I had the exact same problem for 2 years on my 88 cherokee. Radiator wasnt leaking, hoses were fine, cap was tight on overflow tank. Or so I thought.
The 88 uses a radiator system that is pressurized through the overflow tank instead of radiator cap (which it doesnt have). Yea stupid design, but it is what it is. What finally solved this problem for me was replacing the actual coolant overflow tank. The tank is plastic, and the threads wear out over time and become slightly warped just enough to cause the cap to not "seat" just right and hence not hold the pressure correctly. Even replacing the cap doesnt work, because the threads on the tank are the issue. The reason you aren't having problems at highway speeds is because there is more than enough air flow to offset the faulty overflow tank not holding 100% pressure. But, once you take away that airflow in regular traffic the thing overheats.
Replace the tank. You can get a good 3rd party one (not the Jeep OEM one) that comes with the cap for about $30 dollars.
Thursday, March 11th, 2010 AT 11:52 PM