Overheating issue

[Lt1_8U]

I have a 1990 gmc sierra with a 5.7 tbi and a 5 speed. A few days ago while going to pick up my gun it started overheating.

I changed the thermostat and nothing
No leaking out of my pump
I flushed the system
I changed the coolant cap
I Noticed that when I give it throttle the upper hose flexes like it is compressing.
My cat isn't clogged

It will normally climb to temp but then it will slowly go to around 230-245 than instantly drop 35-40 degrees in a few seconds. I'm going to purchase a digital thermometer to see what temp it reads.

Thanks guys!

[ThoR294]

I wonder if you have a clog somewhere. how did you do the flush? when exactly does it overheat?

[Lt1_8U]

I put a t fitting in the heatercore line and ran water through it. It seems to heat up after I drive for a couple of miles but it doesn't shoot right up. I'm going to see if i have a clog in the radiator after work.

[ThoR294]

Yeah that would be my next step

[chrisfrom nj]

jim did you change the fan clutch my blazer did the same thing till i changed the fan clutch

[Lt1_8U]

No not yet, ibelieve it's fine. Update. Drive home from work and it stays normal temp. I even sat there with the a/c on for 10 min and it didn't go above 200. Very strange, maybe a clog that cleared in the radiator. I cleaned it out yesterday and have not driven it much.

[chrisfrom nj]

jim change the water pump

[Lt1_8U]

I see it circulating water when the cap is off. I'm going to check to see if my radiator is clogged first before I buy anything.

[townsend]

sounds like air trapped

[Lt1_8U]

Put a new radiator in and it fixed the issue. When I was taking the old one out I had both hoses off and I heard a small whistle sound in the upper hose. When it made that sound the radiator finished draining. Definitely clogged up in there!

Thanks guys for the tips