Cadillac Repair: 4.6 L engine overheats while driven, ebay ebay, northstar engine

Question
I have a 1996 Caddillac Concours 4.6 L northstar engine. within the last 18,000 mi. its had new head gaskets,intake manifold,oxygen sensor,new bolts,thermostat,hoses. For the last two years it has run great. Then a week ago engine overheat warning begins,so I crawl to my mechanic about five miles away.We put new thermostat in,new water pump,new radiator,fans are working,engine oil is clean no water,no white smoke when first started.Engine idles with air conditioner on,for an hour and temp remains a constant 206 degrees. Then you drive it about 7 mi. and it overheats, no liquid on the ground, our lost anti freeze in the resevoir. What are we overlooking? We would greatly appreciate your insight thank you R. Fuller

Answer
Hi Rockey,

      The steps to check the cooling fans eithor with a scan tool or with the onboard diagnostics would be too lengthly to print out of for me to type up. Basically what happens is both fans should turn on slow speed at around 106 degrees C and high speed at 110 degrees C. If you only have low speed then there could be a problem with the hi speed relay or wireing.

      If you need more info that that I would suggest you contact the other Caddy expert and he might be able to send the info you need. Otherwise a GM shop manual or a subscription to AllData.com will give you the info that you would need to diagnose this yourself. Ebay, Ebay Motors, manuals is a good sourse of GM factory manuals. Make sure to get both manuals for your car because engine and trans is in 1 manual and everything else is in the other.

     Sorry I can't be anymore help than that. Bill