Our '03 Infiniti I35 lost large amounts of coolant each time on 2 long trips but when we did a pressure check, we couldn’t get it to leak anywhere we could see and no visible signs of where it may have leaked while driving (no drip lines, puddles, etc.). We’re not sure where the water pump is located, but have been told it’s internal to the engine, so how can we tell if it’s gone bad? Would the coolant burn off with no visible signs? We changed the radiator cap but no change. We use a trusted shade tree mechanic, but he’s stumped.
If there is a head gasket failure, then the coolant can leak into the engine and be vaporized out the exhaust.
Cheapest, Easiest Things, First ?
How About That Radiator Cap, Eh ?
NEVER Remove A Radiator Cap On A Hot Radiator. Wait Until The Radiator Is Cold. Otherwise, You Can Be Burned.
In a 2/6/04 three-page Technical Service Bulletin ITB04-007 (with pictures and arrows), written to help Nissan/Infiniti technicians diagnose cooling system problems in 1999 and later vehicles, are tips for inspecting radiator caps.
The bulletin was issued because some caps were found to have “swollen” (enlarged) seals that can result in coolant loss and overheating because they don’t vent properly.
Basically, there are 2 things to check for and the cap should be replaced if either is found.
1 - The cap has two black rubber seals. Look at the one on the “plunger” (usually spring loaded). The rubber seal’s outside diameter should not be greater than the brass plunger disc that it sits on (swollen condition if it is) . You should be able to see a little of the brass plunger showing around the seal.
2- Deposits of waxy or foreign residue or material on the gasket or its metal retainer.
Always wipe out the filler neck before installing a radiator cap.
If in doubt, you could always purchase a new cap and try it. I’d recommend a Genuine Infinity part. The pressure requirement and size of the cap is very important.
CSA
They already changed the radiator cap:
"We changed the radiator cap but no change."
@98caddy, " They already changed the radiator cap: "
Oops ! Why didn’t you stop me before I went through all that work ?
What’s the matter with me ?
CSA
Water pumps can leak only when the engine is operating, through the seal aruond the shaft that turns the impellar, making them not always easy to verify. I believe on that engine the pump is located under a cover at the front of the engine and driven by the lower timing chain. Others will correct me if I’m wrong.
In addition, you could do a leakdown test for a possible blown headgasket.
Also, theres a flourescent dye that can be added to coolant to find leaks. Add the dye, run the engine, look with a blacklight and the leak will light up in front of you. It should be available at any parts store or over the internet.
@common sense answer - At least it was an informative write-up even if if didn’t apply to the situation at hand 
Was a high temp the reason you checked for coolant loss? I am wondering if high temp and pressure on the long drives are causing you to blow away coolant. Prime suspects would be radiator cap, done, sticking thermostat, or clogged radiator. A compression test can be done to check the head gasket.