Random Cylinder Misfire???

[car12586]

Well after finishing the stroker install all seemed good. I had a check engine light which i knew would be caused by the lack of the two downstream O2 sensors. So i went ahead and checked the codes to make sure everything was as expected. To my surprise i found 3 codes i wasnt expecting. I had a cylinder 3 misfire, cylinder 5 misfire and a random cylinder misfire code. Im not sure what would be causing these codes as everything seems to be running fine. Anyone else experience these codes, stroker or not? I have another coil pack im going to try in the morning and i need to get some larger injectors for the stroker still. After that im not sure where else to look for issues. Any help or input would be appreciated and thanks in advance.

[Mgardiner1]

make sure your map sensor is hooked up to a vacuum line. When i first started my stroker it ran like poop and it was mostly due to the map being disconnected

[seanyb505]

My 97 has said its had a misfire both before and after the stroker. Im chaulking it up to a stupid persistent computer, as the 4.0 was running fine, and the 4.6 runs fine as well.

[car12586]

Anyone else had this issue?

[SilverXJ]

Are the codes persistent or did it only happen once like during cam break in? The cylinder misfire on 3 could be related to the fuel injector shielding TSB for injector #3. It has to do with a hot soak issue, where after driving the engine sits and heat soaks the injector. On subsequent driving you may get rough idle for a bit and the misfire code for #3. The fix is putting heat shielding on injector #3.

[car12586]

I've cleared the codes a couple times now and find them keep coming back. I cant seem to tell when the misfire is happening. Could be a heat soak issue i guess.

[SilverXJ]

When does this happen? Describe the events prior and during the mis fire. The #3 could be the heat soak issue as I described. The #5 shouldn't be. Verify that the injector connectors and spark plug rail are seated well.

[car12586]

I haven't been able to determine exactly when this happens because i cant tell when an actual misfire happens. The CEL will come on at anytime during a trip which could be caused by the misfire or missing O2 sensors. All i can do is check it once ive stopped. What actually detects the misfire? Could it be a faulty sensor of sort?

[gradon]

Mine has a random "make specific" misfire CEL as well. I blew the original 96 pcm by letting the US o2 harness kiss the Borla. I then ran a 98 auto pcm(no tach or a/c/e-fan engagement other than the auto turn on @230*), a 97 ax15 pcm(got tach back, but still no a/c engagement and the cel and gen were 1/2 as bright but on all the time w/ no codes registered). I then got a free 96 pcm from NukeProof over on NAXJA from searching the Sierra chapter's Karma thread(free parts, just pay shipping and return the favor to keep the Karma going). Everything works with this pcm even though it's not the same p/n as the original, but I keep getting that misfire code. I also don't ever detect a misfire when driving, so obviously some sensor is tripping out, or the pcm doesn't like it's new body(it should be glad it's being put to use and especially in a stroker).

[car12586]

So you dont know what was causing the misfire code then? Do you know what sensor is responsible for the misfire code?

[gradon]

I don't know(even though I should by now), but would look at the cam pos sensor. I chalk mine up to the pcm since there is no noticeable misfire. Search around NAXJA and you might find some direction.

[SilverXJ]

I believe it is the crank sensor detects the misfires.

[1bolt]

How can either of them detect a misfire? I would suspect the only thing that could detect a misfire would be some knock sensor like microhpone or something detecting fuel in the exhaust or overpressure in the intake manifold... Hmmmm How about the MAP sensor? If it gets hit with an intake backfire wouldn't the voltage be seen at the ECU? The timing could tell the ECU which cylinders to flag?

Also the thing most likely to cause an intake backfire is a faulty MAP... Having had two MAP's that went wonky, both of them caused bad backfiring.

Also Gradon... have you ever looked at the O2 sensor heater fuse in the power distribution block? Maybe you didn't fry your PCM, maybe the wires shorting blew the o2 fuse and your XJ is running only off the MAP (no closed loop mode)?

Yeah its got a fuse, I found this out on my 97, when the o2 wires melted against the header... after splicing them together it ran like ass occasionally after startup and threw CEL's, then it would run fine until you stopped and started it again. It was trying to go into closed loop and would backfire until it went back to "limp mode" using only the MAP. Of course the intake backfires probably made the MAP go south.

Anyway maybe worth a look, but I suspect your MAP at least needs to be replaced anyway.

[gradon]

I'm running stock map now(for at least a month longer), until I retune the psc1. I am using the lm2 wideband o2/obd2 scanner/data logger and it tells me when I'm in closed loop and open loop. Everything functions correctly from the open loop cold startup running a little rich and idling higher to the closed loop 14.7afr. I didn't have a misfire cel until I installed this replacement 96 pcm a month or so ago--I'm not worried at the moment. When I switch to the psc1 map then I will see if by bypassing the stock map the cel will go away. All the sensors save the coolant temp are probably 12 years old, so maybe one or more need replacing. Now another thought is: I have cut the tangs and moved the distro clockwise a bit(I did this long ago). Maybe mine isn't sync'd up correctly and at some rpm it's off just enough causing the cam to set the misfire cel(even though driveability is good)?

[seanyb505]

Could be, but mine insisted on a misfire even before I did the stroker, back when the engine was stock and I had never touched the dist body.