89 k1500 gets warm only on the highway

[Nopainnogain445]

So I have a 1989 gmc k1500 and for the last couple weeks I’ve noticed my temp guage slowly creeps up once I get on the highway doing about 60-65mph to about the 230 mark and one I slow back down to 30-40mph the temp drops back to normal, I replaced the thermostat and water pump was replaced a couple years ago, the coolant that is in there is brown and there is buildup around the inside of the radiator, also I have no heat what so ever so I am thinking maybe it’s a clogged radiator and heater core but I would think that would cause overheating at lower speeds, any help would be much appreciated.

 

[CumminsFever]

A proper flushing of the cooling system MAY get things unclogged, antifreeze shouldn't be rust colored.
Your actual problem may well be the cooling fan clutch. However, if your radiator is internally plugged, your fan clutch can't "sense" the water temp.

 

[movietvet]

I am guessing that it is a clogged radiator and heater core problem. Theoretically, you don't need a fan working at highway speeds because of the volume of air flowing over the radiator at that high speed. The fan clutch would pretty much go to neutral at highway speed and work much harder at slower speeds to pull air across the radiator.

 

[Nopainnogain445]

A proper flushing of the cooling system MAY get things unclogged, antifreeze shouldn't be rust colored.
Your actual problem may well be the cooling fan clutch. However, if your radiator is internally plugged, your fan clutch can't "sense" the water temp.

I’ll pick up a new rad tomorrow and throw it in and flush the whole system I’ll check back tomorrow and let you know if it still overheats

 

[GoToGuy]

When doing very complete flush remove heat exchanger hoses, and use your garden hose to moderately power flush, both directions. And watch the crude blow out. The exchanger can like a settling pond , filling with crude . When flushing , block drains should be removed also, to flush low areas in block. Is top radiator tubes filling with crude reducing flow? Pull thermostat, in pan of water boil it , at what temp does it open? (Physical test of thermostat.) Test radiator cap, hold pressure, seal is good. That's a start.

 

[movietvet]

If you are doing the radiator, do the thermostat. The brown fluid also means that the inside of hoses are being eaten away. How old are the hoses? Temp sender?

Hell, as easy as it is, I would look at the impeller of the water pump. What quality is that 2 year old water pump?

 

[CumminsFever]

A complete flush is an absolute MUST! I redneck it. Take off a heater core hose, put the garden hose in the rad cap, start it and rev it up. Run until the thing is chucking clear water. Drain the water and fill with antifreeze. It may not get the low areas of the block, but it sure gets alot of scary stuff gone!

 

[movietvet]

A complete flush is an absolute MUST! I redneck it. Take off a heater core hose, put the garden hose in the rad cap, start it and rev it up. Run until the thing is chucking clear water. Drain the water and fill with antifreeze. It may not get the low areas of the block, but it sure gets alot of scary stuff gone!

Easy enough to get to. Pull the block drains.

 

[Schurkey]

Flushing a cooling system without pulling the block drains is almost but not quite a waste of effort.

"I" would use a flush 'n' fill kit.
www.amazon.com/dp/B000CCFY5W/?coliid=I2S26W9IAZJJ30&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

1989 gmc k1500

Either a V6 or a small-block. Left side of block has a drain plug, 9/16 hex head unless it's been changed. Right side of block has the knock sensor screwed into the block drain hole. Be sure to properly seal and torque the knock sensor when you reinstall it.

 

[618 Syndicate]

Block drains may require some unclogging with a pick to get them to flow.