4.0L #3 cylinder misfire story and possible fix

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 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
118,000 miles

good fix, but your problem did seem to be the o-ring and not the heat soak issue where insulating the fuel rail from heat build-up when the engine is cools.

89xj said:

good fix, but your problem did seem to be the o-ring and not the heat soak issue where insulating the fuel rail from heat build-up when the engine is cools.

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Yep, thanks for pointing that out. I'm not suggesting that it was a fix for a legit heat soak issue (long idling/hot ambient temp/shut down and restart) but it may be for misfires during idling with known good ignition system.

My o-rings were nice and tight when I replaced my injectors with ford ones. It was actually pretty tough gettin em out. The new injectors were a PITA to get in but I got em in. I still haven't figured out the heat soak crap. I first replaced the injectors, then plugs, then coil rail. Next I replaced the torn up heat shield that was there from the TSB. And now I just put heat shield tape over the fuel rail and injectors. I still get the rough idle sometimes after restarting it but it hasn't been as bad as usual. I think we need to put a large shield pan under the intake manifold cuz that's where the headers are spreading the heat.

I also posted to the OEM Tech forum, check out Joe Peters reply and my replies that include a link to a redesigned injector from AZ. If only one of your cylinders is misfiring and the cause is heat soak you may want to consider upgrading to the redesigned injector.