Lincoln mark VIII has no water leaks but gets hot after running for a while. What else could the problem be?

Well, it could be a bad water pump... That is the most likely culprit. Hopefully it is just a sticking thermostat or a clogged radiator or something like that, but the water pumps on those cars are powered by the timing chain and will not usually show signs of leaking externally.

though not advisable, recommend running completely without the t-stat in place, that way will be WIDE open to it's cooling potential...if this changes nothing would get a thermometer gun and point it at the radiator to see where the hot spots are then....replace the radiator (if you suspect clogging, and labored flow)~

Your mechanic can hook a good scan tool and watch the temperature reading to see if the engine is actually getting hot. It could be a faulty indicator or just that it looks like it's running hot. Overtemp at low speeds is usually a inoperative fan or water flow problem. Your Mechanic should also do a leakdown and compression test if everything else fails. Does it run ok? Don't let hum open the motor unless he's Certified and insured. That motor is mostly Aluminum and takes training and special tool to disassemble and reassemble properly.

It could be a very simple fix...I just had the same problem on my '95 Mark VIII. After about 5 minutes the temp gauge registered hot... the needle wouldn't move for the first three minutes or so but once it did went to the hot level. I had a radiator shop put in a new thermastate, flush the radiator and engine and put in new coolant mixture of 50/50 antifreeze. The car now is running normal. One thing to always watch for. Since our cars have a sealed system, keep your eye on the coolent resorvoir. It should always be full when cold. If you see it is below then always add coolent to the top of the fill mark on the container. only do this when the engine is cold. Hope this helps

The top center fill valve (open with hex) may have to be removed to fill with coolant; rest of system may be full but not water jacket.

Regarding the Waterpump. JoeF was wrong about how the Waterpump operates on the 93- 98 Lincoln Mark VIII. He said "powered by the timing chain and will not usually show signs of leaking externally." Thats incorrect. Mark VIII's had a 32V 4.6L Intech motor, later to be know as the Cobra engine in the 1996 Mustang. The water pump runs off of the accessory serpentine belt. If it leaks you will see signs. But it doesn't have to "leak" for it to be no good. The pump impeller could be rotting away inside. I've seen the impeller disintegrate off into nothing. So the belt will spin and it will appear to be working and actually be overheating. Its a really easy job on that engine, well only on the RWD cars. Not on the FWD Continental,its a knuckle buster, lots of stuff in the way. Check and or replace the pump for piece of mind. The bearings do go and can get noisy after alot of years/miles. That coolant x-over tube has a cap thats a wrench breaker to remove. That tube always has to be topped off with coolant for the system to work rite.

there is a temperature control that operates the cooling fan on my '93 MKVlll this is part of a computer that when it fails causes the engine to overheat. have a mechanic check it out Lincoln Bob