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4.0L Jeep Six: Cylinder #1 Misfire Trouble Code


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My TJ has an occasional cylinder 1 misfire indicated by the check engine light and an occasional lurch at highway speed.  I have noticed the misfire when cruising at highway speed after driving about 20 minutes and feeling a sudden lurch, or when sitting at a red light idling.  The engine runs fine aside from the occasional lurch and running rough at idle.  I suspected a sticky lifter so I ran Sea Foam in the oil for about 2k miles and then changed the oil and filled with 10w-40.  I never noticed a misfire on the highway after this, but it idles rough at stop lights and the check engine light came back on at a stoplight.  It will probably turn off if I drive the highway a few times.  Wondering if there's an interim fix to get by another 6 months or year without getting stranded by this thing.  It's a 1998 with 250k miles on it, but I'm trying to put off replacing the engine due to the expense and I'm not sure it's worth it on a vehicle this "used".  Is there any chance an even heavier weight oil is a temporary fix?  Or is this engine shot and needs overhaul or replacement? 

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Bamafan1...The IAC should throw a malfunction code P0505 if any of these conditions exist:

  • IAC motor connector is damaged (check connector and pins for damage, and moisture)
  • One or more IAC circuits open or shorted to ground
  • One or more IAC circuits are shorted to voltage
  • IAC has failed
  • PCM has failed

The IAT will throw a P1192 or P1193 code to share that the circuit is reading either high voltage or low voltage.  Most sensors can be readily tested with a quality digital volt-ohmmeter for proper continuity and ohms resistance readings.  Since temperature is usually involved, tests should be run with the right simulated temperature on the sensor to expose an "open" that may occur only when the sensor is warm/hot.

 

I poked around on the internet to see how folks now approach troubleshooting.  Of course it's impressive how troubleshooting can be helped along by self-interrogating electronics systems like the PCM's ability to troubleshoot and throw codes.  However, I also have experienced PCM issues where the PCM cannot interrogate its own problems and either fails to throw codes or throws them rampantly and indiscriminately with no real substance.

 

Weighing the popularity of this #1 Cylinder Misfire topic, I'm going to do a very basic troubleshooting HD video (maybe even a short how-to series) on quickly confirming the fuel, spark and timing functions of a modern Jeep MPI engine.  There's a tendency to either overthink problems or limit troubleshooting to a dependency on codes thrown by the PCM

 

Most important is whether the engine runs right or not...I'll help here...This is basic.  For a Jeep 4x4 used in the back country, you should be able to troubleshoot quickly on the spot.  Seldom do several "devices" fail at one time, and pinpointing the crippling trouble area can often be done readily.  We'll save the nitpicky, nuisance engine check light codes for when we get home—the idea is to get home.

 

Moses

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Thank you for addressing the code question. However, if the engine is running and I unplug the IAC and/or IAT should the engine stall like it does when I unplug the MAP?

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Another subject I'd like an opinion on is mpg, I run 33x10.50x15 BFG MT's (on it when I bought it). Mostly in town driving as far as street use goes and I checked mpg ob last tank and averaging 12.6 which seems significantly lower than average of 15 mpg. Just wondered if "misfire" is contributing to fuel consumption too?

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Here's my take on your question, Bamafan1...At a stone steady idle, especially with a warm engine under no load, you may not notice the IAC function.  With the IAC plug disconnected, try setting the E-brake and place your transmission (manual?) in 2nd gear.  Keep your foot off the gas pedal and gradually let the clutch pedal up from the floor, just enough to create a load but not enough to stall the engine under normal conditions.

 

Normally, engine rpm will begin to drop then instantly attempt to stabilize toward the factory idle speed.  If the engine flatly lugs or dies, you'll get a sense for how a functioning IAC compensates for engine load to keep the idle speed stable.  (Normally, if you depress the clutch pedal when the engine has this load, the rpm will momentarily flare up.)  Obviously, if you let the peddle up too high with the brake set firmly, you will stall the engine.  We're talking about load, not a dead brake against the engine.

 

Unplugging the IAT of an idling engine, again not under load, would not make a noticeable difference—especially during cold warm-up where the CTS (coolant temp sensor) holds sway over the IAT function. 

 

Also, the IAT, even though it does have a role with injector pulse width settings by the PCM, is not as "important" as the O2 sensor.  If the upstream O2 sensor is defective, you'll always get an engine code and maybe limp mode at the same time!  (You'll get a code whether the up- or down-stream O2 sensor is defective, though the downstream should not throw the system into limp mode.)  The O2 sensor is the major player in air-fuel ratio settings and control of the injector fuel flow.  For emissions purposes, manufacturers are very concerned about the O2 function, and so is the EPA.

 

Again, if you suspect a sensor is defective, a quick ohms-resistance test of the sensor makes for quick troubleshooting.  We're trying to avoid unnecessary parts replacing...

 

Moses

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I haven't seen the MIL for a while now.  I am starting to wonder if it improved after I added coolant to the overflow bottle.  I noticed it was really low, so I filled it back to the full line.  Fortunately, it's been holding at the full line, so I don't seem to be consuming coolant.  And oddly, the MIL lamp has stayed away.  What's the likelihood that these two things are related?  Or is it just coincidence? 

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If the radiator was also low, this could be an issue, possibly creating air blocks that could affect the coolant temperature sensor. The engine would likely be overheating by that point, though—maybe not in ultra-cold weather.  This is a longshot in any case...I believe some of the earlier work that we discussed has actually helped.

 

Your initial topic post has now generated substantial traffic, this is one of the longest threads at the forums and very popular.  Thanks for hitting a nerve, we've aired quite a few issues within the context of this #1 Cylinder Misfire code.  Exchanges have been thorough and thoughtful, you started it all!

 

Moses

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