To start with, I have searched, and havent been able to find the same issue. But I feel like (maybe more so just hoping) that this is a relatively easy fix and I'm just an idiot.

98 1500, 5.9L with 150k. Just finished the plenum less than 3 months ago. Replaced the T-stat, water pump, belt tensioner, belt...etc

I live in Iowa, and its cold as **** here right now. The temp gauge before would get to about half way to the middle, so 1/4. Im at work right now and not looking at the truck to know what the actual number is.
Now, it runs ALL OVER THE PLACE. For a few days I was thinking it might be the fan clutch. The temp would skyrocket while I was idling, but rarely hit the red. (i think it hit it once) Then when I would start driving it would cool off. Meanwhile, still blowing cold air.
Now its doing basically the same thing, but it will skyrocket when im driving too. Then when i give it some gas, I hear the water sound in my dash, the temp drops back down to the 1/4 mark, and the air turns nice and warm.

Does that make any sense to anyone? I wanted to try and get a video of the temp gauge while I was driving but havent had a chance to do it yet.

I apologize ahead of time if I missed something on here where this is a common issue, or for just being a dumbass. But any help would be appreciated.

Thanks!

 

burp it...

 

^Ninja'd me.


bubble in the cooling system.

 

^^^arg!!

Hey it makes sense.

You may still have air in your system. Burping the system will help this. Have you checked your coolant reservoir?

Burping by getting the air out is putting the passenger side up a bit higher or just getting the whole front end up, like slanted driveway for example. Idle the engine and add coolant. Get yo weight on the front of the engine bay to bounce it for moving some air. I also will squeeze the upper rad hose. Get the rad coolant level to just start coming out.

Other thing is that you may just need to flush the heater core.

 

hah...that makes sense! You think theres any chance that the t-stat is bad too? ive heard people getting bad ones with the hughes plenum kit.

 

Thanks! I honestly havent really checked anything yet. It only gets driven once in a while, and I was somewhat afraid to start digging thinking I was going to find the worst case scenario. I will try and burp it this evening when I get home.
And if I understand you correct, just make sure its uphill, add coolant to the rad, squeeze the hose, maybe bounce it a bit...

what else should I look for?

 

mr. whiteknuckles did a good job explaining... I'd venture away from the stat until proven otherwise, though bad stats seem to be the norm nowadays..

two other tips: leave the cap off.. when you reach operating temperature, slightly rev the engine and watch the level drop, and top it off..

it can take a while to get a good burp... you definitely have air in the system as you can hear the tell tale trickle.. that is the good news.. the two nuggets of bad news are: burping can be a pita, and drastic and quickly changing temperatures is indicative of blown head gaskets.. ruh-roh.. that said, i'm willing to bet you simply have a big ol bubble..

 

trickle? are you talking about the water in the dash sound? I've had that for a while now...maybe thats why the temp never got up to the 180!? Makes sense now!

Hopefully I can get this resolved tonight! Thanks for all your help

 

something like that...

your temps were likely 180*+, but where it's measured is likely sitting in a bubble (the temp port is one of the highest points in the cooling loop).. the fluctuation is because the sensor probe is likely periodically being splashed with hot coolant.. the trickle is the heater core, which likes to absorb the air and hold onto it like it's floggin' gold.. it's important you get the nose of your rig up higher than the heater core to get a good clean (hopefully quick) burp..

leave the cap off, run the truck to operating temp, add 50/50 coolant, rev engine, add more, rev engine, add more, rev engine, add more, rev engine, add more, rev engine, add more... you get the point.. it can take a while..

air in the system can promote rust forming.. let's hope your heater core isn't clogged up with particles picked up from the rest of the system..

 

The burping is an important step, and likely to fix your problem.

Another option, one that I tried once on a whim that fixed a fellow's issue...same as yours...was simple. Replaced the wonky temp sender. It's cheap, and fairly easy to get to behind the alternator. I replaced it and the fluctuating temps stopped.

The loss of heat in the cabin is most likely a clogged heater core, but the blend door operation might be compromised. There is a motor that spins and stops with resistance and as it wears it stops controlling the door itself.

You can test it by getting on the floor and watching the motor / door connection as you change the temp on the dash. Watch it turn and stop as you change the temp. If it keeps spinning and spinning...it needs to be replaced by a metal one...which can be found on Ebay.

The bad news is that if the heater core itself is clogged or even leaking in the case...it needs to be replaced and that requires a complete removal of the dash from the truck, and that is NOT an easy task.