Searched around and can't find an answer. I just got my 2000 F150 4.6L back from having the engine replaced a few weeks ago. Ever since the day I drove it home I've noticed the temp gauge will get almost all the way to red, then back down. It doesn't do it on short trips, like my drive to work (4 miles) and does it more/faster with the A/C on. It doesn't overheat, and the temp will generally come back down to around normal, but it always climbs back up, usually going back and forth very slowly. Also, once it gets hot the A/C kicks off (my guess is the compressor kicks off as the fan still runs). The first thing I did was change the thermostat, but it doesn't seem to have helped. The radiator is brand new (replaced along with the engine), and I don't think it's the fan as the truck doesn't get hot at all when sitting in traffic, only when moving (i.e. foot on the gas pedal). I'm thinking it's the water pump (used engine). Also, I have no heat. I replaced the heater core before the engine swap and the heat would come and go, but I think that was due to the fact that the coolant was leaking into the oil (reason for the engine swap). What I think is happening is the water pump isn't putting out enough pressure to fill the heater core and also to properly circulate coolant once it gets hot. Any ideas?

 

the only water pump failures I've seen have been due to the blades rotting apart. Modular engine got a pretty good water pump.. Could also be you have an air pocket theres a way of burping but what I've done is just run the motor with the cap off with the front of the engine raised up like on ramps or jackstands. thats worked for me..

 

Thanks for the reply. I thought about an air pocket, but kind of thought that it would have worked itself out by now. I haven't tried burping the engine yet, but I could give it a shot when I get a chance.

 

when it gets warm, can you hear the fan roaring loudly?

I'd start with burping it, and it very well could be a fan clutch as well. But the symptoms you're having are backwords from what they should be with a weak clutch.

If those don't solve your issues, its worth having someone check for exhaust gasses in the coolant system, you could have another engine with another blown headgasket. They make a special tester for this purpose.

 

You have an air bubble circulating through the system. It could be from a blown head gasket but unlikely with a new engine. More likely its like patman and jethat have discribed ands since the system has just been refilled because of the new engine and radiator I would suspect the system needs burping. The air bubble gets very hot and when it gets to the temp sender it spikes the sender, the bubble moves and the temp gauge drops. With these cooling systems it's not uncommon for this senario to happen but get it burped soon. If by chance burping doesn't solve it then by all means get a hydrocarbon test performed or rent the kit and perform it yourself before serious damage to the engine occurs. Good luck

EDIT:
I should have said that with these systems it's not uncommon to get air in it IF the coolant has been drained for any reason and the system refilled. It can take a little time to work the air all out sometimes.

 

Still haven't had a chance to burp this thing. It really seems like having the A/C on has some effect on it. Drove to work and normally don't use the A/C when I'm driving alone, but it's unbearably hot so I ran it. The temp went up to normal, paused, then continued to climb. Normally it doesn't get past normal on my way to work, but the the air on it did. As soon as I stopped at a red light the temp came back down to normal, then when I hit the gas it slowly started to rise. I always thought with an air pocket it was a bit more erratic in the fluctuations. I could be mistaken. I don't think it's a head gasket, but I'm also hoping it's not. I'll try to burp it this weekend and are if it helps.

 

thats odd. Cools down at a light. strange. Sure you have the heater core hoses on the right way? every car I've ever had that had cooling issues it got worse at a light cooled down while moving. New radiator new thermostate.. odd deal you got going on there. open the overflow with the front of the truck lifted to let the air rise forward try that..

 

headgaskets are more likely to leak combustion under load. and normal cooling demands will climb with RPMs

I worked on a 2000 MY Discovery that ran all day long at normal temp in my stall, but running it around the block within a mile it would be puking coolant everywhere. It had a warped block/heads

 

Hmm - that DOES sound like an eaten up water pump.

 

Definitely not puking coolant. Levels seem perfect. It is very odd. That's why I think it could be the water pump. It's pushing enough coolant to stay cool at idle RPMs, but at cruising RPMs it's not able to keep up with the engine heat climbing. It's not overheating at all, just climbs and falls. The first day I got it back I drove it on the highway at 75 mph constantly for about 25 miles and it would heat up and cool down, never overheating. Seems if it were pushing combustion gasses into the coolant the problem would get worse and worse, eventually overheating. It's not getting any worse, but not any better either. Also, not sure if I mentioned this, but the truck gets to operating temps normally. Not to quick and not too slow.

 

Is your degas bottle cap holding 16 psi? Another thought that can drive you crazy is a radiator hose that collapses at higher RPM restricting coolant flow.These water pumps can lose one or more impeller fins and lose efficency as well as others have stated. Also check the radiator hose for any bends or kinks that may have happened when the radiator was installed.

 

Radiator hoses look good. Haven't checked too see if they are collapsing, but I don't think they are. They seem to be in great shape, but that doesn't mean anything. They are the original hoses, so at 12 years old they may be no good. Still haven't had a chance to attempt to burp. I've got to find a place where I can incline the truck and also not disturb too many people. One of the tailpipes rusted off the muffler and I haven't had a chance to get it welded back on, so it's a little loud. Plan on going fishing today, so I might take it up one of the back roads near the lake and give it a shot.

 

Speaking of water pumps losing find, I replaced the water pump on the old engine when it was giving me trouble and one of the fins had broken completely off. It wasn't rusted or damaged in any way, other than a fin completely missing. Have no idea what happened.

 

That was supposed to be fins, not find. Stupid iPhone.

 

Also, I'm not sure how to check the degas bottle cap. I know it holds pressure, as I've taken it off with the truck warm (not hot) and it will release some pressure.