2006 F250 88,000 miles, egr delete, tuned. I recently replaced my factory ford thermostat with a 200 degree Mishimoto. My factory t-stat seemed to have been stuck open and was not reaching normal operating temps. Heat would blow hot while accelerating and then blow cold at stop lights and blow hot again after accelerating again. My new Mishimoto t-stat wouldn't even get close to 200 degrees (180 tops) I contacted Mishimoto and they claimed it was a dud and seemed to have been stuck open. Sent 1st Mishimoto t-stat back and replaced with another new 200 degree Mishimoto t-stat. Still having same problem. Took a 3 hour round trip on interstate (80 mph) and what not and still not getting anywhere close to 200 degrees. Coolant level is full, recently replaced coolant cap with a new oem coolant cap. Let my truck run idle in driveway with coolant cap off for roughly 30 mins. to bleed any air out that may have been in there. Before and after t-stat changes I would have coolant residue around degas bottle and around drivers side engine bay. Delta's have always been 10 degrees or less. After 2nd t-stat change I did notice at one point delta's reached 16 degree difference briefly, but returned to normal. Is this a sign of headgaskets? Oil cooler? I would greatly appreciate any and all advice! Thank you in advance!

 

That seems kind of hot, but then again, my truck isn't tuned or moded in any way except blue spring. My water temperature hardly ever goes over 192.

 

If you don't have the right housing it gonna do that. I believe the last number on the housing has to be a 92. Also the t stat has to be boiled in a pot in the housing to seat the t stat in the housing. I went through all that in both of my trucks as I run the Mishimoto too.

 

Forgot to mention I did boil both new t-stats to properly seat them. I will take a look at the housing in the morning. I was under the impression that the 05-07 had the updated housing. I'm hoping that is the issue as it would be an easy fix. Would you recommend an oem or Mishimoto housing?

 

I was lucky because I had access to all the housings. I'd get theirs if your not sure it's seating properly. I'd check the number on the housing, pull it and boil it to make sure it's seating. You can try drilling a bit of the housing where the pin goes in but not too much.

 

The housing to get is Motorcraft RT1169. I had a 2005 and needed to get it, I had the other one in my truck. It's probably available at local parts stores, but I got mine from Amazon. It's $24 there. I just did that and the Mishi Tstat and am running low 190s with an empty truck. I don't know why it's not closer to 200, but it works well holding around 192-193.

 

You report exact same issue with both thermostat and are reporting a symptom of head gasket fail or water pump fail

It warrants a pressure test of coiling system and or cylinder compression test

​​​​​​​Is this truck tune on bolts?

 

The trucks cooling system was recently pressure tested and satisfactory. Yes tuned on bolts. I have a mechanic buddy who will perform headgaskets, studs and all necessary upgrades for $4-4.5k if needed whenever I'm ready to pull the trigger.

 

I'd check the status housing first, it's alot cheaper. Then if your sure it's seating do a head gasket check. I had the same issue with my 06 with the proper housing. I had to modify it and all is good now. If your still in doubt. Get a factory International t stat, they are a better quality than Motorcraft.

 

Thanks Per4mance. I'm going to check and update housing this coming week and hope that does the trick. If not I'll be taking the truck to mechanic buddy to check everything over and go from there.

 

Just installed a brand new 200 degree Mishimoto t-stat and housing. The t-stat was already seated in the housing with the o-ring attached. Everything went smooth and I'm beginning to uninstall and reinstall everything in record time having done this multiple times now. Fingers crossed it was the housing. I will find out on my commute tomorrow morning on the interstate. I will be sure to update. Thank you again for the feedback.

 

The t-stat housing was the culprit. Truck warmed up to 200 degrees operating temp within 10 mins. 50 degree ambient temp. During my commute on the interstate I was seeing ECT 202 and EOT 208-209. Thanks again!