The (apparently) dreaded misfire

[SCTexasTJ]

Hello to All! New member here with a new to me 97 TJ. Owned a CJ7 many years ago and finally have gotten a replacement! My problem is a bad misfire, setting codes P0300 and then P0301 thru 306. Obviously every cylinder. Misses at idle and on the road. It also idles up and down between 750 and 1000rpm and then will gradually decrease till it dies. If I unplug the MAP sensor on the TB the idle variation stops but misfire is still there. Guessing that the computer is using that to try to compensate for the misfire but don't know. IAC is new as is the TPS. It appears that the PO had replaced lots of items chasing this problem (I knew it had this going in). I even put a new crankshaft sensor in knowing that the one it came with looked new. Coil seems to check out good (internal resistance, signal to it, etc) From what I have found while researching this problem it seems that these are very sensitive to having OE sensors so I'm thinking that's it but wondering about the PCM (or ECM, not sure which is the correct term). I've been fooling with it for the five days I've had it (long time I know!) and am about at the point to take it to a mechanic to get it diag'd. Sure don't want to drop the money on a PCM and reluctant to buy another crank sensor from Mopar for $140 without knowing fairly sure this will fix it. Thought I would reach out to y'all here since there is obviously a huge amount of combined knowledge and helpful members on this site!

Thanks for any help you can offer!

 

[mots]

Have you checked the plugs? Do you have the correct OEM plugs installed and are they gapped correctly? Improper plugs can cause misfires.

 

[SCTexasTJ]

Yes I have. Autolite 985.

 

[Jeepn_Texas]

Hello to All! New member here with a new to me 97 TJ. Owned a CJ7 many years ago and finally have gotten a replacement! My problem is a bad misfire, setting codes P0300 and then P0301 thru 306. Obviously every cylinder. Misses at idle and on the road. It also idles up and down between 750 and 1000rpm and then will gradually decrease till it dies. If I unplug the MAP sensor on the TB the idle variation stops but misfire is still there. Guessing that the computer is using that to try to compensate for the misfire but don't know. IAC is new as is the TPS. It appears that the PO had replaced lots of items chasing this problem (I knew it had this going in). I even put a new crankshaft sensor in knowing that the one it came with looked new. Coil seems to check out good (internal resistance, signal to it, etc) From what I have found while researching this problem it seems that these are very sensitive to having OE sensors so I'm thinking that's it but wondering about the PCM (or ECM, not sure which is the correct term). I've been fooling with it for the five days I've had it (long time I know!) and am about at the point to take it to a mechanic to get it diag'd. Sure don't want to drop the money on a PCM and reluctant to buy another crank sensor from Mopar for $140 without knowing fairly sure this will fix it. Thought I would reach out to y'all here since there is obviously a huge amount of combined knowledge and helpful members on this site!

Thanks for any help you can offer!

Are you near me, I live down in Katy, TX. I might be able to help!
I have some pretty good diagnostic skills and I might be able to find the Ghost in the machine.

 

[SCTexasTJ]

Are you near me, I live down in Katy, TX. I might be able to help!
I have some pretty good diagnostic skills and I might be able to find the Ghost in the machine.

Thanks for the offer! Unfortunately I'm about 2.5 hours east of you, but I do appreciate it!

 

[SCTexasTJ]

Thanks for the offer! Unfortunately I'm about 2.5 hours east of you, but I do appreciate it!

And I apparently can't read a compass cause I meant west!

 

[Jeepn_Texas]

And I apparently can't read a compass cause I meant west!

That's not such a bad trip, Wife and I go to Gruene from time to time...

 

[SCTexasTJ]

If I can't get it figured out I may take you up on the offer!

 

[mots]

Yes I have. Autolite 985.

Cool, the newer TJ’s (00-06) with direct ignition can be picky about plugs, but yours is a 97 and you are running properly gapped 985’s, copper, platinum, or iridium, I would think you should be good for a 97. Since you are getting the misfire in all 6 cylinders, I figured I would ask since it wasn’t mentioned. Might still want to double check the gaps if you aren’t sure.

Good luck with the troubleshooting! Let us know what you figure out.

 

[RMETeeJay]

Check the camshaft sensor and OPDA. That was the cause of my recent P0300 (and a couple of P030* cylinders).

In my case, there was lateral play in the sensor ring at the top of the OPDA which messed up the sensor (there should be no play). On mine, it was bad enough to damage the sensor and create metal shavings that stuck to the magnet.

It’s more common for the OPDA to go bad on 05 and 06 years, but mine is an 04. Sometimes they make noise when they go bad, but mine did not.

Finally, I have to add that my codes would still not clear after replacing the OPDA and sensor. In the end, what worked for me was resetting the adaptive memory in the PCM. That procedure can be found here.

If the OPDA and camshaft sensor check out, then I would test the coil next. If you don't already have the FSM, you can download it here. Details on my 97-04 OPDA replacement is here.

 

[SCTexasTJ]

Cool, the newer TJ’s (00-06) with direct ignition can be picky about plugs, but yours is a 97 and you are running properly gapped 985’s, copper, platinum, or iridium, I would think you should be good for a 97. Since you are getting the misfire in all 6 cylinders, I figured I would ask since it wasn’t mentioned. Might still want to double check the gaps if you aren’t sure.

Good luck with the troubleshooting! Let us know what you figure out.

Definitely will. The plugs that were in it were the weird triple electrode type but changing for the correct ones didn't change anything, but those things needed to come out anyway!

 

[SCTexasTJ]

Check the camshaft sensor and OPDA. That was the cause of my recent P0300.

In my case, there was lateral play in the sensor ring at the top of the OPDA which messed up the sensor. It’s more common for the OPDA to go bad on 05 and 06 years, but mine is an 04.

The camshaft sensor has been replaced, but once again it is a non OE part so....

Thanks for the reply!

 

[SCTexasTJ]

Check the camshaft sensor and OPDA. That was the cause of my recent P0300 (and a couple of P030* cylinders).

In my case, there was lateral play in the sensor ring at the top of the OPDA which messed up the sensor (there should be no play). On mine, it was bad enough to damage the sensor and create metal shavings that stuck to the magnet.

It’s more common for the OPDA to go bad on 05 and 06 years, but mine is an 04. Sometimes they make noise when they go bad, but mine did not.

Finally, I have to add that my codes would still not clear after replacing the OPDA and sensor. In the end, what worked for me was resetting the adaptive memory in the PCM. That procedure can be found here.

If the OPDA and camshaft sensor check out, then I would test the coil next. If you don't already have the FSM, you can download it here. Details on my 97-04 OPDA replacement is here.

Thanks for the link about resetting the PCM. I will do that this evening and see if it yields results.

 

[RMETeeJay]

Thanks for the link about resetting the PCM. I will do that this evening and see if it yields results.

I should have said before that in my case, there was no apparent missing going on when I did the reset. It was running very smoothly, but still had the codes. If your engine is still obviously missing, I'd bet that the reset won't matter. You have to figure that out first.

The reset removes the PCM's stored data about how your system is functioning. After you reset it, it will re-learn over the next 500 miles and store the new data. If you haven't addressed the missing, then that poor running data will be stored, and you probably don't want that.

 

[SCTexasTJ]

I should have said before that in my case, there was no apparent missing going on when I did the reset. It was running very smoothly, but still had the codes. If your engine is still obviously missing, I'd bet that the reset won't matter. You have to figure that out first.

The reset removes the PCM's stored data about how your system is functioning. After you reset it, it will re-learn over the next 500 miles and store the new data. If you haven't addressed the missing, then that poor running data will be stored, and you probably don't want that.

Probably won't change it but it's free and easy to try so can't hurt.

 

[eastbloc]

Yes I have. Autolite 985.

Which 985?

First set of iridiums was causing strange fuel consumption issues, as well as being picky to a certain quality of fuel. I was trying to make sense out of it for over a year. Unfortunately i had O2 sensor issue at the same time, so i didn't even think to suspect spark plugs.

Switched to double platinum before a long road trip to DC just to satisfy my curiosity. 2000 miles there and back, including two weeks of in town driving was nothing but a smooth ride. Engine was working like a quality German sewing machine.

Upon returning, i purchased a second set of iridiums, just to make sure it wasn't my imagination. Rougher idling from the start. Took a 30 minute drive in one direction without issues, on the way back i stopped for a snack. After a crank, engine started choking and threw a misfire code(i think it died too). Mileage dropped to 10mpg, didn't even get 200 miles on the full tank. Here we go again with this iridium shit....

Another spark plug related issue - take the coils off, and wiggle tips of spark plugs. There should be no wiggling, if it does, you broke it.

 

[mots]

Which 985?

First set of iridiums was causing strange fuel consumption issues, as well as being picky to a certain quality of fuel. I was trying to make sense out of it for over a year. Unfortunately i had O2 sensor issue at the same time, so i didn't even think to suspect spark plugs.

Switched to double platinum before a long road trip to DC just to satisfy my curiosity. 2000 miles there and back, including two weeks of in town driving was nothing but a smooth ride. Engine was working like a quality German sewing machine.

Upon returning, i purchased a second set of iridiums, just to make sure it wasn't my imagination. Rougher idling from the start. Took a 30 minute drive in one direction without issues, on the way back i stopped for a snack. After a crank, engine started choking and threw a misfire code(i think it died too). Mileage dropped to 10mpg, didn't even get 200 miles on the full tank. Here we go again with this iridium shit....

Another spark plug related issue - take the coils off, and wiggle tips of spark plugs. There should be no wiggling, if it does, you broke it.

He has a 1997 TJ, so there isn't a direct ignition coilpack assembly with 3 coilpacks like your 06 has. Instead, it has a distributor, different type of shared coilpack, and spark plug wires. The older TJ's with distributors are less picky about the particular spark plugs they can tolerate from what I understand. Even copper 985's would likely be fine, they just won't last as long. For the newer TJ's, we need the double platinum or iridium 985's.

When I got my 2005 TJ, I accidentally used the AP985 (single plat plug) because a parts store said they were the correct ones. I didn't notice the one letter difference from the APP985 I set out to purchase. They misfired like crazy. I figured it out when I read a post by @Jerry Bransford that also applies here for the OP:

• For the OP, Autolite's AP985 or Champion's 3034 single-tipped platinum (platinum on just one side of the gap) is the best choice for yours since yours has a distributor.
• The more expensive APP985 and Champion 7034 (double-tipped, platinum on both sides of the gap) is only needed for newer distributorless 4.0L engines and there would be zero benefit to running the APP985 in yours.

FWIW, I am now running Iridium 985's (XP985) for my 2005 TJ without issues for about 5000 miles and going on 2 years.

 

[mots]

Check the camshaft sensor and OPDA. That was the cause of my recent P0300 (and a couple of P030* cylinders).

In my case, there was lateral play in the sensor ring at the top of the OPDA which messed up the sensor (there should be no play). On mine, it was bad enough to damage the sensor and create metal shavings that stuck to the magnet.

It’s more common for the OPDA to go bad on 05 and 06 years, but mine is an 04. Sometimes they make noise when they go bad, but mine did not.

Finally, I have to add that my codes would still not clear after replacing the OPDA and sensor. In the end, what worked for me was resetting the adaptive memory in the PCM. That procedure can be found here.

If the OPDA and camshaft sensor check out, then I would test the coil next. If you don't already have the FSM, you can download it here. Details on my 97-04 OPDA replacement is here.

It won't be from 97 to 04. It should be from 2000-2004, which won't apply to the OP. The 2000-2006 have an OPDA and direct ignition. The 05-06 OPDA are a bit different than 00-04. The 1997-1999 TJ's have a distributor that slides into side of the engine in the same spot. Both are geared to run off the camshaft in the same way.

But still, related to what you said, perhaps the engine timing is not set correctly and could potentially be causing a problem. Worth throwing into the bucket of things to consider.

 

[eastbloc]

It won't be from 97 to 04. It should be from 2000-2004, which won't apply to the OP. The 2000-2006 have an OPDA and direct ignition. The 05-06 OPDA are a bit different than 00-04. The 1997-1999 TJ's have a distributor that slides into side of the engine in the same spot. Both are geared to run off the camshaft in the same way.

But still, related to what you said, perhaps the engine timing is not set correctly and could potentially be causing a problem. Worth throwing into the bucket of things to consider.

Does he have anything that would retard the engine to not get past certain rpm?
In my 06 i messed up timing while messing with OPDA, engine would run like OPs but power would be cut off to not get past ~2000ish rpm.
If so, that would be a good indicator of bad timing.

 

[Jerry Bransford]

Which 985?

First set of iridiums was causing strange fuel consumption issues, as well as being picky to a certain quality of fuel. I was trying to make sense out of it for over a year. Unfortunately i had O2 sensor issue at the same time, so i didn't even think to suspect spark plugs.

Switched to double platinum before a long road trip to DC just to satisfy my curiosity. 2000 miles there and back, including two weeks of in town driving was nothing but a smooth ride. Engine was working like a quality German sewing machine.

Upon returning, i purchased a second set of iridiums, just to make sure it wasn't my imagination. Rougher idling from the start. Took a 30 minute drive in one direction without issues, on the way back i stopped for a snack. After a crank, engine started choking and threw a misfire code(i think it died too). Mileage dropped to 10mpg, didn't even get 200 miles on the full tank. Here we go again with this iridium shit....

Another spark plug related issue - take the coils off, and wiggle tips of spark plugs. There should be no wiggling, if it does, you broke it.

I've been running Autolite XP-985 iridium plugs in my '04 for years and it's a commonly recommended and highly praised plug. The XP-985 is suitable for any model years of the TJ 4.0 and 2.5 engines, not just the versions with the coil pack ignition system.