2001 Montero Sport ES 3.0 Prod date 11/2000 139k miles

I’d been battling cooling issues for a while. Radiator, radiator and tranny cooler hoses, thermostat, belts, have been replaced, system bled mid-winter. Stopped watching the temp gauge after weeks of all good. Recently overheated again. Topped up coolant and drove 5 miles home. Two problems since.

1- Heavy coolant leak behind the motor. Streams run down from tranny case when rad cap is tight; drips when loose.
2- Heavy misses and stalls after coming up to normal temp. Won’t restart til it cools. OBD P0301, P0134 codes apparently point to Right Upstream O2 sensor. Got the part, can’t get the old one off. Looking for 7/8” crowfoot. Wondering if that could get it off w/o a sharp whack to break it loose. No room to swing a hammer or get leverage. Dipstick looks OK. No evidence of coolant in the oil.

My searches aren't finding anything to get me past this point. Any information would be greatly appreciated.

 

Check the heater hoses running to the throttle body, one of mine was loose and would do the same thing. It sprayed coolant just right onto the driver side of the transmission and poured onto the ground. Another possibility is that one of the O-Rings in the crossover pipe that runs underneath the intake manifold has developed a leak which fills up the valley of the engine and then pours out the back. The O-Ring problem is fairly common after the water pump has been replaced because most people don't think about them.

From what I know, a faulty O/2 sensor in rare circumstances can cause the engine to stall once it's warmed up but so can a faulty temperature sensor. A faulty temp sending unit can trigger the P0134 code, I've seen it happen a couple times on multiple different makes. I'd also look into vacuum leaks, this can also trick the ecm into thinking the O/2 sensor is faulty in the same way the temp sending unit could.

If you're wanting to replace the O/2 sensor, make sure to soak it in bolt loosener like P/B Blaster for several hours before attempting to remove it. Also parts stores will have a. oxygen sensor wrench kit you can rent from them and you'll get your money back when you return it. Another trick is to get a cheater bar which is a piece of metal tube/pipe and put it over the end of wrench. Further helping this process is to warm up the O/2 sensor and surrounding exhaust pipe with a heat gun prior to removing, this will also heat up the P/B blaster which will cause it smoke and smells horrible but it shouldn't catch fire unless you get it too hot.

 

Get a good flashlight and a small mirror. Use them to examine the engine valley, all little hoses to TB and the back of the engine. There is a crossover pipe on the back of the heads. Some people reported it getting corroded or develop small cracks. There are two o-rings in the back at pipe connections. You could have a loose bolt. Between the head and crossover pipe too. It might be helpful to get engine warmed up and while it's running, get under the car to watch the back of engine (above transmission, in front of the firewall).

 

Thank you Naeos_Valkarian and HunterD for responding. I have a guy coming tomorrow to pull the plenum. Hopefully the problems will become clear when we can see them. Regarding the stalling problem, codes have been cleared. It seems fine idling long after temp gauge hits its normal spot. Short test drive only lasted a few hundred yards.
Naeos_Valkarian- Check Engine light is still dark so the O2 sensor can be ruled out. I've not known a vacuum leak to happen only when hot. Is that possible?