JBwithaJC

I recently purchased a 99 XJ with 163k miles on it.

1 week ago it would not start, error code read P0340 (bad camshaft position sensor). This was replaced and the jeep now starts like a dream. However it now overheats. I don't think these problems are related I just figured since my history with the vehicle is so short I may as well include that. The way it overheated was while driving - temp gauge slowly and slightly went up over 210 and then when I came to a stop it was in the red instantaneously. Pulled over and allowed engine to cool, limped it home with the same method. Found a small plug of orange/red material (looked almost like fur and gravel) clogging the hose from radiator to coolant overflow tank. Today I am performing a coolant system flush. I disconnected hose that leads from water pump to radiator and allowed that to drain into bucket. I used a garden hose to spray water from the radiator cap and into the radiator from where the thermostat housing hose connects; these both drained very well out of the down hose with more bits of orange gunk. I sprayed backwards into the heater core and allowed it to flush until the water ran clean. I then sprayed through the hose that runs from heater core to water pump until water ran clear. Everything was reattached and I filled the radiator and overflow tank with distilled water. I allowed engine to run until up to temp (with heat on) and then shut off. I drained the water and am now in the process of filling with antifreeze and burping the system. I am unable to run the engine without the radiator cap off - coolant will come erupting out of the radiator cap. If I place the cap on and run the engine until it's up to temp and then shut off I have to wait until engine is completely cooled before removing radiator cap, otherwise coolant will come bubbling out. I believe this to be abnormal but am unsure. I definitely have not gotten all of the air out of the system yet. The manual states that the coolant system can hold 11.4 L and I'm only at about 7 L so far. I think this is all the info I have to cover. Any help would be very appreciated.

Other notes - the prev owner stated that the water pump was new (it does appear to be) but I don't know how new; the radiator was replaced 1 month ago. Both fans in the front are fairly new and working great. The serpentine belt is not worn and it is in proper place. Thank you for reading all of this.

tjwalker

It is possible that the previous owner put some type of "stop leak" product into the cooling system of your XJ. That small plug of orange/red material clogging the radiator to the coolant reservoir is suspicious.

If after you finish burping the system, it still overheats then you may want to perform a compression test and or perform a block test, where you sniff the coolant for the presence of hydrocarbons. The compression spec for the 4.0 liter engine is 120-150 psi, with no more than a 30 psi variation between cylinders. Along with being a good diagnostic, the compression test is a good snapshot in time of the internal condition of your engine and is pretty easy to do on your engine. Many parts stores will rent you this tool inexpensively if you don't have one or access to one, call around for availability.

I don't see a connection to the P0340 code which you resolved with a new camshaft position sensor. Good luck and keep us updated!

cruiser54

also a very good possibility the radiator is clogged up. Can you see a factory tag on it with a part number? If it's original and has had stop leak put in it, it's time for a new one along with a new Mopar thermostat.

JBwithaJC

Thanks for the advice from both of you and the direction about the compression test TJ. The coolant flush has worked so far. Previously I couldn't make it 4 miles without overheating and I did 56 yesterday. I forgot to mention that I had already replaced the thermostat before trying the coolant flush. It's not a mopar one but it's 195. The radiator had been replaced by the prev owner last month. A friend of mine brought up that maybe he used some stop leak stuff too. So far so good though with the coolant flush. I'm definitely going to need to utilize this forum again though, the vehicle definitely needs more work.

awg

this simple test will be very indicative of a head gasket leak

remove rad cap and start vehicle, allow to idle to operating temp

if needed add coolant or water so it is visble in rad neck

Once engine thermostat is fully open, there should not be bubbles of gas apparent in the coolant visible in rad neck

this is not 100% definitive, but close, as if head gasket is failing, exhaust gas in coolant may result (cause overheat)

The other almost definitive test is salad cream inside oil filler cap

both of these are easier than a compression test, which would be next

Roler

Buy yourself an exhaust leak test kit online.
And if it turns from blue to yellow like here, you'll know.