Project Cherokeeper: Blown Head Gasket! Time to Rebuild the Jeep 4.0 Engine Top-End
An overheating issue really had me stumped for a very long time. It had the characteristic of a mild blown head gasket (it would only run hot on the highway, no other problems), but non of the physical symptoms - no coolant visible in the oil, no oil in the coolant, no smoke in the exhaust, compression was great on all cylinders (cold engine), it passed the combustion "block test" both with the rental kit from a parts store and when I took it to a local garage with an emissions test machine, and it passed the cooling system pressure test. Finally, I sent an oil sample to a lab and they said there was signs of coolant in the oil - not a lot but enough for concern. Off with the head!