Ok y'all here's what's up.
When I drive on the interstate around 65 to 70 and then get off at the exit my temp gauge goes up as far as 235. This happens while I'm sitting in the drive through line at wherever I'm grabbing a bite. Within seconds of getting back out on the road, it moves back down to 195 or so.
Now I've replaced the radiator, fan, fan clutch, thermostat, and water pump within the last year.
I feel like it's not the water pump because there's no leak.
I think it's either the radiator is partially blocked or the fan clutch is not functioning properly.
May I get votes/ opinions from all who are interested/care to help please?
Thanks!!
p.s.
When sitting idle, I can get it to cool down just by revving it up to a little over 1000 rpms.

 

What does your coolant look like? Any residue on the bottom of the cap or visually in the radiator when you remove the cap?


Even within a year Dexcool can turn into a rusty looking slurry that will clog up a radiator and kill it's cooling capacity...


If there's any sort of nasty looking stuff in there, I would suggest a full flush (a few of them to be sure you get everything out) and refill with fresh coolant and see what happens. Sometimes you can loosen up the junk in the radiator and get it to come out with a flush or two, but some times it's just crammed in too tight and the only thing you can do at that point is replace the radiator again unfortunately. The radiator (and heater core) is going to have the smallest passages for crud to get stuck in, the engine block & heads and all the hoses should be fine and clear out nicely with a few flushes, but it's the radiator that doesn't always come clean.

 

Yeah the cap has some gunk and looking down inside the radiator I see some buildup. I think I'll run some system flush for a while then some water behind that and go ahead and switch to the green stuff. Sick to death of having issues like this.Thanks man!

 

Took radiator cap off, removed top hose and placed several buckets underneath to catch runoff. Put garden hose in radiator at a nice slow flow and started up the engine. Once temp was reached abd tstat opened, I watched the flow from the upper hose until it ran crystal clear. This took about 30 minutes or so. Ran the a/c to help it reach temp quicker. It stayed at around 195 during the entire portion of this operation.
Once I was sure all the dexcool was out of the system, I added system cleaner to the radiator, put the upper hose back on, radiator cap on, then flushed out the recovery tank thoroughly.
Drove it down the road a couple of miles, came back, and it was boiling over as soon as I got in the driveway. Showed a temp of 226 as I cut it off. Ambient temp was still in the 70s. I feel like it shouldn't have overheated like that.
What am I missing?

 

I had similar issue when I would run the AC on one of my Blazers. I found a small crack on the radiator would open up when it got to operating temp.

I would pressure check the system if you haven't then I would make sure I got the air out of the system.

The preferred method is to jack up the passenger side so the radiator cap is the highest point and start the truck. Add water until you have no more air.

Have you cleaned the fins on the AC condenser with my issue if I didn't run the AC it wasn't a problem.

 

I did a pressure test with my mighty vac. It held around 18 psi for 20 minutes. This was about 2 or 3 weeks ago.
Drove it to work today with that cleaning agent still in it. Seemed to do better. Will run it a couple more days like that and flush out to the green stuff by weeks end. Will keep results updated.
Hosed off the radiator and condenser fins too, btw. 👍

 

So this morning I flushed the cleaning agent out of the system and refilled with the green stuff.
Drove the heck out of it for a couple of miles and then let it idle in the driveway with the air on.
Sure enough, the temp started creeping up and it began to boil over into the recovery tank.
HOWEVER, when I pressed down on the radiator cap it stopped boiling.
Did I:
A. Find the problem
or
B. Simply override the function of the cap?

 

Alright well I stopped and bought the cheapest available radiator cap from auto zone and installed it right there in the parking lot. Drove to work and now have been sitting here for 20 minutes with the a/c running and the temp hasn't moved up more than 2 degrees.
I believe the problem is solved.
Note that the failed cap is an AC Delco OEM part that's only been in service since the end of January of this year.
I'll be sending it back under warranty tomorrow.
Thanks for all the input ya'll!!

 

Well the new cap solved the boiling problem but the temp creep persists. Observed range on dash command was from 180 to 230.
Seems like 50 degrees is a pretty broad range. I have an old radiator laying around that I'm going to pressure test and thoroughly clean to swap out and see if it makes any difference.
Will post results.