Need some help diagnosing a coolant leak in my wife's 2000 P38 4.6. A month or so ago we were driving around one afternoon, country roads and one relatively smooth gravel road. When we stopped for gas, we noticed a large puddle of coolant under the front end with heavy dripping from the thermostat / radiator area. The expansion tank had gone from full to about half full, so we topped it off and drove 30 minutes home. I assumed I would be able to crawl under and easily diagnose it at home, but when we got home and parked it, everything was dry. No leaking, and the expansion tank stayed full on the 30 minute drive home. Ever since then, I have been checking the coolant level regularly and adding coolant as needed. I haven't been able to find where the leak is coming from, and it only ever seems to drip slowly, and only on occasion. We took the Range Rover down the interstate 3.5 hours this weekend and then around town for an hour or so, and when I checked the expansion tank, it was empty. Filled it up, drove around, drove 3.5 hours home, level was still good when we arrived.

On the rare occasion that I can actually see it drip coolant, I can only locate it as near as somewhere by the passenger side of the radiator, or possibly just above the thermostat. It drips off of the radiator and the thermostat, but I never know if it has dripped down or been blown from somewhere else.

Where should I start?? I'll replace whatever I need to, but all new hoses is $400, a new radiator is $150-200, etc. etc. I would rather figure it out prior to replacing the entire system. Thanks!!!!

 

Does it have catch pan/panel beneath the engine that might steer you towards it leaking fwd of the engine? Regardless of where you see a leak doesn't rule out other areas that may be contributing to the coolant loss.

Have you replaced the reservoir cap? And carefully checked the reservoir for leaks?

Usually evidence of leaking is much easier to spot than a drip. Find those, do some cleaning and check again. I usually start on my back from below with a bright flashlight. Then from above with the same light and a inspection mirror. Water pump and it's weep hole a good place to check. Fwd and aft end of heads are where many external HG leaks are found. Take your time and check all hose connections and your radiator. Some leaks can be found when at temp and others may only drip at cooler temps or when cold. Removing the fan/clutch, belt and radiator cowling will give better access for doing a decent inspection. Make sure your cabin floor isn't wet from a leaky heater core.

If you can't find any leaks doing the above you can use a pressure tester that may help the find the cold leaks when under pressure. If no leaks found after that and it doesn't hold pressure pull your spark plugs and check for evidence of coolant in the cylinders. Good luck.
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