Hopefully I can explain this correctly. I have a 2001 Jeep Cherokee and I recently had a P0302 code pop. I've removed and cleaned the coil rail and installed new rubber boots and springs, I've also replaced the spark plugs. I'm still getting the code. I've moved over to the fuel side side and the injectors seem to be good, I've sprayed starter spray on them while the engine is running and there was no increase in RPM.

Here is my question when I first start the vehicle (because I know the PCM clock is ticking). When I pull the electrical connection on injectors 1, 3, 4, 5 and 6 I get a noticeable dip in idol and RPM, when I pull the power to the #2 injector there is no noticeable difference. So does this mean the there is no power going to the 2nd spark plug on the rail side or no power to the 2nd injector?

Take and move the #2 injector maybe #4 or trade it with any other cylinder and see if the misfire moves also .

You could also swap with the adjacent injector connector. #2 and #4 are on the same coil if he's got the rail, although there was a recent post that it' possible to have just one spark plug drop off with the rail (I assume due to an internal short to ground inside the coil)

Okay I swapped injector #2 with #4, and there was still no drop off on #2 when I pulled the electrical connection, but there was a noticeable drop on #4, so I'm guessing the injector is good. Now it's either faulty wiring to injector #2 or the coil rail on plug/cylinder #2 is bad. Also and I don't know if this makes a difference, when I was pulling the injector connections during my test I got a P0202 code.

Since it didn't kick the code till you unplugged injector 2. I'd say that Injector 2 is bad. probably a physical internal failure. I have an injector on my project car that electrically tests ok but is physically stuck wide open, yours sounds like yours might be stuck shut.

Thank for jumping in!

It actually kicked the code after I did the injector swap of injectors #2 and #4. So if injector #2 was bad, I would think that it would have shown the error on injector slot #4? I got the same results on the #2 injector slot, which was no idol decrease, using the original injector and the one from injector #4#. I'm thinking the P0202 code (which I've never seen before) was due to me continually removing the power connectors on each injector a few times. This morning I pulled the spark plugs I had installed on Tuesday from cylinder #2 the potential source of the P0302 code and then cylinder #3, to see if there was any non firing evidence, and #3 looked almost brand new and #2 already had a brownish ting to it, like it hasn't been firing. I'm getting ready to do a rail firing test to see if each plug location creates a spark. This Jeep has 195,000 miles on it and my gut tells me that the rail has gone bad.

Well I'm back at square 1. I did an ignition test on the coil rail and all the plug holes produced a spark. So I'm thinking it has to be the lack of electrical current going to the #2 injector. Does anyone know a way of testing this as I don't own a noid light kit? Can I test it with my VU meter? Or is there anything else that I can use in it's place? Also, once I determine that injector is not getting any electrical juice, is it as simply as tracing the wires to see if I can find a short?

The fuel injectors get constant 12v, you can check that with the engine running and plug 2 off. The ground to fire injectors is what cycles and it goes on and off so fast you almost cant see the blink. I dont think a meter would react fast enough to show anything meaningful. You can build your own noid light with any small 12 v light and 2 lengths of wire that will fit into the connector sockets. If you disconnect #2 injector plug you get a fault...that means the computer sees an open...if you don't get the same fault with it connected then the wiring and injector is probably good. Have you done a compression test on #2 cyl

Thank you for the info on building a noid light, it worked perfectly. So I was able to get a strong flashing signal from injector plug #2, so I'm guessing an electrical issue involving the injectors can be ruled out? That being said, I still don't hear any RPM dip when I pull the plug off of the 2nd injector like I do when I pull the plug off of any of the other injectors while the engine is running. I have not done a compression test yet. I will say that it idols smoothly, and when I rev the engine while its in park it revs up and down very smoothly. The only time you can physical tell that something is wrong is when you drive up a hill, then you can feel the loss of power. Is there an audible sound I could listen for indicating one of your cylinders has low compression?

When you pull any other plug the engine bogs because you now have 2 dead out of 6...when you pull 2 its only 1 dead out of 6. You must do a compression test

Okay, I was able to do a compression test today and everything tested within range. I tested each cylinder twice:

Cyl. #1 at 140 and 150 psi
Cyl. #2 at 150 and 160 psi
Cyl. #3 at 140 and 150 psi
Cyl. #4 at 150 and 150 psi
Cyl. #5 at 150 and 150 psi
Cyl. #6 at 150 and 155 psi

I also did a voltage test on the constant injector plug wires coming from the ADS. With key on but with the engine not running, I get a reading of 6v. With the engine running I get a reading of 14v. To test the ground, I discounted the wiring harness going into the PCM and did a resistance test from the connector plugin block to each wire at the injector plug and I got a reading of continuity.

One other thing the noid light is working sporadically or at times not at all on injector #2, but works perfectly on all of the other injector plugs. Does this sound like a PCM driver issue, a bad ground wire from the PCM to injector #2, or has the ADS gone bad kicking out too much voltage?

Hook the noid light from power on the injector connector to any good ground..should light steady with engine running...shake the harness...should remain steady. Then hook the light up to the battery pos terminal and the other leg to the injector connector ground and run the engine. Should blink steady and quickly. Shake the harness again. Whichever side does not give desired result is the bad leg/ wire or something since you said the noid light was intermittent. But earlier on it was strong and steady..14 is 12...its whatever your batt voltage is at the time . The ASD relay only has 1 or 2 outputs that run dozens of things at once, so if its bad then many things are bad. No way it could fail just #2 injector circut

Intermittent means it's not likely the computer. A blown driver there would just die and not be intermittent. Intermittent sounds like a wiring issue to me. It could be the injector itself, the plug, or wiring in the harness. Look especially at the harness where it jumps from the rail to the firewall as that can flex and fatigue the wires inside.

Well guys I'm at a loss. I hooked the noid light up to the constant (ASD) side of the injector, grounded it to the body with engine running and got a good steady light, then I switched it over to the connector ground side coming from the ECU/ECM with the engine running it began flashing without having to connect the other lead of the noid to a power source. I also moved the harness around and it didn't seem to alter the flash. Next, I disconnected the #2 harness (the white one) from the ECU and tested each designated pin/wire to its corresponding injector plug wire (with my ohm meter) and all test for steady for continuity. I also moved all the wires around to see if I could break the continuity, and it did not.

After all of this I reconnected the injector control harness to the ECU and began to strip back very carefully the tape and wire loom on the wiring harness all the way to the brake power booster unit on the firewall. I isolated the connector ground wire the goes from the ECU to the injector plug of #2, (It's very light brown) and inspected it for outward signs of compromise and it looked fine, as a fact all the wiring looked great especially at the end of rail where it then turns towards the firewall... I then move it all around with the engine running and noid light insert into the injector plug #2 and the light continued to flash with no observation of erratic behavior when moving the connector wire.

Now here's where the testing went awry. I was going to move the noid light to one of the other injector plugs to see if I got the same flash pattern as #2 when my jeep just cut off. It wasn't a slow choking stall it was as if some had turned off the key. I was able to crank back up immediately, it idled fine, but within a minute or so it cut off again, and again and again. One quick observation, a few times when I restarted the engine, I would rev it a bit, the RPMs would increase to say 12000 then dive down to say 600 then jump back up to 1200 and when I took my foot off the gas pedal settle in at around 750, where within a minute it would cut off. That's where my testing ended

I must tell you I feel like I've somehow made things worse. Before I started working on the potential issue being in the wire harness, it would at least start and idle smoothly until I turn it off. Any idea what might be going on now.

I disconnected the battery and let it sit for a while, I also sprayed a small amount of contact cleaner into the 3 plugs that go to the ECU and then sprayed them dry with an air compressor. Jeep started right up but staled within 30 seconds. Started right up again but I had to keep revving it to keep it from stalling, The revving sounded very labored like there was a timing issue. CEL came on, I hooked up the scanner, and I've never see this many codes in my life.

This whole adventure started with a P0302 code, then a P0202. Now I have no less than 12 of them.

P0122 - Throttle Position Sensor Voltage Too Low
P0123 - Throttle Position Sensor Voltage Too High
P0201 - Injector # 1 Control Circuit
P0202 - Injector # 2 Control Circuit
P0340 - No Cam Signal At PCM
P0700 - TCM DTC Present
P1698 - No Bus Message From Trans Control Module
P1694 - CCD Message From JTEC Failure
P1687 - No Cluster Bus Message
P1686 - No Skim Bus Message Received
P1388 - Auto Shutdown Relay Control Circuit
P1391 - Intermittent Loss of Cmp or Ckp

As I stated earlier I was very gentle in removing the tape and looming around the injector wire harness. Some of the wires were stuck together as if the plastic casings has melded together or they had been glued together especially as I got closer to injectors #3, #2 and #1. I did not try and pull them apart, the only wire that I carefully isolated from the others was the ground wire from Injector #2 that ran to the ECU.