coolant. My indy shop doesn't think that it's the head gasket; although they cannot find the leak, anywhere. He talked to a mechanic at the dealership and the mechanic mentioned something about a coolant jacket modification...has anyone experienced a loss of coolant? What was the repair? Her truck loses the reservoir within 3 weeks. I guess, I’ll end up taking it to the dealership, but any insight would be appreciated.

It's the water pump.

I had a similar problem on my 550. It was losing enough to trigger the "add coolant" light every 3-4 weeks which required adding a few cups of coolant to fix the problem but there were no obvious external leaks or coolant under the vehicle. I got the "couldn't replicate the problem" response the first couple of times I told the dealer about the problem. After I mentioned that I had almost used up a gallon of MB coolant (plus equal amount of distilled water) refilling it, they finally fixed it. I wasn't footing the bill, so I didn't get as detailed an explanation as I would have otherwise, but it sounded like it was a problem with porosity of the head from the explanation the service writer gave me. I think he said the fix was to block off one of the cooling passages in the head, but I'm not sure.

It was a pretty involved fix since the heads had to come off, but fortunately I was still under an extended warranty so I'm not sure what the cost was. Good luck.

The car isn't CPO, but I have an aftermarket warranty for the car...guess I'll see how good of a warranty it is. In regard to the water pump; nothing is wet up front...it must be internal.

Check the front of the engine after driving for a while. Check if you can see any flid come out or any dampness. If not, it is probably what they are telling you and it is leaking into a cylinder.

I would try adding UV leak detection dye which you can purchase at any auto parts store.

The indy shop ran UV leak detection dye through the engine; they didn't find anything external.

Is there any sign of coolant on the dip stick?