What tells my 2000 XJ to go from open loop to closed loop? I have been chasing down an odd and expensive misfire in my cherokee and had a buddy with a fancy scanner tell me that what ever is telling ( or not telling ) my jeep to switch loops is my problem. Thanks

 

A functioning O2 sensor(s) is what "closes" the loop. The PCM (computer) is always in charge/controlling the motor.....open or closed loop.

Picture a triangle......on one point is the motor and it's various sensors. On the second point is the PCM (the computer). On the third point is the O2 sensor(s). In open loop, the motor's sensors and PCM are talking back and forth to each other and the engine is running based upon pre-set programs in the PCM. The "loop" closes when the O2 sensor(s) starts signaling the PCM, then the PCM is able to fine tune the info sent to the motor based upon info provided by the O2 and sent to the PCM.

The O2 sensor (in the exhaust) must reach an operating temp of +/-650F in order to start signaling the PCM (loop closes). Heated O2's can reach operating temp within seconds of cold motor start-up, thus the loop can go from open to closed loop within seconds of cold motor start-up.

Numerous operating conditions (WOT for example) will cause the system to momentarily re-enter open loop and then return to closed loop under normal throttle/driving conditions.

 

Well I keep getting this crazy Cyl 4 misfire and it seam like its happening around when it goes from open to closed.... I replaced just about every sensor I can think of both upsteam o2's, cam, crank sensors, TPS, coolant what others control it? also replaced everything else that deals with spark and fuel and checked just about everything else

 

On a single cylinder misfire, where you never see ANY other cylinders misfire, you must focus on everything that is related to that particular cylinder.

Plug, plug wire, distributor cap, fuel injector and wiring, internal condition of that cylinder and the hardware on the head for that cylinder.

This is not likely a "common" component problem.

 

sounds like a bad/dirty valve, Try running some seafoam through it and see if it helps. If it helps then it is highly possible that a valve just needs cleaned.

Also you should check your temps sensors, that could definetialy cause the jeep to go in and out of loops. If the PCM sees the engine is cold then it will think it is just started and will confuse itself.

 

I did replace the coolant temp sensor and on the scanner the IAT has good readings. I ran seafoam twice though it and plan on going to a jeep dealer to get some good stuff that they recommend. Jeep has 144k on it but found out it has a 2005 jasper replacment engine in it, so I would think it would have under 100k on it and less of a carbon build up

 

replaced or swapped all things that deal with just that cylinder, and did a compression test with it at 150 psi

 

0331 Head Plague, Me Thinks!

 

It could be. Have you replaced the head or had any coolant consumption?

 

The NLEV 4.0L's do have a slight problem with gumming up from excess blow-by and just the fact that they are trying to make a super inefficient engine get good emissions and not heat the world up.

My 2001 gummed up baddly after 12k miles of me driving it, but I think it was an abused city jeep in it early life...

 

no coolant lose or consumption and a compression test had good results

 

No mention of checking under the valve cover.Check the casting # on the head.
Read the plug out of cyl #4. White, Black or Tan. Do you smell gas or coolant when you remove the plug.
It's only happening on one cylinder, so trouble shoot the one cylinder.
Open or Closed Loop shouldn't make any difference for just one cylinder.
I think valves or head.

 

I'm leaning that way as well......

 

Closed loop can be reached within seconds of motor start-up or it can be a minute or so after start-up.....kinda depends on a number of variables. A misfire is either a momentary no spark at the plug (for several reasons) or an momentary insufficient injector squirt of fuel (for whatever reason).

Again, closed loop" is simply the O2 sensor joining in with the PCM and motor/other sensors. I wouldn't think closed loop has anything to do with a specific cylinder misfire (no spark/no fuel).

Is it always cyl #4? May want to revisit post #4. I think I remember tj suggesting a while back that if all of the ignition electrical is good (but still getting a misfire at #4), try switching injectors #3 and #4 or #4 and #5 to see if the misfire moves to #3 or #5. If the misfire moves, it's probably an injector, if it doesn't move, probably something causing no spark at #4.

Hope I didn't mis-quote tj.

 

Good call djb - for some reason I thought that had already been done.