A buddy has a 99' cherokee w/ a 4.0L. He has been having overheating problems. He called last week and said he replaced the thermostate (Cheap, easy try for him) and that did not do anything to help. I talked with him over the phone and asked him to fill the radiator and have some fire up the truck see if the level of coolant/water drops in the radiator. He said it didn't move. Next he pulled the return hose to the radiator and fired up the truck and nothing flowed out and he said there seemed to be no pressure. I'm assuming the pump is the problem, but I have not worked on a newer 4.0L and am not aware of the cooling system. Does this seem like the problem or does the motor build up heat before the pump kicks on? Have not actually seen the truck so am seeking some advice to pass on. Thanks in advance for any help.

The pump should not just turn on and off. It's belt driven so it should be running all the time. It could be the pump or a blockage in the system.

The thermostat keeps the Coolant in the engine till 195* is reached and then opens and allows the coolant to flow.. What does "overheating mean" Does it puke coolant out?
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

Bingo, Jim. The coolant will start flowing when the engine heats up (at which point you won't want to be grabbing the hose!). I'll bet the radiator is partially plugged, and probabaly needs to be replaced. I spent just as much trying to 'flush' and repair mine, than I ultimately had to spend on a new unit.

Hey guys thanks for the responses. Sorry its third party, but its a friend a few hours away that is relaying info. I asked him exactly what was happening. He stated that his temp guage would start to rise above normale operating temp and then at stop signs it would be hesitant and stalling. Last time out he pulled over b/c it was running rough and opened the hood to white smoke. I asked if it smelled like coolant, but he doen't remember it smelling like coolant. Any thoughts for him? Thanks in advance.

MANY thanks for the good reply! The guy has a coolant leak in a place that is spewing the coolant onto his exhaust manifold! The white smoke is the proof of the leak.
His temp is rising because he has not not ENOUGH coolant in the engine/radiator!
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

Thanks for the replies. I went up to my buddies late yesterday to have a look. We fired it up and within the first minute of driving (15 - 25mph) the guage started to rise. Less than 5 minutes it was over 200 degrees. Radiator was boiling over. I was assuming the pump but someone may have suggested the system and/or radiator could be clogged. What do you think. Thanks again in advance.

It should run at are near 200 degrees at all times rising slightly on hills or when idleing perhaps.
I think I would look for a blown headgasket.
Park the vehicle in a position so the radiator CAP is at the highest point of all the coolant.. Fill to the top and then start the engine LEAVE THE CAP OFF. Looking into the radiator keep it full to the top and watch for small bubbles while it is idleing. You may have to add coolant to keep it full. If their is a compression leak into the coolant it will appear as small bubbles. You will not be able to see these if the radiator is NEARLY full.. so keep it full to the cap and keep watching.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

Thanks Jim. This guage raised like a stop watch. I've never seen temp guage move that fast. If it were a blown head gasket, wouldn't there be some coolant in the oil. Thanks

No there would not be coolant in the oil! Sometimes there is but it is rare..
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />