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4.0L #3 cylinder misfire story and possible fix

weebur

NAXJA Forum User
I thought I’d throw this out here for the guys that are getting persistent codes for #3 cylinder misfires on the 4.0L engine.

After getting tired of resetting the codes and rough idle I checked around and learned that the problem was most likely “Heat Soak” whereas after running the 4.0L in a hot environment and then shutting down would cause the fuel in #3 fuel injector to vaporize and cause a lean mixture/misfire. The factory came up with a hokey fix that involved wrapping the injector with insulating tape. I tried wrapping mine with aluminum foil and blocking off the spaces in the intake manifold that may have allowed heat from the exhaust manifold to heat up the injector(s)…no help. I should add that my engine would throw a code while idling, not just after shutting down, which should have provided enough airflow to prevent heat soak.

I checked with my local part stores and saw that AutoZone offered original replacement injectors and a “redesigned” one for several bucks more. I had them pull the redesigned version and saw that it’s felt heavier and appears to be made of stainless steel. I was low on cash and decided to just go with the original injector and replace the “O” rings on the other injectors.

After pulling the fuel rail/injectors and replacing the O-rings I noticed that the new #3 injector with a new O-ring slid into the recess easily, a LOT easier than the other injectors. All of the recesses in the intake manifold where cleaned and lubed so I have to think that the hole for #3 injector was bored too big at the factory. What I’m thinking is that when my intake manifold got hot it created a vacuum leak past the O-ring on #3 injector causing a misfire/code.

For what it’s worth, I suggest that anyone with a rough idle/#3 cylinder misfire issue consider just replacing the fuel injector O-rings before spending several hundred bucks on new injectors. It’s an easy job and hell, the O-rings are only a $1.50 or so for two O-rings. Ten bucks in O-rings and an hour labor might just be the ticket. My engine is running smoother and so far no codes/misfires.


'97 XJ Country
4.0L/AW-4 auto
118,000 miles
 
I hope this works for you.

The proof, however, will come when the weather warms up again.

If the problem reoccurs, may I suggest that you add a timer to the efan circuit to run the fan for 3-5 minutes after engine shutdown.

A relay: http://www.at-fairfax.com/P1786-ELK-960.htm
 
Thanks for your input, Joe. Your previous replies about the timer was in my mind when I was fooling around with this issue. I should have made it more clear that I don't believe that this possible fix was for a legit heat soak issue (high ambient temp/long idle period/shut down and restart). It may apply to engines that have good ignition systems and throwing codes while idling like mine did.

You're right though, the proof will come next summer. If it starts throwing the same code I guess I'll just have to pony up for the redesigned injector (I'm assuming the redesign was to deal with the heat soak issue).
 
Thanks for your input, Joe. Your previous replies about the timer was in my mind when I was fooling around with this issue. I should have made it more clear that I don't believe that this possible fix was for a legit heat soak issue (high ambient temp/long idle period/shut down and restart). It may apply to engines that have good ignition systems and throwing codes while idling like mine did.

You're right though, the proof will come next summer. If it starts throwing the same code I guess I'll just have to pony up for the redesigned injector (I'm assuming the redesign was to deal with the heat soak issue).

I happened to re-read your original post, and I am somewhat surprised that heat soak would be an issue with your 97.

The groundwork for the heat soak issue was set in motion with the elimination of the fuel return line from the fuel rail. It came to full bloom with the re-designed manifolds on the very late models. The TSB applies to 99+ Cherokees:

1803103 - 4.0L ROUGH ENGINE IDLE AFTER RESTART FOLLOWING A HOT SOAK
Date: 09/05/03 (supercedes 1802702 dated 09/06/02)
Model Year(s): 1999-2004

I'm thinking your problem may just be a poor injector, as you suspected. In that case cooling will probably have a minimal effect, and you could expect the problem to get worse over time.

Did you happen to get a part number for that redesigned injector? Might be useful for others if you could get the number and post it up here.

Good luck.
 
Hey there, Mine was acting up and doing the same junk... Ever check the Primary O2 sensor :?

No, I didn't bother because it looks like the previous owner replaced the entire system, including new O2 sensors. And I passed emmisions with flying colors. The misfire was only #3.
 
Now that makes sense. The fuel recirculating, however small, might help keep the fuel cooler. Anyway, here's a link to AZ for the redesigned injector: http://www.autozone.com/autozone/ca...&navValue=14900120&itemId=120-0&parentId=49-0

If the link doesn't work the part number is: 800-1256N
$65.99 Made by GP-Sorensen

Thanks for the TSB info, now I can forget about that issue if it starts acting up again.

Yeah, the link didn't work so I looked up that part on the GP Sorensen website. It is listed for the 94-96 Durango, full-size Ramcharger, and full-size van. A further search of the website showed no fuel injectors for the 97 Cherokee.

Vatozone strikes again? I wonder what the specifications on that injector are?
 
I happened to re-read your original post, and I am somewhat surprised that heat soak would be an issue with your 97.

The groundwork for the heat soak issue was set in motion with the elimination of the fuel return line from the fuel rail. It came to full bloom with the re-designed manifolds on the very late models. The TSB applies to 99+ Cherokees:

1803103 - 4.0L ROUGH ENGINE IDLE AFTER RESTART FOLLOWING A HOT SOAK
Date: 09/05/03 (supercedes 1802702 dated 09/06/02)
Model Year(s): 1999-2004

I'm thinking your problem may just be a poor injector, as you suspected. In that case cooling will probably have a minimal effect, and you could expect the problem to get worse over time.

Did you happen to get a part number for that redesigned injector? Might be useful for others if you could get the number and post it up here.

Good luck.
I'm surprised, but not very - IIRC, the fuel return line was eliminated after the 96 model year, or perhaps late in 96, and I know a 97+ has only a vapor return line for the evap system. So the groundwork for the issue was laid in 97 or late 96 (forget which) but the issue really only reared its ugly head for most in 00 or 01 when the pre-cats exacerbated the fuel rail heating issue.

If my understanding is correct (I have a 96, so my 97+ fuel system knowledge is only firsthand concerning the tank+pump assembly, the rest is learned from NAXJA and the MOPAR parts manuals, etc) the old fuel return line and rail-mounted regulator system helped in two ways - it kept the fuel cooler, and it also helped to flush vapor out of the fuel rail when it did boil.
 
I wouldn't be surprised, but the one I held in my hand looked the same except the body may have been a little thicker and like I said it appeared to be stainless. I didn't buy the original one from AZ, I went down the street to O'Reilly's.

I'm going to try and paste the info from their website, if it turns out crappy, please excuse.

Gp-Sorensen / Fuel Injector For your 1997 Jeep Cherokee 4WD 4.0L MFI 6cyl











Price:
$65.99



AVAILABILITY:

STORE PICK UP
SPECIAL ORDER
Visit your local store to special order.



SHIP TO HOME
AVAILABLE
Ships within 2 business days.



















Part Number:800-1256NWeight:0.2 lbsWarranty:1 YR Notes:New Improved design
Type:Mfi InjectorIncludes:Caution Sheet And Instruction SheetOEM Replacement:YesQuantity:1 Per BoxShipping Information:Overnight and Two Day shipping are not available for PO Box, APO/FPO/DPO or US Territory addresses.

Features & Benefits:
Injectors Are Manufactured To Oe Fit And Function. Manufactured In Usa To Ts16949 Specifications.
 
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