what would cause the temp to rise and fall ? just tried a new thermostat still the same .

 

bad sensor?

 

Bad water pump, fan clutch, need rad flush?

 

was woundering about water pump

 

air pocket in the cooling system, after you replaced the thermostat and refilled you created an air pocket.

 

If it's really fast, I'm with air pocket.

 

try replacing the GROUND STRAP from the motor to the firewall ( I had the same problem and that to care of it ) .... I replaced t-stat and temp sensor but still had the same problem so for $5 i did the ground strap... 1999 Sport 4.0 L

 

My '99 does the same thing. The needle on the guage moves up and down slightly at just below the 210 mark. I assume it's just showing the thermostat opening and closing. My cooling system has been flushed recently, but didn't have a lot of rust or solid debris in it. My water pump and stat are unknown. I've only had the vehicle a few months. I'm not worried about it. I'm nowhere near overheating.

 

.......but isn't it that "unknown" stuff that should be replaced/maintained, especially on any 10+ year old vehicle?

 

if it's moving slowly , like heating up at a red light or moving down slowly when your driving that's normal. just the engine heating up and cooling down.

Now if it's constantly shooting up in the 240- 260 range your cooling parts need to checked completely. From the fan, to the radiator cap, and the water pump with everything in between getting the once or twice over. There is nothing in the cooling system that is real expensive so might as well replace most of it.

 

if the temp really goes down by a lot when you get on the highway, fan clutch.

 

That's one way to look at it, but if age is what determines replacment, where does one stop? After all, the whole vehicle is over 10 years old.

My thinking is that I will look at replacing the water pump and thermostat when there's evidence of failure. Right now, the temp never goes over an indicated 210 and the coolant is circulating properly. I don't see any good reason to tear into a system that appears to be functioning correctly.

 

its like its the thermostat opening and closing nomatter how fast your driving never going above 210 . thats why i replaced the stat.

 

CHeck the CONNECTOR on the Water Temp Sender on the back of the block.

My 95 was having wild fluctuations and I found that the connector ON the sending unit was broken causing the connection to be intermittent.

It made no sense. When I accelerated the temp would DROP .... when I cruised or was at idle it was normal.

Sometimes the the most easy solution is the right one.

You could spend hours getting the air out of the system that isn't there, or changing thermostats, when it only takes a minute to check that connector.

 

I just look at it from the stand point of maintenance of some sort is always required. The manufacturer requires maintenance even when the vehicle is still covered under factory warranty.

My luck is, stuff never breaks/leaks/overheats/leaves me stranded when I'm in my driveway, on a Saturday afternoon, when the parts stores are open, and it's convenient to repair. Preventative maintenance makes for a more reliable DD.