4.9 inline six temp gauge fluctuation

[cavf150]

Hey,
Can anyone give me advice on why my temp gauge has gone on the fritz? At start up all is fine , 5 miles down the road the gauge rises to the O in the gauge then suddenly goes back to normal. Then it climbs slowly again then back to normal. Fluid level is at the top and shows no leaks. The oil is w/o freeze in it and is at full. Can a bad cap cause this, or a sender unit? Where is the sender unit located on the engine? Engine has a/c. Too cold here to turn it on. Heater throws hot air and the top rad hose is hot. Could a air lock have developed somehow? Engine is not overheating!
Thanks,

[andy2005]

I have heard that after 1988 Ford gauges are basically a light with a needle hooked to it. My guess would be that it everything elese seems fine, its probably a sending unit.

[mustangmike351c]

check your thermostat it may be sticking

[ModMech]

The temp gauge is a real gauge, I suspect either the thermostat is sticking, or the sender is bad. Since you need to drain the coolant some anyway, I would replace both with FACTORY replacement (Motorcraft) parts.

[cavf150]

Originally Posted by andy2005

I have heard that after 1988 Ford gauges are basically a light with a needle hooked to it. My guess would be that it everything elese seems fine, its probably a sending unit.

ANDY,
Thanks for the input, I am starting with the sender
cavf150

[cavf150]

Originally Posted by mustangmike351c

check your thermostat it may be sticking

Mike,
Sounds like it. does a thermostat constantly open and close?
cavf150

[cavf150]

Originally Posted by ModMech

The temp gauge is a real gauge, I suspect either the thermostat is sticking, or the sender is bad. Since you need to drain the coolant some anyway, I would replace both with FACTORY replacement (Motorcraft) parts.

ModMech,
Seems to be the consensus of all replies, I am changing the sender 1st
The thermostat change is pretty straight forward and I'll do it next.
cavf150

[J-Ri]

Originally Posted by cavf150

does a thermostat constantly open and close?

Not unless it gets cold coolant flowing by it. It will fluctuate a bit until the engine's been running for awhile.

[mustangmike351c]

Originally Posted by cavf150

Mike,
Sounds like it. does a thermostat constantly open and close?
cavf150

its should only open at the temp it is made for but if its sticking it will not open until it gets hot and then open util it cools back down and it may stick open and over cool making the temp go all the way up and back down many times