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Have a 96 Ford Areostar with a 4.0 engine temp is fine during normal driving(has 185 themostat)but when the A/C is on, in stop and go traffic and the outside temp is above 90, the temp gauge will crawl up to the 3/4 mark about 250-260. Have changed or tested all cooling system parts( new rad,clucth fan,fan blade, hoses, tested themostat and cap has 50/50 mix of coolant) once out of traffic temp goes back to normal. Is this normal ( I know fan balde is spinning slower in traffic) or did I miss somthing, also at what temp do you start to destroy the engine Thanks Joe

 

First suggestion, and the easiest ...

Try cleaning the dead bugs and road grime off the A/C condenser. This is the first place to get airflow into the engine compartment.

Borrow a Friend who smokes........with the engine running, blow smoke at the front of the truck, It should suck straight in......Try this before and after cleaning the bugs out.....Note any improvements.......

 

With 50/50 mix and good cap it is good to 265 before boiling.

You have a fan that does not come on when it calls for it, electrically?

IF, say, the fan was NOT the issue, then you could leave out your thermostat to experiment to see if this fixes your problem. If it did, either your stat is not opening all the way, or the coolant passages are gooked or limed, perhaps worst problem in radiator.

You could also try flushing the cooling system.