My '97 with 120k mi. just started some weird cooling system related issues.
I will be driving and my heater is blowing hot air - the temperature gauge is in the proper range, etc. and suddenly - there is no heat and my temp. gauge drops to cold - within a few minutes the engine overheats and you can hear the coolant boiling. I get a code P1299. Then I can start it up and it will be fine for a day or so before doing the same thing again. I had it in the shop for a new alternator and asked them to check out the cooling system and they couldn't find anything wrong.
Would a stuck thermostat cause this? I don't understand why the temp gauge would drop down to cold and then peg back up to HOT++??
Anybody else ever have a thermostat that got stuck intermittently? Hopefully this isn't a heater core issue.
Thanks for any help

 

if the T stat opens and floods the engine with cold coolant then it can do that. or the gauge is fubar. but either way it sounds like your stat is stuck or sticking. if it sticks open after flooding the engine with coolant then it just recirculates and has no chance to recool then you overheat.

though i could be totally wrong too.

 

Sounds like you are having a headgasket problem. Have a pressure test done.

 

code P1299 - Cylinder Head Over Temperature Protection Active.
I would agree that you need a pressure test and ECU Scan to verify any/all codes. There is also a cylinder head leak test that can be done at any respectable garage. What it consists of is a special fluid that is put into two glass tubes with a squeeze bulb on top. It is then placed over the running vehicle's radiator where the cap goes, and the vehicle is run for about 5 minutes. The bulb is squeezed, sucking up VAPOR, not fluid. Then the colour of the fluid is monitored. Snap-on's specialty fluid is naturally blue, turns yellow with the presence of hydrocarbons (leaking head gasket). Hope this helps - Jimi

 

Check for a coolant leak and the system not holding pressure.

 

I put a new thermostat in over the weekend and there hasn't been a problem since. I'll keep my fingers crossed and hope that was all it was. The shop pressure tested the system for an hour and a half when they put in the new alternator and said it held pressure fine.
I think there may still be an issue though since I notice my coolant smells like fuel. How hard is it to replace the head gaskets on the 4.6?

 

Also, are there any other symptoms/signs when the head gasket is on it's way out?

 

have you at any time/times had to add coolant when serviceing

I have seen head gaskets blow and lose coolant through the exhaust. and at times there is not enough coolant to circulate through the system causing the hot cold cycle you described.

 

Both my wife's 1991 5.0 Mustang LX and my 1991 F-150 5.0 are just about ready for new thermostats. As the engines warm up from a dead cold start the temp gages go past mid range, then drop down to normal. The thermostats are beginning to stick. I'll replace them this spring when I do my annual cooling system flush. Mel

 

Well.............It's baaaaaaaaaaaack!
Thought I got away with the easy fix but alas - it has resurfaced. I put the new thermostat in and it worked fine for a couple of weeks. I was driving home from work a couple of days ago and it did the same thing again. The heater stopped blowing warm air, temp sensor pegged. I stopped to get gas and when I started it again the temp was normal????
Also, when it does this, coolant starts leaking out of the pressure cap on the reservoir. The coolant doesn't seem to be very hot either??
I was suspecting a temp sensor issue but I don't see how that would cause the coolant loss, heat loss, etc. I have driven it since the last episode without incident?
Help!

 

It seems as if there is plenty of coolant for the system - it is never low in the reservoir anyway. I have not noticed any coolant loss other than what has sprayed out of the pressure cap when it has this issue.

 

That is strange enough. Man it don't sound good.
i don't know if its a head gasket or not. but it was sugested by someone here that you may have a blokage in your heater core.

It maybe somewhere in block. You need to have the hydrocarbon test that was posted above. That would probably be the next step

 

Well, I can say that I had the EXACT same symptoms, and it was either a cracked head, or head gasket. I had a radiator shop do the chemical test to determine if there were exhaust gasses in the cooling system... ($50 to buy the test kit and do it myself, or $60 to have the shop do it)...sure enough, the chemical turned colors in about 5 seconds. I priced how much it would cost to have head gaskets done (gambling that the head wasn't cracked), and decided that it was only about $500 more to just have a used motor put in instead. Whole thing cost me about $2200.

I drove it for months with the symptoms though, since it was intermittant, and seemed to go away when it heated up. I figure either the gasket swelled when it got warm, or the head swelled, sealing it back up.

 

It is still intermittently having the problem. The heater goes cold, the oil light comes on followed by the temp gauge headin' north. If I pull over, shut it off and turn it back on - the problem is gone. Instantly the temperature is in the normal range - the oil light is gone and then it's fine until next time?? I had it in the shop again and they can't find anything wrong. I could do the hydrocarbon test I guess but this seems more like an electrical issue with the sensors or something since it goes away instantly like this.
Anybody have issues with sensors, etc. that might cause this?

 

As far as, it being electrical, I don't think you would be getting the fluids boiling out.

My money would be on a blockage (had a 91 chevy do the same thing and had a blockage, gotta love stop-a-leak ) or a blown head gasket, or cracked head. You did mention that your coolant smelled like fuel? That would be a very good indication for a head problem.

Keep us posted! Good luck!