Tony the Liger
NAXJA Forum User
- Location
- USA
A quick change in temps is virtually impossible on an engine due to the shear thermal mass. What is possible is a quick change in temp of the sensor caused by air in the system. When the sensor is covered with coolant, all is fine, but get an air bubble and presto the sensor reads way hot.
Now comes the question, why is there an air bubble? Have you replaced any hoses or the like where you drained coolant? If not, you may have a head gasket leak or a cracked head letting exhaust gas into the coolant. The first quick and dirty step is to let the engine cool and then pull the radiator cap. Start the engine and allow it to warm up. Do you see bubbles in the radiator? You will see the level surge as the thermostat opens, but look for a bunch of bubbles.
There is a kit that allows you to "sniff" the radiator coolant for hydrocarbons to detect a leak of this type.
Yeah, I mentioned that above. When replacing the valve cover gasket, we disconnected and moved some hoses, so when I added coolant I did so through the hose in order to flush out any air. It stayed cool when I drove it after that, but I didn't drive far or long, nor have I driven it since.
My concern is with the coolant that was lost prior to it overheating: was it due to a leak, or a head crack? The oil isn't milky/frothy, best I can tell; I haven't seen any steam/white smoke coming from the exhaust, but I haven't viewed it from behind while it's been on the road, either - only while idling.
I haven't driven it since that first drive after topping off the coolant. Today I noticed a couple new (dried) wet spots on the cardboard I placed under the XJ, so there's definitely a leak, albeit a slow one. I also noticed one green drip on the bottom of the oil pan that was ready to drop. Anyone care to take a guess as to how it would've gotten there?
Not gonna get my hopes up, but I want to take that as a good sign. Coolant seeping through a crack in the head would either pass through the cylinders and into the exhaust, or mix with the oil in the motor, correct? Is there a way that coolant from a head crack could wind up on the outside and bottom of the oil pan?