2005 Peterbilt 379 - Cat C15 ACERT - Heavy Fuel in Coolant

Question

I've got a literal mess on my hands. My 2005 Pete with a Twin Turbo C15 has fuel in the cooling system. I opened the surge tank and it's full of black, diesel-smelling sludge. The truck isn't overheating yet, but I know this will eat my radiator hoses if I don't fix it fast.

I'm worried it's a cracked head or a spacer plate issue. I've already drained the coolant and flushed it twice, but the fuel comes back within an hour of running. Is there a common failure point on the C15s that causes fuel/coolant mixing without putting coolant into the oil?

KenworthKai · Accepted Answer

✅ Confirmed fix: Cracked injector cup (sleeve). On the C15 ACERT, the injector sits in a copper sleeve surrounded by coolant. If the O-rings fail or the cup cracks, fuel sprays directly into the water jacket. Replaced all 6 injector cups and O-rings, flushed the system, and the coolant has stayed clean for 5,000 miles.

MackMarty · Answer

Wait, check your fuel cooler first. If your truck is equipped with one, the internal tubes can leak, allowing fuel (which is under higher pressure than the coolant) to migrate into the cooling loop. It's a much cheaper fix than pulling the head.

ShopBossMike · Answer

Could be the oil cooler too. Usually, that puts oil in the coolant, but on some configurations, the pressure differentials can make it look like fuel. I'd pressure test the oil cooler before tearing into the top end of the engine.

2005 Peterbilt 379 - Cat C15 ACERT - Heavy Fuel in Coolant

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