When you put the t stat in are you using a gasket or just silicone? You should use a gasket and make sure the t stat sit's tight in the hole. Any corrosion around it? Any slop and coolant will go around the t stat. Feel the upper rad hose just before the t stat opens, It should be cold. If you feel any warmth then it's leaking past it.

 

This is about the only thing I can think of, that would explain the problem, given all that's been done. Coolant has to be bypassing the stat somehow.

 

So, I've researched this. This is a somewhat regular problem with the 2nd gen. At first I thought it might be the thermostat gasket, thin paper is original. I was thinking it would be a rubber one that went around the thermostat and the wrong type of gasket was being misused. Paper/gasket material that is thin is the right one. Air in the system seems like the culprit. I'd try to Burp it on a steep hill. I'm stumped. This fixed a few of them and no feedback of what worked for the others.

 

If it was an air bubble, it would either have poor heater performance, over it would tend to overheat....... at least, until it worked the air bubble out. (heater performance may still be poor though, as the core is notoriously difficult to get all the air out.)

Any holes drilled in the stat?

 

sorry for the delayed response, work and all ya know. there is no weep hole in the thermos and it is using a paper gasket. I want to try to hold the hose as it warms to see if it slowly warms up or all at once when the thermos opens. When we burped it we used a very steep hill. now that its been running awhile, I will try to burp it again tomorrow evening after work also while holding the hose for feeling the temp. I seen a lot where this is a common problem, but like you said, no one seems to post a cure. I know 2 others who did the same thing to the plenum seal and now they have the same problem. I am hoping burping a third time will help out, an air pocket steaming by the thermos would keep it open if the steam is hot enough, but with the location of the thermos, once open, it should release the air pocket. but if everything acted like it should when we fix it, I wouldn't be here now huh. I will try these suggestions and make some observations. I agree its like water is getting around the thermostat, but damn if I can figure out how, each time I replace it, it is in the well pretty darn good. I am using a gasket with a light coat of silicone.

 

BADASR1
thanks for the link, like the fix. I had my unit apart to replace the core, while I replaced the core I also replaced the evap, and inspected all box components. I wish I seen this before I put the dash together as I may have put it in anyway to avoid a future problem.

 

Yeah, it still sounds like an air pocket to me. You have redundantly done way too much already to not have any heat. I drilled a 1/16 hole on each side of 195(or 190 can't remember) degree stock stainless T-stat. I did have this issue, no heat, once and put up with it until i was able to revisit that area again due to a leak. Fixed the leak (it was the brand new bypass hose, it had a slit at end and water was leaking) and modified the T-stat. Never had any heating issues after that. I too have a 2001 with a 5.9 and 180 thousand miles.

 

Glad im not the only 1!! Mine will get toasty if im on the freeway. Takes 4ever to warm up in town.. Never gets hot! Ive been told its air also...01 5.9 195,000 miles...runs like a beast.. New flashpaq 4845 tuner also! No more death flash!

 

hell yeah, mine runs good too. i been losing water ever so slowly for a while, took alternator off not long ago and was able to see some wetness around that damned water neck. will have to replace the gasket and sealed it up again eventually.

 

well, ive bleed the coolant system 4 times, and drove over 1000 miles, I get heat, but the heat temp reflects what the gauge reads, hot air around 181, warm air when gauge drops to around 177, then when gauge goes up, heater temp goes up. temp gauge never hits 195. no water seems to be rushing past thermos when closed, engine just never gets to operating temp when in the winter, blocked rad for now and it helps, but still has never been the way it was before replacing plenum seal, runs great, ill have to deal with it until I can think of a logical reason why 7 new thermos all open to early.