Just got my jeep back from the shop this morning, they replaced the downstream O2 sensor and fixed some burnt out coils and spark plug boots, and flushed my coolant system. It ran like a champ all day, drove all over town with no issues. Temp rose a little higher in the stop and go traffic than I like but tolerable. Normally operates at 210 but rose to around 220 for a bit.

On the way home the check gauges light came on and the temp went up but I know it wasn't running hot. It went back to normal for a bit and did it again a couple more times. I'm getting 4 codes saying the voltage to the throttle position sensor is low (Just replaced it last week), a misfire in cylinder 5, an evap leak (large), and O2 sensor heater circuit.

Long story short, there's water vapor in my breather box and getting into the intake (Not separating?), the PCV valve and hose have sludge in them (Pretty sure needs replacing). I'm guessing the CCV probably needs replacing, maybe the coolant temperature sensor as well. The electric fan comes on at the right time and goes off when it should. Wondering if I should also replace the charcoal canister near the fuel tank and possibly the emission control canister under the hood too.

The water vapor issue happened prior to putting it in the shop, they said the head was fine and also checked my catalytic converter at my request and said it was also fine. Any ideas? This is driving me nuts.

Edit to add the codes:

P0161
P0455
P0305
P0122

Hoo boy. Okay. First things first. Go into Google and type this:

"site:cherokeeforum.com 0331 head crack"

Read some of the threads. 2001 + running hot = very bad. You need to know what's going on with your engine before putting more effort into it. Judging by the clogged CCV lines it probably has not been maintained...

Your codes suggest an electrical fault in the wiring harness. I'd start there by inspecting wiring before spending money on sensors.

After you figure out if you've got a cracked head...

P0161 is the heater circuit for your downstream O2 sensor. If they actually replaced it, you may have some chafed or broken wiring. The first place to start with that is to climb up under there and inspect your wiring to see if you can see some damage. The "generic" sensors are actually longer than stock and can smash up against the underside of the Jeep. If they used one of those, they may have kinked a wire.

P0455 is a failure in your evap system. A bad gas cap can actually cause this, but you probably have a cracked hose. You can take it to a shop and have a smoke test done to find the leak.

P0305 is a misfire in cylinder #5. Did you have your plugs replaced when you had the coil rail replaced? FYI, your Jeep has a coil rail that has the spark plug boots built into it, and it has to be replaced as a unit. You might want to check your #5 plug and also check to make sure that the boot in the new coil rail is actually seated on the plug properly.

P0122 is throttle position sensor. If you already replaced your TPS, there's something else going on. Did you clean your throttle body?

Regarding your temperature spikes, that could be a bad sensor. It could be air trapped in your coolant system that hasn't worked its way out yet.

But it could also be an indicator that you have a head crack that is opening intermittently and venting exhaust gas that is boiling your coolant. Before you had it flushed, was the system full of reddish brown mud? Those temp spikes were one of the symptoms of my cracked 0331 head.

There are several things you can do to test for a cracked head or leaking head gasket. One is to get a pressure test. You should also test for the presence of exhaust gases in your coolant. You can buy the test kit yourself from Amazon for about $50, but a lot of shops have them and will do the test for you. You can also send off an oil sample to Blackstone to test for the presence of water in your oil. Given the temperature spikes on a 2001, I would most definitely test for exhaust gas in the coolant at a minimum.

You may not have a head problem. But I really wish I had known about the 0331 head issues a lot sooner and had been able to diagnose it earlier.

Thanks for they input, I've been messing around with her since this morning and found some of the problems. I'm still getting code P0122 but the others have not returned.

When I started checking vacuum hoses this morning, I noticed the two that connect to the Vapor Canister Purge Solenoid weren't even connected. I may have done that by accident when replacing the spark plugs last week. It wasn't long after that the water vapor in my air filter box started happening. Thinking that may have addressed my evap leak (large) issue too.

Both hose assemblies and the PCV Valve needed replacing as well but I had to order those. The CCV Valve and grommet were in stock so I replaced them immediately. I also replaced the coolant temperature sensor this afternoon. On test driving, the temp did go up a little but the fan kicked on and lowered it as normal. I'm still not comfortable with that though.

Hoping things will straighten out once I've replaced the faulty PCV and hose assemblies but if not, I'm looking at possibly the MAP sensor or PCM. The Vapor Canister Purge Solenoid seemed to be working but if not, it's a lot cheaper to replace. That's where I'm at right now. Thanks for the replies thus far.

Oh and also disconnected was the vacuum line going to the reservoir, which I reconnected and could be where the air was coming from in my coolant system.

I'm with Salad: check for a cracked 0331 head. Google up the inspection method and let us know what you find.

These 0331 heads are just poison.

Yes eliminate the head issue first off before spending more money, you need to know if it has a problem.

The most obvious and most consistent clue for a cracked cylinder head is unexplained, e.g., no external leaks, etc., loss of coolant from the reservoir bottle.

When my head gasket was leaking it was leaking around the cylinder so when the piston would come up the pressure from it would push all of my coolant out of the engine into the overflow tank and out the overflow tank. Also because it was leaking at the cylinder it was over pressurizing my coolant system and caused two radiator hoses to bust after I parked it.


It is very important to try not and over heat the I6. Heat is its worst enemy.
Also check your oil to see if you see coolant in the oil. also if you do have a head gasket leak I would go ahead and replace your water pump and thermostat because its all ready apart and that's all part of your cooling system.

That's true. If the head gasket is blown between cylinders and into the water jacket, you will get an overheat condition and coolant blowing out way before the engine itself would get hot enough to do that if, say, your water pump was weak or bad.

Would you believe it was a bad radiator cap all along?

A radiator cap can't control engine temperature.

Sure it can. If a pressurized cooling system can't maintain pressure the cooling efficiency is reduced. Same thing can happen with a trapped air bubble .

You are discussing pressure, I stated temperature, dif.

You have only delayed the inevitable.