Cooling issue. Ideas??

[Sleek26]

95 Buick Roadmaster Estate Wagon. Same running gear as an Impala SS.
LT1 engine. It doesn't overheat around town but does on I25 at 75 mph
after about 10 miles. Temp gauge climbs slowly but keeps going right on up. I can get off the highway and back onto sidestreets run 55 and it cools back down.
1. replaced thermostat-- no effect.
2. replaced water pump-- no help
3. replaced lower radiator hose (thought it might be collapsing at higher RPM's) no help.
I have two of these cars. My wife loves them and this one is rusting away so,
I bought her another one. Thought I might take the engine out of this one and put it in my 87 El Camino, but not until I figure this out. The heater core started leaking afew months ago and I just took the heater hoses off at the firewall and connected them to each other with a piece of copper tubing. Is it possible that so much of the water is simply running through these hoses that not enough is going through the engine , at higher RPM's, to cool it???
Any ideas???????????

[Fiat48]

What kind of fan? Does the car have a shroud on the radiator?

[Sleek26]

Stock clutch type. Yes a full shroud, by that I mean it is all there. The upper and lower halves. I thought about the fan clutch, but if it were bad I would expect it to overheat around town,when your relying on the fan to move air through the radiator, and not on the highway at 75 mph when you shouldn't even need a fan.

[1BIGJIM]

Have the radiator checked to see what the flow rate is. Any Radiator shop can do it in a matter of minutes while you wait.
My bet is it's not flowing 100%

[Fiat48]

I think I would go with radiator restriction also. If it was air flow then getting it off the highway would not cool it down. Sounds like everything else is in order.

[78Eliminator]

When you replaced the lower hose, did you bleed the air out of the system? Some cars need to be air bleed.

[HALLETT BOY]

Radiator...especially if the car has a lot of miles. Just replace it, don't have it repaired, the price of new radiators is very reasonable these days

[Beer-30]

Radiator...especially if the car has a lot of miles. Just replace it, don't have it repaired, the price of new radiators is very reasonable these days
Agreed. If the heater core went, some of the deposits (if not all) that corroded away are stuck in the lower rows of the radiator. There is enough cooling capacity for idle, low rpm, but not enough for fwy power. If it was a fan problem, it would start heating at idle and get worse.
Forgive me if I am wrong, but doesn't the LT-1 have a cam-driven water pump and electric fans? No provision for clutch-type.
http://my96impalass.home.comcast.net/images/My_96_Impala_SS/P5030042.JPG

[Sleek26]

The one in question 1992 still has a regular distrbutor and water pump. but is listed as an LT1. It has TBI injecttion. I questioned this and a guy at Advance Auto parts looked it up in his rebuilt engine book. I had asked him if I would get a genuine LT1 if I bought a rebuilt engine from them. He looked it up and it was listed as an LT1.My 1995 Buick Roadmaster Wagon has the LT1 like the one pictured above. It has TPI injection, a cam driven water pump, The distributor is behind the water pump. The fan however is still belt driven. I may try replacing the radiator.

[Oldsquirt]

A quick and simple way to test for radiator blockage....Run the engine til it is at normal operating temp, then shut it OFF. Reach inside the shroud past the fan and feel as much of the radiator surface as you can, top to bottom and side to side. If there are spots that are colder than the rest, you have restriction in the tubes in that area. If so, you can get the radiator "rodded-out" or you can replace it.
Beware of the low priced replacement radiators. Few, if any, are as good as OEM. Often times the fit is so poor they require modification to fit. As a tech, I hate it when a service advisor sells one of these instead of an OEM unit. Time is money and modifying-to-fit comes out of MY pocket. As far as the quality goes, you get what you pay for. Now, if you only want to use it long enough to get you to the point of swapping the engine into the EC, cheap is probably the best choice.

[Sleek26]

Thanks for the tip on how to tell if the radiator is partially blocked. I'll try that. No I didn't bleed the air off and can't really say I would know how. I refilled the radiator with the engine idling until after the thermostat opened and I had lots of water flowing. Then I put the cap on and filled the reservoir to the mark. I may just pull the radiator out and have it rodded out. I just put it in new about 2 - 21/2 years ago, but as someone said it may be partially clogged with crap out of the heater core. Thanks for the ideas I let you know what I find.

[slotracer]

daughter's x boyfriend had same problem. it would overheat. ran it around brought it back to house and i showed him how to check the radiator. half of it was cold as could be. no flow.
pat(slotracer) ;)