I have a 1999 LHS 3.5 Litter, back in Jan '14 the top radiator hose split while driving. Had to drive about two more miles to the house. After replacing the hose it started overheating. Since then I have replaced the Thermostat, Water pump and Radiator. The car overheats in just a few miles after starting cold, maybe 9 to 10 miles. I have done a hydrocarbon test, ( three times) all proved negative. I have had a cylinder leak down test that result was good. The spark plugs all look good, no fouling like its burning antifreeze. But it still overheats, if I turn the heater on high and the blower on it will cool down. Also if when it heats up past normal and I keep the RPM's at 1800 or above it will cool down. But after I let it come back to an idle the temp goes back up again until the heater is turned on or the RPM trick. A mechanic said he thinks the block is partially plugged up. So I have run a double dose of flush for about four weeks, and it is doing the same thing. The car runs as good as it ever has, so the mechanics and I do not believe there is a head or head gasket problem. Has anybody got any ideas. I like the car a lot and don't want to give up on it..

 

Im gonna bank on the heads being warped when the temp gets to 215 or more the headgasket can not hold back teh pressure so it pushes out the coolant and thus u over head. Easy test Funnel where the radiator cap it fill up system up then let a little bit of coolant in funnel then let engine heat up when it gets to 210 watch for bubbles in the coolant and see if it starts to push it out if does then ya know =)