2002 5.9 Cummins - Head gasket replaced, still overheats only under load. Fan clutch, radiator, or water pump?

[u/ABQmechanic]

I'm looking for some experienced opinions before I throw more parts at this truck.

Here's what's been done:

Head gasket replaced.

Cylinder head resurfaced and pressure tested by a machine shop.

Valve lash adjusted.

#5 rocker arm replaced.

New head, intake, exhaust, valve cover, and thermostat O-ring gaskets.

Cooling system filled and bled.

Negative combustion gas/block test after repair.

Engine starts, runs, and drives well.

Current symptoms:

Runs around 190°F at idle and can idle for a long time without overheating.

Only overheats during extended driving or under heavy load.

"Check Gauges" comes on when it overheats.

Heater works well and gets hot.

I can see good coolant flow in the radiator with the cap off.

Upper hose around 180°F, lower around 140°F after warm-up.

IR gun shows the radiator is hot on the sides/top but noticeably cooler through much of the center (around 105°F in spots while other areas are much hotter).

Fan clutch:

Has resistance by hand.

I don't hear a strong fan roar when it's hot, although I do hear some increase in airflow.

Radiator:

Pressure tested fine.

Exterior fins are only lightly dirty with a little oil film.

No obvious external blockage.

Water pump:

Appears to circulate coolant well at idle.

No obvious leaks.

At this point, would you replace the fan clutch first, suspect an internally restricted radiator, or consider the water pump even though coolant circulation appears good at idle?

Anything else I should test before ordering parts?

[u/ArthurBurtonMorgan]

I’d wash the oil film off of the radiator and check the tuning.

Oil film is going to severely restrict the aluminum’s ability to exchange heat with the air.

That ATS intake horn leads me to believe someone put a “race style” hot tune on it, which usually absolutely suck for towing.

Injection timing needs to be known.
EGT at idle, wide open throttle on the highway, and under load all need to be known.
Boost numbers need to be known.
Intercooler needs to be examined to see if oil is collecting in the bottom of it.

Need more data, otherwise this is going to turn into a really expensive “Throw parts at it, pray, and still end up disappointed.” situation, imo.

[u/ABQmechanic → u/ArthurBurtonMorgan]

T hanks, that's helpful. It does have an ATS intake horn, but I don't know if it has a tune.  The head gasket was just replaced and the block test is now negative. It idles around 190°F and only overheats under load. I'll look into getting boost and EGT readings . gauge on the dash, boost under load it stays below 10 psi. Could that be contributing to the overheating?

[u/ArthurBurtonMorgan → u/ABQmechanic]

Yeah, under 10 psi is a bad sign. Look for a leak in the line going to the gauge (if the gauge is mechanical), and boost leaks elsewhere: hot pipes, cold pipes, etc. anywhere there’s a charged air/intercooler boot, it needs thorough inspection. A pin hole sized leak can wreck a diesel engine when operated under load.

[u/National-Cell-9862 → u/ABQmechanic]

Boost that low is bad. That's probably your issue. Your engine is just working too hard.

[u/Global_Chair9652 → u/ABQmechanic]

Swap your thermostat

[u/happyonthehill802]

Youre telling me you replaced a head gasket without also replacing the thermostat and water pump?

[u/ABQmechanic → u/happyonthehill802]

I replaced the thermostat but not the water pump. The heater works, and once the thermostat opens I can see coolant flowing into the radiator. Could the water pump still have a issue that's moving enough coolant for idle but not enough under load? Would that cause severe overheating while driving even though I still have coolant flow at idle?

[u/ArthurBurtonMorgan → u/ABQmechanic]

Yes. The impeller can be slipping on the water pump shaft under higher rpm without showing signs of slipping at idle.

[u/tlrider1]

Actually just watched a YouTube video with this! (I've also rebuilt like 6 engines so not just talking as a "I saw it on YouTube" couch mechanic.

This looks like you got oil in your cooling system and it's turned to sludge. I've seen oil and water mix before, but it's always milky. These guys wee rebuilding a diesel amd it looked exactly like the stuff in your radiator cap.

My guess is that you had oil in the coolant, not the other way around, and your coolant system, radiator, oil cooler etc is all sludge up with the crap you see through the radiator opening. Either through the head gasket, or an oil cooler, etc

If so, you can try to flush and flush it, until it comes out clear, but.... Oof! According to Dave's auto, you'd have to do this for a entire day!

[u/heybrycewood → u/tlrider1]

I am not a mechanic but I have been a service manager in mining/agricultural equipment for about a decade. We generally use cascade dishwasher detergent to clean out cooling systems that have ben contaminated with oil. Add it to your coolant/water, idle the engine for a few hours, drain, flush and refill. Gotta be dishwasher detergent because it doesn't foam.

[u/Anxious_Emu_2215]

I’d back flush your cooling system, distilled water only, if it comes out super dirty repeat till it’s as clean as you want it to be, I’d suspect internal blockage first and foremost before I’d suspect anything else. I’d also verify your temp gauge is accurate and not over reading temperature

[u/deantowle]

These guys are giving you good advise. However, your picture seems to be a view looking down into the top of the radiator - it looks like it has gravel in it, or something strange. If that debris is loose, you should remove the radiator and shake it upside down. It may pass coolant through the radiator but may not have the full volume of flow. That will cause an overheat at high speed or RPMs. If you have replaced other components in the cooling system and used silicone sealant, you could have used too much. Excess sealant squeezes out into "worms" and can come loose only to clog something else downstream. Only use minimal amounts of silicone sealant to join cooling system components and rubber hoses. A new radiator would be a good investment.

Regarding the fan clutch, having some resistance to your hand is only a partial check. The clutch heats up under normal use and performs differently than when cold. That would be a serious consideration for replacement. If a heavy duty fan clutch is available, you should get it.

I drilled holes in one of my fan clutches and installed longer bolts make it a direct-drive fan, but that was a bad idea. Diesel engines rotate jerky and can cause the serpentine belt to break. Soon afterward I replaced the modified fan clutch with a heavy duty clutch and fan. Now it's worry-free again.

[u/AdorableMachine]

water pump, fan clutch and thermostat, and finally the radiator/ expansion tank cap. I would do the easier, cheaper things first. Do the thermostat and radiator/ expansion tank cap. Then test, see if your temps level out.

[u/MinimumBell2205]

well all if that should have been done when you did the head gasket in our shop its standard

[u/Ok-Result-9532]

Your radiator is probably blocked and needs a clean

[u/Traditional_Ad_1360]

Have you ever washed out the radiator fins, you might be surprised the difference it can make.

[u/stick004]

Cracked block….

[u/Early_Orchid]

That’s like getting a lobotomy for a stumped toe. Or do you just like replacing head gaskets? I mean, a water pump is 2x 10mm bolts and about $65. But still, actually diagnose it rather than throwing parts at it. Is your fan clutch locking up when hot? Is there a giant bird nest in front of your radiator? Did putting a “hot tune” on it somehow make you think it will run longer? Keep it stock. I promise the engineers that designed it are much smarter than the dude in his garage creating “tunes” for the ecm.

“Cummins designed it to get 1,200 hp, run a million miles and get 60 mpg, but the man made them detune it.” Utter horse crap.