i had a post about 2 weeks ago related to a similar problem i was having. At that point I had no heat at idle but when driving I would get heat back. At that point I determined i had a bad water pump. That was the 3rd replacement pump from for so i bought a napa lifetime pump this time. Anyway, that one had a cracked impeller, about .5 of one fin was missing, could only find small pieces of black plastic in drain bucket, never found actual piece. so i replace the pump, i have the 100mm pump on my truck. My heat seemed ok for the first few days, now I can start my truck and it will bake you out of the cab at idle but once you start driving about a half mile in it starts blowing cold for about a mile, then it gets warm again for about a mile. it will usually repeat this for about 10-15 then seem to have normal heat for the most part. i do notice that my coolant was fluctuating. when i got in after a 10 minute warm up(about 20 degrees here) it was hot in the cab and the coolant was about 175, after driving it went up to 190, then started to drop which is when i lost heat, it would go down to about 170, then climb to about 204-210 and i would get heat back, it would only stay that hot for about a second then start dropping back to 190. at that point it would repeat itself. at this point i have the heater valve tied open to allow constant flow. any help would be great.

 

i forgot to mention when i did the water pump i replaced the thermistat with a new one from napa as well.

 

Have you looked at the coolant level when it is blowing cold? Almost sounds like it is low on coolant

 

coolant is about equal with the min line on degas bottle, can really check when its blowing cold since its when im driving down the road. it was blowing cold about 30 seconds before i checked it last time.

 

Could be that piece of impeller is now stuck in the thermostat or the passage between the pump and theromostat. Since it did not come out on replacement it was floating around in the system.

 

i guess anything is possible, i did look with a scope and couldnt find anything, im kind of leaning toward a bad new thermastat my self

 

anyone have any other ideas, anyone think it is a bad thermastat? really want to get this fixed, sucks not having heat.

 

This is a good possibility. I have personally remove pieces out of a water control valve myself, orings plastic and other crap love to get stuck in them!

 

Bad thermostat was what I was thinking before you said that you'd already replaced it. I wonder if that piece is floating around in the area of the thermostat also, but I'd think that's a serious stretch. My concern would be for it to end up in the oil cooler blocking flow there. Have you taken it for an extended drive of say 30 or 40 minutes to see if it stabilizes? It almost sounds like the thermostat is cycling "too much". Closing for too long and then opening too long. Like you are getting a huge shot of cold water then hot water then cold water, etc.

 

the longer i drive the truck the better the less it seems to happen. the longest i have driven is about 30 min and it seemed ok at that point. i have not driven that long in a few days or since it got colder. as for the new thermastat i will say my old ford one would sit at 190 all the time once warmed up, i only replaced it since i was there and figured why not, as for this stat, i noticed since the day i put it in it seems to vary from about 184-196 once warmed up. as stated now the last week it seems to float a lot more than that. as for the piece i pretty sure it came out in the little pieces i found in the drain bucket.

 

Since your old thermostat was working fine why don't you replace it and see if your problem goes away. If it does I'd be hitting Napa? for a refund.

 

that was where i was planning on going next at this point, i will post back an let you know what i find, wont get to it till this weekend but hopefully ill have heat at that point. thanks again.

 

Have read on here that new thermostats can be bad, I like ncpartsman's idea of putting old one back in.Plus you will get a chance to look and see if there is any blockage.Hopefully you will find your problem, let us know how it works out..

 

some one told me possible head gasket, mine have been done and truck was studded at that point, i dont have any signs of gaskets, i did loose a little coolant after the pump but i overfilled it. you guys think that could be it? i know i still havent put the therestat in so it could still be that.

 

I don't think it's headgaskets