I have a '97 MC with a 3.4L. I noticed when I bought it with 94K miles that when I was driving, the temperature would go up, idle and it would go down. It has a new water pump, thermostat and hoses. Upon further inspection I finally found the problem. The upper radiator hose collapses when I open up the throttle. Only does it when you give it throttle. Can't figure it out. Any one have/had this problem?

 

Sounds to me like an air bubble in the system. Maybe a clogged radiator. Try bleeding the system at the bleeder valve on the thermostst housing. Might need a new hose if it's real soft. It shouldn't be spongy.

http://www.cardomain.com/ride/2319272

 

also i know a great fix for this.
the same thing i want to do on mine to just make it more reliabel.

FLEX HOSES!!! they are copper hoses in diffrent colors but they are not cheap . but this will make it hard for them to do this and also will make them pretty much never need maintnace again other then the short rubber collors on both ends.

 

have the system flushed really well, and make sure all the air comes out of it, getting air out of the system is sometimes hard, could be a clogged heater core causing it, could just be an air bubble.

 

I would go the flushing route as well. It costs more, but you don't want that system clogged up with corrosion.

 

Thanks for the input. I found the problem. Despite the previous owner installing a new water pump, it was not OEM. The fins were bigger, thin aluminum and it was churning (beating)the coolant so hard it was cavitating and creating air bubbles despite continual bleeding of the system. Not knowing this, I took a chance and tried a new waterpump (OEM) and saw the difference when I got the old one off. Put on the OEM water pump, runs like a champ.
29-31 MPG highway with the 3.4L - outstanding and powerful besides. A cols air intake and I will be a happy camper.