engine swapped now overheats

my 99 grand voyager has a junk yard engine swapped 300 miles ago, now it overheats, but only when driving. When idling the guage is in the middle, but drive 3 blocks, and the guage spikes & red light & alarm goes on. Stop & idle, and the guage returns to middle. No heat inside passenger compartment. Top radiator hose hot, bottom radiator hose cool. I tried a new thermostat, and no change. Before the engine swap, I never had an overheating problem. I think the old engine swallowed a valve at 115,000 miles. The van sat idle with no engine all winter. The junk yard engine sat in the yard on a pallet all winter also.
The mechanic who installed the engine mentioned some problems with hoses I think to the rear passenger heat, or maybe to the transmission, I'm not sure which. I cannot find a leak anywhere.

Talk to the mechanic and find out exactly what he said about hoses.

There are a couple possible explanations for your problem. First, air in the system will cause the symptoms you describe. You can bleed it yourself or have the mechanic do it for you. I am concerned about the hose issue as it could be bad hoses that are not allowing the water to circulate properly. Finally, you could have a bad thermostat or water pump although it should overheat when you are sitting with these problems. I would look at air in the system first! Good Luck!!!!

Thanks, I will ask what hoses he replaced, he claimed the existing hoses were leaking, it cost me another $150 for hoses from the dealership.

How do I bleed the air out of the cooling system?

If you have a radiator cap, leave the cap off and allow the engine to run until the thermostat opens, once it opens you should see a drop in the coolant if there is air in the system. Refill the system with coolant, watch the system to see if there are any additional drops and top it off again. If no drops, put the cap back on and put coolant in the overflow bottle. If there is any additional air it should vent itself. The other issue here is that you may have a blocked radiator. If the top hose is hot but the bottom hose is cool then the radiator may be plugged and will require backflushing or replacement. You can replace this yourself if you are mechanically inclined and you can do the hoses then. Good Luck!

I left the van in the parking lot behind my office in Southeast Washington D.C. because of the overheating, and it was stolen, and found abandoned in the ghetto, passenger door lock popped, and the ignition switch destroyed. I hope the bandits didn't ruin the engine. I'll get it back tomorrow, and take your advice re. bleeding the air, and/or repairing the radiator, that is, if the engine still runs.