is the simple things. Don't overlook a change in the airflow over the radiator.