Cooling system woes

250,000 mile 05 z71 with an overheating problem. Started as a leaky fitting to the heater core so I dumped in the fix leak because I was lazy problem solved, so I thought a year went by no problems . Fast forward to a month a go and truck is getting warm in traffic (less than 15 mph)but never went into limp mode. I did get the engine coolant temp warning but it went back down. Make it home no problems at highway speeds. Truck sits for a week or so and hop in to go run errands, 35 45 mph for less than 5 miles and truck is at 3/4 mark on temp gauge stop run errands. Truck only makes it 4 of the 5 miles home and overheats rapidly to the point of limp mode. Immediately shut it down and wait. Get home and change thermostat. No help I think I put a 186 degree one in it, auto zone special, so new water pump , just got done doing that today and not making it run at 1/2 way on temp gauge. I pressure tested while the old pump was on and it was leaking, pressure tested the new one and it held 15 to 16 lbs for 10 min before I got impatient and gave up for the night. I did try to idle with heat on full blast to get the air out of the system but as I added more it started getting hot (the coolant) and bubbling out of the overflow lid. I'm baffled but that doesn't take much , got a shade tree neighbor coming over tomorrow to help but hoping for some help here as well. If it helps I don't have a mechanical fan just a double electric I bought it that way.

Do the efans work?

Do you have heat?

How did the old coolant look?

How's your coolant level?

SnowDrifter said:

Do the efans work?

Do you have heat?

How did the old coolant look?

How's your coolant level?

Click to expand...

E fans do come on
Still no heat front or rear
Old coolant was watered down but still orange
I'm had it to the cold fill line when I started it with cap off to burp it and as it went down I filled it

At first gander it sounds like an air lock. Worst case though is the stop leak plugged the heater cores and or radiator.

Let's start with the simplest first:

First level sanity check: did your thermostat have a jiggle pin / small hole on it with a metal pin sticking through it? Was it oriented correctly? Tstat should be installed such that the jiggle pin is at the highest point, and such that the metal plate behind the thermostat is facing the water pump. Jiggle pin aids in air locks and helps with the thermostat seeing a small portion of coolant even when closed. Plate at the back is a recirc/diversion valve to either sent coolant back into the engine or push it through the rad

Inspect your reservoir and insure there are no deposits or obstructions in any of the connecting hoses.

Then disconnect your upper radiator hose at the radiator, then fill the reservoir until coolant comes out of the hose/rad. This will help with making sure your water pump isn't airlocked.

Put the hose back in, fill the reservoir up, then start your truck and bring it to a high idle. Top of the reservoir, then keep the high idle. You should have heat at this point. Once at operating temp, give the thing 7 or 8 brisk revs to help purge extra air. Top up again.

You shouldn't have any overheating issues during this process. If you do, shut the vehicle off and let's reevaluate.


Other bits while you're in there: are your heater core hoses hot? Upper rad hose? Bottom rad hose? Note: it's normal for the upper radiator hose to be lukewarm due to convection and turbulence. Once the thermostat opens up, it'll get piping hot, at least until the rad fans kick on. Then it'll drop to uncomfortably warm

Thanks a ton SnowDrifter I knew the little jiggle pin was correct so I skipped straight to flooding it till it flowed out of the top hose/rad connection. Front heat works awesome and no overheating issue,back heat not so much, not sweating that though. Thanks for the help. Now anyone wanna buy this thing... I gotta buy a mini van for the wife.

Wait so is it fixed? Like, completely?

Want to dig into the rear heat or nah?

Yep overheating issue and no heat issue is so far (the 5 mile trip) fixed. Also the rear has always been a bit wonky, both of the small lines on the outlet side of the pump are hot with heat on full blast. Cycled the heat zones in the rear and the temp , with the cap open and couldn't get the rear to work . If (big if ) I were going to start that project aka rear heat where would I go

does it blow cold instead? if so, i'd check the blend door or actuator that switches between hot and cold air, there's tons of threads. I'd even try the reset procedure first. Sometimes that's all you need

Yeh its definitely colt out of the rear roof vents

Try the relearn process. If that doesn’t work, you can take it all apart and manually flap the door the door the other way. Closed one way is cold, the other way is hot, part way is mixture.