I am experiencing a very strange problem with my 1997 F-150 4.6L. When idling I lose heat, temp gauge looks fine, no sign of overheat. When I rev the engine higher, the gauge swings up to the high side for a couple seconds and then back to normal.
This is clearly a flow problem but I am wondering if it is the radiator or could a water pump possibly have weak performance. The reason I am wondering is that the truck won't overheat with some rev's and standing still. I would think if it was a plugged rad it would still overheat. Has anyone ever had a water pump go bad but not leak?

BTW-coolant flushed this summer and belt change, new t-stat, 112,000 miles, no coolant contamination or oil contamination.

 

was it a complete flush?? block ,heater core etc...grab the fan blade see if you can move it up and down. if you can pump is going. so its fine when driving around? heats fine, temp etc..???. a pump can go bad with no apparent leaks, the fins inside wear out. so doesnt pump as well. that is a rare though. you have 112k? thats not high miles for these trucks.i have 2 4.6s crown vic,f150both over 120k still om original pump,keep fingers crossed

 

Yes everything is normal unless the engine idles, doesn't have a problemm when sitting still when revved to 2000, checked the tensioner and belt, tight, no play in the water pump or fan, clutch fan is spinning, I am going to have the radiator tested, maybe the pump is having a tough time pushing coolant at low revs. May also replace pump both to rule it out and because it does have a lot of miles on it.

 

well if you plan in keeping the truck for a few more years go ahead and do the pump.fairly easy job. pressure test the radiator

 

Radiator shop says there is little flow, looks like Thats it even though it visuaaly had looked ok.

 

So I'm new to the whole changing out radiator fluid/flushing the entire system. Any suggestions on how to go about that? Sorry to take the focus away from your low flow problem, but I thought I would just ask since it's the same subject. THANKS!

 

you have to take out the block drains to do a complete flush.all you do us take your garden hose and flush all the crap out. when you have clear water coming out your all set.you can by flush kits at wal- mart or any parts store. im sure others can give you a few pointers

 

Just something else you may want to check out. Have someone watch your radiator hoses while you apply various RPM. You may have a radiator hose collapsing and restricting the flow. Sometimes when they get older they will become weak and collapse.

 

hes right i forgot about that.i have a buddy he has a dakota. anyway at idle his top hose was flat it was so flimsy.we replaced it...all set now