That does make sense, but everything you have described sounds like a bad headgasket. As far as the sending unit, that could cause the issue with the temp gauge, but it won't stop you from getting heat. The two are not related. You have coolant running through the heater core at all times. What determines if you get heat is a blend air door that opens one way for heat and the other way for AC. The only other thing I can think of is you are getting pockets of air in the cooling system from engine compression (bad headgasket) and it is causing the temp to rise quickly until the pump forces coolant to flow through again as well as the heater core getting an air pocket that the pump can't clear. All in all, he is there and I'm not. If he feels the sending unit will help (I don't) it may be worth the try. It is a cheap part, and you could install it yourself to save money. It just screws into the engine. I hope it is something that simple, but I don't think it is. Finally, remember, it is possible for compressed air to get into the cooling system and for you not to loose to lose coolant. The cooling system could actually (burp) it out while the engine is running. Keep in mind, he said he had heat, but you don't. Something is stopping the flow of coolant through the heater core. When you have no heat, have you ever felt both heater core hoses to see if they are both hot and not just one? Have they ever checked engine compression to see if there is a low cylender?
Let me know.
PS: Sorry it took a couple days to get back to you. With Christmas, kids, wife. Well, you know. I hope you had a good one.
Sunday, December 26th, 2010 AT 6:31 AM