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post #1 of 60 Old 03-13-2014, 02:56 PM Thread Starter
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Misfire Diagnosis Guide

I've recently notice a lot of 4.0 misfire threads popping up and I find myself linking a thread I made a few years ago about my misfire issue and the result. Since there are many possible causes to a 4.0 misfire, I figured I'd compile everything here in one thread. While some of these are geared towards the 4.0, this guide can viewed as a 'rule-of-thumb' for the 4.7 and really any internal combustion engine for that matter.

If you have any suggestions on something that I've missed (or made a mistake on), please comment below.

Also, I wanted to mention before we even start that if any sensor is replaced, be sure to use genuine Mopar parts. WJs are very specific about their sensors. If you replace a sensor with a dollar store knock-off, you've really done yourself no good in resolving a misfire issue.


Your very first step should be to pull the codes. To do so, turn your key to the On position (without starting), Off, On, Off, On. This will read the codes and display them in your odometer window. When you see donE, there are no more codes to list. If it didn't work, try again, this time putting the key in the ignition. If codes are present, you're possibly one step further in diagnosing the problem.

The following list is ordered by cheapest fix and most likely problem. Hopefully, your issue is resolved before you make your way to the bottom!


Unplug Battery/Learn

When working with anything electrical, always unhook your battery (after checking for any codes first!). Not only will this save you from possibly shorting something out, it will clear the system. It may take the computer some time to relearn the sensors but sometimes, this is necessary to clear out some possible faults.


Idle Air Controller

This can cause issues with rough idle and possible misfires. While the IAC can be replaced, typically a good cleaning will clear up some issues. To do so, disconnect the battery (the IAC will need to 'relearn' when you've finished cleaning), removed the throttle body and the IAC and scrub off the carbon build up on both the throttle body and the IAC. Reinstall both and reconnect the battery.


Throttle Body Cleaning

See Idle Air Controller.


Vacuum Leak

Vacuum leaks ultimately add more unregistered air to the combustion chamber in the engine. Since this extra air is not monitored, the computer does not know to add extra fuel. Therefore, a lean condition is created. If the air/fuel ratio leans out too much, a misfire situation can occur (usually in multiple cylinders). Inspect all vacuum hoses for wear and replace if questionable. A trick I've learned is to spray some carburetor cleaner around the engine bay while the engine is running and note any fluctuations in idle. Just be careful to not set your jeep on fire!


Wiring Harness

More specific to the 4.0, check the wiring harness on the rear drivers side of the engine that wraps around the valve cover. It is located in such a way that it tends to rub on a stud and wears through the wiring. If this is the case, repair the wiring. It is also recommended to slip some vacuum hose over the stud to prevent future wear.


Bad Fuel

Clearly, bad fuel/incorrect fuel can cause running issues as well as misfires. If you suspect your fuel may be the issue, either add a fuel stabilizer to the tank or drain the current tank of gas and add new, good gas.


Fuel Pressure

Inadequate fuel pressure can cause a low supply of fuel to the cylinders. Test the pressure by hooking a gauge to the Schrader valve on the fuel rail. If the pressure is low, verify your fuel pump is working and consider replacing your fuel filter/regulator.


Spark Plugs

Are they old? Replace them. They aren't very expensive and unless you're current on maintenance, they probably need replaced anyway. Be sure to gap them properly.

4.0
Champion RC12ECC (P/N 438). Also rumored from the TJ guys, Champion RC12PEPB5 (P/N 7034).
Gap: 0.035

4.7
Champion RC12MCC4 (P/N 439)
Gap: 0.040

4.7 H.O.
Champion RC7PYCB4 (P/N 3340)
Gap: 0.040


Injector(s)

If you have a specific misfire code, switch that cylinder's injector with an injector from a cylinder that is not misfiring. If the misfire persists at the same cylinder, the injector is good. If the misfire follows the injector, it's bad. Cleaning/replacing the injector should solve the issue.


Coil

On the 4.7, swap the coil pack with a cylinder that is not misfiring. If the misfire follows the coil pack, the pack is bad.

On the 4.0, replace the coil rail and see if this resolves the issue.


Throttle Position Sensor

The TPS can cause rough idle and possible misfire issues. Replace the TPS with an OEM unit and see if this resolves the issue. Unfortunately, the TPS is almost a maintenance item.


EGR

An issue with the EGR can cause a misfire in that a stuck EGR valve can act as a vacuum leak. If your EGR valve no longer holds vacuum, it should be replaced.


PCV

Similar to the EGR, a PCV that isn't properly functioning can cause a vacuum leak. Be sure the vacuum hose(s) and elbows to the PVC valve(s) are not split, the PCV valve itself is not cracked or broken, and the valve is the proper size. If in doubt, replace the valve(s)/elbow(s)/hose(s). They aren't very expensive.


Injector Heat Soak - TSB 1803103: http://www.wjjeeps.com/tsb/tsb_wj_1803103.pdf

This TSB describes an issue with the fuel vaporizing in injector #3 after the vehicle has been warmed up, turned off, then restarted which results in a misfire in cylinder #3. To resolve this issue, wrap injector #3 in an insulator sleeve.


Carbon Build Up - TSB 0900303: http://wjjeeps.com/tsb/tsb_wj_0900303.pdf

Carbon can build up on valves and cause misfire issues. Running cleaner through the intake to break down the carbon can help resolve the issue.


O2 Sensors

O2 sensors don't typically cause misfire conditions though they can contribute to the situation. If an O2 sensor is on its way out/bad, typically the air/fuel ratio is enriched in order to keep the engine from running too hot. If the mixture is extremely rich, you could foul your spark plugs and cause a misfire. Moral of the story, if you have a misfire and are also throwing a code for an O2 sensor, look into replacing the bad sensor(s). A little piece of side advice, buy your O2 sensors from Kolak. WJs are extremely sensitive to O2 sensors. Kolak has the proper sensor and sells them at a good price.


Compression Test

If you've eliminated some of the above problems, now would be a good time to run a compression test. To do so, remove the Fuel Injector fuse (#26) and Fuel Pump relay (#29) as well as the coils/coil rail and spark plugs (be sure to disconnect the coils/coil rail).



Install the compression gauge in a spark plug hole, crank the engine until the cylinder compresses a few times, and record the results. Repeat twice more and average these results. Do this for each cylinder and note any irregularities. This test can also be repeated with the addition of a teaspoon of oil in each cylinder. These results compared to the previous results will reveal piston ring condition.

If low compression is found, there may be an issue with cylinder rings, valves, head gasket, broken pistons, cracked head, etc. Removal of the valve cover/head are probably your next order of business.


Camshaft Position Sensor (Distributor)

A bad or off-synced CPS can cause misfire issues. Typically you'll have multiple misfire codes and possibly a CPS code if this is the problem. If/when replacing, follow these steps:

1. Set the engine to TDC on Cylinder #1.
2. Remove the CPS and rotate it to line up the hole in the casing to the hole on the inside ring.
3. Insert something in the holes to keep it steady. I used a drill bit. Tape it in.
4. Slip the CPS back into the block and clamp it down. Remove the tape and drill bit.

The above will get the timing/fuel sync close. The fuel sync will still need properly set with a DRBIII tool, typically found at the dealer.


Crankshaft Position Sensor

The Crankshaft Position Sensor reads the location of the crankshaft as it rotates. If the sensor is bad (or going bad), it will not read correctly. In this case, the PCM thinks the crankshaft is not rotating at the proper speed and 'assumes' there is a misfire (even if one is not present).

[Edit]: 2004 4.0 Crankshaft Position Sensor Replacement


Clogged Cat

Typically, a clogged catalytic converter won't cause a misfire (in reality, a misfire can cause a cat to go bad). I only mention this because in an extreme situation, if exhaust cannot escape the engine thus is goofing up the air/fuel ratio, there is a possibility a misfire will result. More than likely, you'll throw a code for a bad catalytic converter well before you create a misfire from one.


Head Gasket

A bad head gasket can cause a misfire due to poor compression, coolant in cylinders, and vacuum leaks. When performing a compression test, note any cylinders with low compression. This could be a sign of a bad head gasket.


0331 Head Crack

If you have the 0331 head on the 4.0 and it cracks, a misfire can occur for multiple reasons (poor compression, coolant in cylinders, and vacuum leaks). Unfortunately, you'll need a new head to resolve this issue.


Valve Train/Lifters/Camshaft

Remove the valve cover and look over the valve train. Verify that there aren't any loose rocker arms and that there aren't any visible issues with the valves. Also, push down on the rockers at the pushrod side and see if you can compress any of the lifters. If you can compress one lifter much easier than the others, that lifter could be bad as it may no longer be able to pump up and open the valves completely, causing issues with the intake of air/fuel or the release of exhaust. At this point, you will need to remove the head and replace the lifters and possibly the camshaft. The following is the write-up I mentioned at the beginning of this thread on how to resolve this issue: 2004 4.0 I6 Jeep Grand Cherokee Camshaft, Timing Set, and Lifter Replacement


I hope that helps! As mentioned above, let me know of any additions/modifications to the thread and I'll make the changes.




~Chris
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post #2 of 60 Old 03-15-2014, 09:18 AM
jeepxj508
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Hey ChrisHager thanks for the write up, going to try doing Carbon Build Up - TSB 0900303: http://wjjeeps.com/tsb/tsb_wj_0900303.pdf later on when I get home.

I've got a 97 XJ that's misfiring on cylinder #1 because of low compression (had about 75PSI) and it looks like the exhaust valve has some pretty excessive carbon build up on it, not letting it seat properly. Wondering if you or anyone else would know if this would work? The valve(s) aren't making any excessive noise and the idle isn't bad, the only way you can truly tell it's misfiring is from listening to the exhaust sputter. I've tried seafoam and it did no good, and I've replaced all plugs and wires when the CEL came on. There isn't any leaking gaskets from what I could see or hear when I had the compression test done. I'm trying to save it without having to rebuild the head just wondering if it'll even be savable and if this would be a good shot. Was also told decarbonizing with water works (possibly better) like any chemical on the market.


Thanks, Nate.
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post #3 of 60 Old 03-15-2014, 09:33 AM Thread Starter
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If it's carbon keeping that valve open, keep running cleaner through it. Definitely do not use water though...

~Chris
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Weird, I didn't realize you could use vomit as coolant.
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post #4 of 60 Old 03-15-2014, 09:40 AM
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See what happens when you leave that BU Mistress alone for a few minutes.. a shear moment of genius occurs, Superb!
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post #5 of 60 Old 03-15-2014, 09:49 AM Thread Starter
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Haha!

~Chris
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Weird, I didn't realize you could use vomit as coolant.
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post #6 of 60 Old 03-15-2014, 09:58 AM
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Which cleaner would you recommend ChrisHager? I just went down to Chrysler and picked up a can of the MCCC. I know there are tons of cleaner products out there. I work at a Kia dealership in parts and we use a chemical made by BG Products for induction services, one of the tech's told me I should give it a shot because he said it works really well.. Anyone ever used it? It's actually two products one is "Air Intake System Cleaner" which is put into a "AIS Cleaning Tool" and then a the other is "CF5" which is an additive to throw in the gas tank. Anyone ever used this on a 4.0?

thanks, Nate.
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post #7 of 60 Old 03-15-2014, 10:27 AM Thread Starter
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Any should work well. Whatever you use, I'd run it once cold and once warm. When the engine is cold, the cleaner will soak into the carbon build up more...
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post #8 of 60 Old 05-09-2014, 12:45 AM
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Thanks a lot for your help
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post #9 of 60 Old 05-09-2014, 05:49 AM
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I'm replacing my camshaft sensor and drive today as an ongoing mission to stop my P300, 301, and 306 codes and rough running condition. When you talk about DRB3 scan tool that's used to set the fuel sync...is this a dealer only item or will ANY DRB3 tool work? Because I see these DRB3 diagnostic tools for $200 and then I see some for $2,500 called "StarScan". Is there any possible way to do this without a scanner?

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post #10 of 60 Old 05-09-2014, 06:07 AM Thread Starter
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I'm replacing my camshaft sensor and drive today as an ongoing mission to stop my P300, 301, and 306 codes and rough running condition. When you talk about DRB3 scan tool that's used to set the fuel sync...is this a dealer only item or will ANY DRB3 tool work? Because I see these DRB3 diagnostic tools for $200 and then I see some for $2,500 called "StarScan". Is there any possible way to do this without a scanner?
Typically, the DRBIII tool is dealer only and costs between $2-$3K. Yes, it is used to set the fuel sync. Now, I have heard rumor of other scan tools being able to do this but I couldn't tell you which they are and if the rumor is even correct.

You can set the CPS close enough to drive but the fuel sync will still need set.

To set the CPS, bring the engine to TDC at cylinder 1 on the compression stroke. Align the gear and cap on the CPS so the holes line up and tape a drill bit or toothpick into the hole to keep the CPS aligned. Slip the CPS into the engine, tighten the retainer clip/bolt, then remove the tape and drill bit. You'll be very close on the timing/fuel sync - close enough to drive it to the dealer.

~Chris
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post #11 of 60 Old 05-09-2014, 05:26 PM
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Replaced the camshaft position sensor today. Made absolutely no change. So now I'm back to the drawing board. This is a new ATK crate engine so I can't believe that it won't run properly and also I can't believe that I can't figure out what the problem is.

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post #12 of 60 Old 05-09-2014, 06:05 PM
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Chris, is the PCV and EGR a V8 thing, because I have 2 WJ 4.0's and neither have these...

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post #13 of 60 Old 05-09-2014, 07:09 PM Thread Starter
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Chris, is the PCV and EGR a V8 thing, because I have 2 WJ 4.0's and neither have these...
I was a little misspoken on that point but I think I'll leave it for others who have a PCV. We have something that's called a PCC I *think*. It's more of an elbow and tube setup instead of an actual valve.

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Quote:
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post #14 of 60 Old 05-10-2014, 09:16 AM
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Took mine to a shop this morning. Tech hooked a Snap-On Solus DRBIII scanner to the Jeep. Cleared the codes and checked it over and it ran fairly good. Then I drove it about 5 miles and got the engine to start missing and reproduce the problem and returned to the shop and he checked it with the scanner again. #1 and #6 were mis-firing. He shut it off and turned it back on and instantly #6 came back but #1 was still misfiring. After a while longer the scanner lost communication with the ECM altogether.


So my diagnosis at this point is a faulty ECM. One of the other guys at the shop said that he's dealing with the same problems on his daughter's WJ right now and he too is pulling his hair out. (No surprise here...I knew WJ's were the most problematic of all Jeeps before I bought this one). When he tried to get the computer flashed they told him that you cannot flash them. The computers must be replaced on these and they have to be programmed to match the VIN. Ended up costing him $440 at the O'reilly Auto Parts locally to get a new computer. (Holy crap.)


My question now is, can I get a computer out of a similar WJ from the salvage yard and plug it up, or do I need to have one flashed for my VIN? Where do you guys source new computers for these rigs?

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post #15 of 60 Old 05-10-2014, 05:50 PM
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Great write up

thanx very much



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