Problems with 2004 Jeep Grand Cherokee I-6

[Roca Fella Bryan]

Okay, so my dad has a 2004 Jeep Grand Cherokee with the straight six-cylinder with roughly ~115,000 miles on it. Recently we've noticed that it's been running very rough at idle sometimes and eventually it threw a CEL after a misfire episode following a warm start.

The codes are as follows:

P0136 (refers to the second oxygen sensor on Bank 1)
P0051 (HO2S Heater Control Circuit Low Bank 2 Sensor 1)
P0031 (Oxygen Sensor Heater Control Circuit Low Bank 1 Sensor 1)


Now, while this may indicate that the sensors have totally failed, I find it very hard to believe that all three of these sensors can suddenly fail at once, leaving only one properly functioning according to the computer. Also, the event that initially trigged the code was a time where we started the car (after shutting it off to go into a store for about five minutes) and it had a misfire that lasted for roughly 10-15 seconds. It went away after the CEL illuminated and after giving it a little gas. It does this misfiring thing every once in a while, but there is no definite repeatability with that. You could fire up the car ten times and it'll maybe do the whole 15 second misfire thing two times out of the ten at ramdom.


Summary:
- Basically, we were going to just start replacing the sensors, but we've heard from multiple sources that something else might be triggering the problem. We have installed new ignition coilpacks and spark plugs that are properly gapped on the car and it still occasionally does the misfire problem. The CEL stays illuminated even after clearing the code.
- To the Jeep guys on here, what the hell could be the problem here? Could one of the injectors be gummed up or could one sensor fail and trip the codes on the others? Isn't it quite uncommon for three out four of these sensors to just suddenly decide to go out at once?

Let me know, and any input is greatly appreciated. As normal, we don't want to start buying unecessary parts when one thing could be the problem.

--RFB

[shdw on th sun]

It's a jeep

[MR2Don]

I think its the #2 hot side 02. I replaced mine in my jeep after similar codes and it runs great now.

[Kisor]

Let me look up some stuffs, heading to church now. They are funny about vacuum leaks being speed density they don't flag lean codes like a mass air car will. Doubt that's what it is but ill see what i can find.

[Roca Fella Bryan]

Thanks guys. I was told to check for vacuum leaks. All of the vacuum lines look fine to us. My dad is removing the o2 sensors now and is going to clean them.

[sxShep]

Originally Posted by Roca Fella Bryan

Thanks guys. I was told to check for vacuum leaks. All of the vacuum lines look fine to us. My dad is removing the o2 sensors now and is going to clean them.

Ask him to change your oil while he's at it... and maybe front you this week's allowance so you can get that new Xbox game you've been bugging him about.

just fuckin whichya

[Kisor]

Never heard of cleaning an o2 sensor. If they are heater circuit codes they are either bad it you have wiring issues. Look at where they are routed be the manifold. I've seen more than one burnt on jeeps there.

So the whole coil assembly was replaced? I'd need to get it and look at mode 6 data to determine the misfire.

[Roca Fella Bryan]

The entire coil assembly was replaced as well as the spark plugs.

[Roca Fella Bryan]

Originally Posted by Kisor

Never heard of cleaning an o2 sensor. If they are heater circuit codes they are either bad it you have wiring issues. Look at where they are routed be the manifold. I've seen more than one burnt on jeeps there.

So the whole coil assembly was replaced? I'd need to get it and look at mode 6 data to determine the misfire.

What is "mode 6"? A data logger?

[Grand National Propain]

Its generally just the sensor on Cherokees. Replace it and see if it goes away. If not, then spend the time to check all the B.S.

[Kisor]

Mode 6 is the factory monitors in all obd2 systems. It logs data for code setting and failure records. Most code setting requires 2 drive cycles. Sometimes a car will have an issue like a miss or something intermittent that won't show a code but you can see the data in mod 6. It's a sneak peek into what's going on without a code having to set.

http://www.jeepforum.com/forum/f310/p0031-p0051-p0136-1049002/

Check this too

[Kisor]

Looks like a common problem is the wire loom at the back of the valve cover rubs through and shorts wires out. Check the fuse for the O2 heater circuit, I think #17 in the engine compartment. Should say on the lid. Also look at the wiring under the manifold for melting.

And no offense GNP, but to throw $100 worth of O2 sensors that may not be bad at it seems a little shotgun diaging. I don't think the heaters would go out on all three at the same time. I could be wrong, but not usually. lol

[Grand National Propain]

Originally Posted by Kisor

Looks like a common problem is the wire loom at the back of the valve cover rubs through and shorts wires out. Check the fuse for the O2 heater circuit, I think #17 in the engine compartment. Should say on the lid. Also look at the wiring under the manifold for melting.

And no offense GNP, but to throw $100 worth of O2 sensors that may not be bad at it seems a little shotgun diaging. I don't think the heaters would go out on all three at the same time. I could be wrong, but not usually. lol

I misread his description of the codes, I thought it was just one. I always verify with a data scanner before replacement. You're right, it'd be highly unlikely to have 3 bad at once.

I usually pride myself on fixing it right the first time and not having comebacks, but its easy to get in the habit of just throwing on an o2 when you see a heater circuit malf. Code.

Also, good call on the loom. I'd never heard of that before till you mentioned it and I looked it up on Mitchell on demand.

[evil8]

Check for rice in the gas tank.

[Roca Fella Bryan]

Originally Posted by Kisor

Looks like a common problem is the wire loom at the back of the valve cover rubs through and shorts wires out. Check the fuse for the O2 heater circuit, I think #17 in the engine compartment. Should say on the lid. Also look at the wiring under the manifold for melting.

And no offense GNP, but to throw $100 worth of O2 sensors that may not be bad at it seems a little shotgun diaging. I don't think the heaters would go out on all three at the same time. I could be wrong, but not usually. lol

Okay, so we're working on it again right now. I checked in the fuse box and the 15A fuse for the o2 sensors is definitely blown. Also, the wiring harness was wedged between the valve cover and firewall pretty damn tight to the point that we had to pry it out. Once it was out, I don't see any obvious abrasions or tears in the wiring harness that could cause a short.

We'll check all of the plugs going to the sensors too, but on a peculiar note, the fuse box itself calls for a 10A fuse and there is a 15A in that slot. I'm wondering if the previous owner had problems with it before and replaced the fuse and it just blew again.

[Kisor]

Looks like a couple others have mentioned larger fuses in place too. I'd pull the loom back and check it out really good back there.

[Roca Fella Bryan]

Checked the harness, found a few cracks in the insulating wrap, but still didn't see any evidence of open wires. Either way, I re-wrapped them and we put the car back together with the new fuse. No CEL and it appears to run fine. I'm going to drive it around for a while and see what happens.


Thank you guys! I appreciate the advice and help!

[bestracing]

Not connected with your O2 problem but If you continue to have a miss-fire problem in just one cylinder, take a look at the connector that goes into the coil. A friend had a similar problem and he swapped coils, plugs and the same cylinder kept showing that it had a miss fire. Turns out that the metal portion in the connector had slid back into the connector housing and was making intermittant connection.

He said that other people on a message board he's on that have similar I6 motors were having the same problem