I disagree, used the right way it can be done with no issue. When I picked up my 87 YJ it had been sitting for 5 years and the motor was severly sludged. I had the head off getting rebuilt. I filled the engine with 5 gallons of kerosene. I let it sit for 2 weeks. I installed the head. I drained the motor out and replaced the oil pan due to a leak. I filled it with fresh oil and filter. Cranked the motor with the coil disconnected to built oil pressure. The motor had 160K on it at the time now it is at 200k and runs great with no smoke.
1987 Jeep Laredo, 4.2L engine, Holley 350cfm carb, TF999 trans, np207 transfer case, Rough Country 4" lift, Mickey Thompson wheels, BFG 33/12.50/15 tires, Ongoing Father and son project.....
there's no harm in using a 5 min engine flush, i've actually done many, and i've torn down motors afterward, and it works. the engine flushes you can buy, you put in the crank case, run for 5 mins and then drain and change oil, are mostly kerosene.
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I recently did a full fluid change on my 89 YJ. The night before I pulled the oil plug, drained the oil, run about a gallon of diesel through it and let it sit and evaporate til the next morning. no problems here. I did the diesel on the advice of a close friend who has been a mechanic for about 20 years, I have experienced no problems and I think it even runs a little better since the fluid change.