Hi everyone, this is my very first post so to get started I want to preface with this:

I hope I’m not in the wrong spot.

I hope I’m not breaking any rules.

If either are true, please let me know.

I don’t mean to be too lengthy, but I’m looking to do a deep dive into this problem that I am experiencing and I feel like context and details are important. I would try to give a TLR but I don't think it is really valuable with nuanced/intermittent issues like I am experiencing.

I have searched the web high and low, even reading several threads on this forum. I know that misfires of various sorts have been covered many times. However, with the seemingly infinite amount of possibilities and odd one off cases you can have in the world of mechanics, there is rarely that perfect post where you read the exact same problem and had the exact same fix. It also seems many posts I read do partial diagnosis or throw parts at the problem until it goes away. I personally don’t like to be a parts changer to the best of my ability, although I know sometimes the educated guesses are easier than actual diagnosis.

I have read through the sticky by GeorgeLG on general guidelines for troubleshooting misfires. I have read other good tips from some of the super members and moderators like rockp2 and LesMyer. I’ve got about 9ish years of both professional and ‘on the side’ technician experience. Lets get started.

I have a 2004 Chevy Blazer ZR2. I bought it in April of this year from the original owner! It has 285,000 miles. Transmission was replaced in 2014, original motor. Here’s a collage of it.



Some time in early June I got a CEL for P0301. At the time I assumed it was probably the known injector issue or something small as it wasn't noticeable at all and drove fine. Also, at this point I bought a 96 Yukon as I needed something with a bit more space so I had parked the Blazer and wasn't driving it daily. I was on the fence of selling this because I like it a lot and know that the ZR2's are getting hard to come by in decent condition but I've got a kiddo on the way and don't need a toy atm. Therefore I decided to post this for sell about 2 weeks ago. At this point I had sort of forgot about the CEL but it didn't forget about me.


Initial Symptoms:

• Intermittent misfire only noticed at idle. Vehicle runs and drives great otherwise.
• While sitting in park, you can notice the misfire ever so slightly as it lightly stumbles consistently with each revolution of the motor.
• I say intermittent because sometimes it will completely go away and idle smoothly.
• As of now, there are no external factors that seem to make a difference in whether it misfires or doesn’t.

Troubleshooting/Diagnosis:
I had already bought new AC Delco plugs, wires, cap and rotor but hadn’t installed because current ones looked fairly new, but knowing these have some history of ignition weirdness, this is where I started:

I took the wire for the #1 cylinder and replaced it with a new AC Delco one along with the new iridium plug. Problem persisted.

I then pulled that wire and used a spare ‘test’ plug to watch for spark as the engine ran, it did have a healthy white/blue spark. Keep in mind, this issue is intermittent, so I am not sure your eye will be able to validate consistent spark. More details on this later.

At this point, I was supposed to show the vehicle the next day so I immediately jumped to fuel being the issue. I knew these have the spider system that everyone has issues with. However, I did not realize that the 2004 model had already gotten the upgraded system (not the poppet style). Either way, I replaced the number 1 injector with a brand new injector from O’Rileys. (I believe Standard brand) and a new fuel filter.

While I had the intake off, I visually verified that the FPR was not leaking. Initially the vehicle started up fine and ran great that evening so I thought hooray! Until the next day of course. Problem persisted.

Except this time, the vehicle did idle a bit lower and at one point stalled when I gave it some throttle. Remember, this is the next day after putting around an hours drive time on it the evening before and it running fine.

• As a note, I did not replace the upper intake and TB gaskets, as the ones in there were still springy and looked great. But more on this later.

Next I went ahead and replaced the TPS and IAC valve. I also cleaned the MAF sensor. Problem persisted.

I returned the new TPS and IAC and reinstalled the old sensors, no change as expected.

I did a quick compression check only on cyl #1 and got 160. More details on this later.

I mentioned re-using the rubber gaskets for the upper manifold, so let's address possible vacuum leaks:

• I sprayed brake cleaner and starting fluid around all areas of the upper intake manifold, intake itself and the throttle body. I disclose more OBD2 data later, but it's worth mentioning now that there is no indicator of a lean condition nor do I have a high idle. Also, it is intermittent and aside from the one time the idle dropped and the 1 time it stalled, I have not noticed a consistent change in the initial symptoms so far with the parts I have changed. More details will get shared later on this but it does appear to be specific to cylinder 1.
• I did find some cracked vacuum hoses and the hose to the vacuum canister near the firewall on the backside of the fender was not connected. The hoses to the HVAC controls, drivers side valve cover, hose to the PCV valve and the PCV valve itself, and the hose connecting those to the check valve to the vacuum canister were all replaced with new components.

Now's a good time to note that the EVAP purge solenoid near the gas tank had been replaced a couple weeks ago as well.

Let's recap:

• The issue started in early June while daily driving vehicle prior to any changes being made whatsoever
• The misfire is intermittent and only at idle and throws a persistent P0301
• New AC Delco iridium plug and AC Delco wire on cyl #1
• New fuel filter
• New #1 injector
• Verified FPR is not leaking
• Addressed bad vacuum lines and connections
• New PCV valve
• Tested IAC and TPS with no change
• Cleaned MAF
• Compression is 160
• New EVAP Purge solenoid

Based on this quick analysis, I realize I have a more nuanced issue. Seemingly good spark, good compression, and good fuel. At this point I realized I needed to do a more in depth diagnosis. So I did the following:

• Performed fuel leak down and got the following results: 61 PSI primed, slowly dropped to 56 and held for at least 10 minutes.
• I performed a fuel injector load balance test and every injector passed except injector 4 which was interesting. Let's put this aside for now. I will address this later.

As of now, I feel like fuel delivery is not the issue:

• Pressure within spec
• New injector 1
• Leak down passed
• Injector for malfunctioning cylinder passed load balance test.

I then decided to go ahead and replace the AC Delco cap and rotor I had already purchased. Problem persisted.

I also swapped out my ICM and coil with a known good one from my OBS Chevy (I verified they were the same part numbers). Problem persisted.

I mentioned earlier that I had been looking at live data with my Innova scanner (5610). Let's talk about that:

• The only CEL code has been P0301.
• STFT and LTFT are all bouncing between +/- 10% range and they are consistent with one another.
• MAF hovers around 6.5 g’s.
• MAP 42-50 KPA.
• One interesting thing about my Innova (which I am not a fan of by the way) is there is a Global OBD2 and the OEM Enhanced data. Global shows P0301 while OEM Enhanced shows P0300.
• Spark advance seems to hover around 15-17* all at idle.

When I went and showed the vehicle to a potential buyer after disclosing the recent issue, we looked at his scanner that did show misfire counts and cylinder #1 had like 1200+ while the others were around 80ish or so.

It is worth mentioning that my Innova scanner for some stupid reason only shows misfire counts for cyl #3 and #4 and both are 0.

At this point with 2 different scanners, P0301 does seem to be consistent based on LD and DTC's.

I questioned my compression test and yes I know the proper way is to do all 6 and take 15% of the highest value and subtract from the lowest cylinder and that’s the threshold. So I went ahead and rechecked #1 cylinder and got 160 again. I checked cylinders #2 and #4 as well and 160 and 155. For consistency sake, I can go ahead and check the other 3 but even if the highest is 180, they all still pass. Additionally, it’s intermittent and only at idle, so I think at this point it’s fairly safe to rule out mechanical issues, at least with that cylinder.

I also questioned the vehicles ability to accurately identify which cylinder has the misfire, so I started to take some steps to validate if it is actually cyl #1 and here is what I got:

• While pulling plugs #2 and #4 I noticed they had normal wear, light brown color on the porcelain/electrode area. #1 however, was black and wet. Leading me to an ignition problem. But this was a new plug so I questioned if it burned differently than the other 2 cylinders that I had checked.
• I replaced cylinder 1 with another new AC delco plug (same part#), along with cylinders #2 and #4. I put several hours of drive time on the vehicle, and pulled those 3 plugs and plug #1 appears to be black again while the other 2 look new as expected.
  • To eliminate any possible confusion, remember earlier I started by only replacing the #1 plug and wire and left the rest untouched. Now I have replaced the #1 plug a second time along with #2 and #4. #2 and #4 looked great after several hours of drive time but #1 looked black and on the verge of being fouled.
• While the vehicle was running and actively misfiring, I did pull the plug wire for cylinder 3 and noticed a significant change in idle as you would expect. I then pulled #4 and got the same result. I then pulled #1 and only noticed a small or no change at all on idle but a significant change when revved past idle.
  • As I am thinking about it, I didn't give the ECU enough time to throw another CEL for #3 or #4 misfire, so that can be on my TODO. However, the test did show that if cylinder #1 was not the misfiring cylinder then you would have expected the same results as I got with #3 and #4. EG if you pull a plug on an already misfiring cylinder, you will get little to no change.

I then decided that I would check for the common plug wire leakage/arcing on exhaust manifolds etc. from the plug wires. I pulled the #1 (new) wire and was holding it with a pair of lightly insulated gloves while watching spark with a spare plug and it shocked the **** out of me. I continued to observe it arcing through the boot and decided I would change it with a 2nd new plug wire. Before doing so I observed the 2nd new plug wire the same way and didn’t see any leakage. I also changed the plug out with now a 3rd new plug to ensure I started fresh. Problem persisted.

I then, being as thorough as possible, took a new good plug wire from my OBS truck and tried that, problem persisted.

Lastly, I went back over all of my components that I have changed and did find that the new dist. cap was not fully seated. I addressed this and the problem persists.

Let's recap:

Ignition Diag

• So far I have changed the plug wire on the #1 cylinder with 4 different plug wires
  • Original plug wire from another cylinder, good new wire from my OBS truck, 2 new AC Delco wires out of the box
• I have changed the #1 plug with 3 new spark plugs throughout each test to ensure I am not testing with already fouled plugs
• I have visually observed spark with the naked eye
• I have swapped the ICM and coil with known good parts
• I have replaced the cap and rotor with new AC Delco components
  • The removed parts appear to be in good shape as well
• Cylinder 1 plug continues to come up with a black and wet plug while others look as expected
• The plug wires do not appear to be arcing or leaking

Mechanical Diag

• Compression has been checked for cylinders #1, #2, and #4 with results in order: 160, 155, 160
• The misfire has been observed as an intermittent issue only presenting itself at idle
  • Runs just fine on the highway or in town
  • Sometimes the CEL will illuminate immediately, sometimes it will stay off for a few days

Fuel Diag

• All fuel tests have passed except the #4 injector does fall out of range:
  • #4 plug does not look lean after several hours of drive time
  • #4 does not show high misfire counts
  • Fuel trims do not show signs of a lean or rich condition
• Fuel injector #1 is brand new
• Fuel filter is new

Conclusion thus far, additional thoughts and questions:

• I don't think there is a fuel or mechanical issue. I think that it is in some way related to ignition. However, why just cyl #1?
  • I have read some info suggesting that the distributor could be worn but would this effect #1 only? Possibly.
• I have considered swapping the CPS with the one from my OBS but what is with the PCM relearn? I do need to inspect it either way. But then my next question is, can this effect Cyl #1 only?
• Are there any other sensors that I am overlooking?
  • O2's appear to be in good shape except B2S2 is showing a LTFT of 99.2% but as mentioned, the overall STFT and LTFT all stay within +/- 10% and at this point I can't see how this would effect cyl #1 only.
• Based on my overall diagnoses, is there anything I should consider walking back through start to finish a second time?
• I do have 2 OBD2 dongles on their way today to try and check CMP retard with Dash Command. If that is off I will lean even further towards dist. going bad.
  • Hopefully one of the 2 work and I can view a lot more live data as well.
• I do plan to go back over my wires and plugs and want to triple check for any arcing.
• What other cylinders besides #1 can show false misfires for #1?
  • I have read some info about adjacent cylinders causing inaccurate readings for cyl #1 but the only one I haven't looked at so far is cylinder 3.

Again, I know this is lengthy but I am just trying to make sure I am covering everything. Thanks in advance.

 

1. Welcome to the forum
2. Step one for any misfire work is getting CMP retard within spec. If its out of spec that can be the distr gear and/or the timing chain. It is very common to be out of spec on trucks this old
3. Any history on the crank sensor? Any work needs to be shimmed as necessary and the relearn performed.
4. Mist your engine in the dark at idle and look for arcing especially the coil, distr, wires.
5. Look under an old distr cap and look for carbon traces
6. Tell me more about the cylinder balance test and the results
7. Fuel trims have to be reported as all 4 values. You add ST and LT together for a given bank. Please provide those values
8. A full compression test is a good idea. That finds most mechanical problems. If we really get out in the weeds on this then a dynamic compression test as often advocated by Les may be in order.

Lets start here.

George

 

Nice writeup Dillonstackhouse!! George will get you going!

 

Well you guys are going to laugh... #5 wire had a worn spot and was arcing on #1 causing the misfire. Classic example of over diagnosing a simple problem.



I had this feeling, after narrowing it down to it being an ignition issue, that I needed to do a more thorough job of carefully inspecting the remaining spark plugs and wires. My initial mistake was only making changes for the #1 cylinder. I knew #1 was where my issue existed but not where it was coming from, and that is the lesson learned.

For anyone else that comes across this and has a similar issue, here are some other takeaways and considerations:

• The tried and true, fire, fuel, compression diagnoses does work.
  • Too often we come to conclusions too quickly and then we think that it must be some super complicated electrical or weird sensor problem. I understand that that very well can be the case sometimes but don't be so quick to rule things out.
• Go back over your diagnosing steps, even if you do it multiple times; and when you do, start over from the very beginning.
  • If you read my entire post, you will see that I went through the diagnosing of the ignition components probably 5 or 6 times. This is a great example of why you should start over from the very top if you are going to go back through your flow chart of steps. There are actually a few good examples here:
    • 1. I replaced the plug and wire and still had the misfire, but then observed a leaking wire (even though it was brand new). Most people would simply change the wire with a 2nd new wire, but consider this. That brand new plug you just had installed but ran it with a bad wire has now fouled. You change to a 2nd new known good wire but now have a fouled plug. You may have very well found the problem with the leaky wire but you now created a new problem and are beating your head against a wall. Again, start from the very top.
    • 2. I did all of this troubleshooting but never started from the very top of the ignition component flow chart. I started in the middle and went straight to cylinder #1.
    • 3. Even with my compression test, I was trying to reason my way out of doing all 6 cylinders because it felt unnecessary. I get that sometimes we can do quick things and take shortcuts especially if you have experience and can make really educated guesses and I would say that is okay to do but only under this condition:
      • If you run through your quick list of things and come up empty handed, then don't try to logic your way out of starting over and digging into other possibilities. You might get lucky going elsewhere, but if not, you will waste a lot more time by trying to be lazy. Another classic example where trying to be lazy actually makes you work harder.
• Step away and come back
  • You've heard it a million times and none of us listen. Usually when problems with our cars come about, we don't have time to step away. We need it fixed right this second. But... if you hit a point of diminishing returns, you have to learn to come back another day and start fresh.

Okay, moving on...

Thank you Les and George! Ironically, we are not quite done yet because I now am having a bit of a rough start/rough idle when cold. This problem existed while dealing with the misfire but now I am realizing the 2 issues were unrelated to one another. I suspect this issue may have been introduced as I was swapping all of the parts and such around trying to solve the other problem.

George, is a relearn required after swapping TPS and IAC? I read somewhere it was but I need to research more. I also wanted to investigate the screens on the distributor for debris and look for signs of moisture.

As mentioned, I have 2 bluetooth dongles that should be here today. I am hoping at least one hooks up to Dash Command as they have been finicky for me in the past. I can use that to check CMP retard and potentially do the relearn?

• I will check the CMP retard when I get dash command hooked up, I already have the PID for it.
• Any history on the crank sensor? Any work needs to be shimmed as necessary and the relearn performed.
  • I think for now I will leave the crank sensor alone since the misfire is resolved.
• Mist your engine in the dark at idle and look for arcing especially the coil, distr, wires.
• Look under an old distr cap and look for carbon traces
• I am going off of memory because I always rent the fuel pressure tester from O'Rileys but cylinder balance tests were as follows:
  • Injector 1, 2, 3, 5, and 6 would all drop from about 60 primed down to 52 +/- maybe about 1 to 2 psi and on each one you could hear a very audible click.
  • Injector 4 only dropped from about 60 to 55 and was hardly audible.
    • It's too bad I broke injector #1 when trying to get it out or else I could easily swap the new one that I bought for #4 and then use the old but good #1 injector for #4. I think since I am selling it, assuming it runs fine and is not related to the above mentioned rough cold start issue, I will hopefully not have to mess with that but will certainly disclose the info with the buyer.
• Fuel trims have to be reported as all 4 values. You add ST and LT together for a given bank. Please provide those values
  • I am not sure this is relevant now that the miss is gone but just for having data for data sake:
  • The values do seem to sort of jump back and forth around -5/+5% for both banks so I took and average of 3 frames:
    • STFT B1 and LTFT B1 = -5.67%
    • STFT B2 and LTFT B2 = -6.73%
• A full compression test is a good idea. That finds most mechanical problems. If we really get out in the weeds on this then a dynamic compression test as often advocated by Les may be in order.
  • After discovering the wire, and eliminating the misfire, I did not perform a compression test for the other 3 cylinders

More to come out of this

 

Misting the engine in the dark would have found that wire arcing.

I believe that the crank sensor is the only one that needs a relearn

How were you pulsing the injectors in the balance test?

George

 

Yes it would have, I should have started with that first.

I used my Innova 5610 to do the test, it will initiate the prime and then the pulse, 1 pulse per each injector of your choice. Then watched the pressure drop with an external pressure gauge

 

thanks for that I did not know that a consumer innova scanner could do the injection pulses for the balance test. I use a discrete box for that, that’s good to know.

George

 

Yeah, I have a love/hate with that scanner. I had to do a firmware update to view OEM Enhanced data and then that allowed me to do the balance test. It will not do it with my 96 Yukon which uses the same style of injector. I am guessing it will do later 90's models and the 2000's stuff fine but that is just a guess.

 

Near as I can tell, there was a significant change in GM Enhanced PIDs in 1998 on these Blazers, during a PCM design change. For example Car Diagnostic Pro GM Enhanced PIDs work for 1998-2005 but not for 1996-1997. This carried over into the bigger trucks as well. Probably you are seeing the same type of thing on your Innova.

 

Yeah I agree with you Les, I bet you are correct. Update on the rough idle when cold: I reflected on the sequence of events and logically, the rough idle on cold start up only happened after I changed the #1 injector. At the time, it was easily lumped in with the misfire issue but in fact it was a separate issue that I believed to have been created by not changing those upper intake gaskets when I did the injector. I changed those last night, started it up and it idled pretty high. Unplugged the IAC and no change so I removed the IAC and inspected and found a small crack underneath the plunger. I replaced that, idle returned to normal. Started and ran perfectly this morning! I think when I changed the TPS and IAC earlier I made the assumption that they did not effect anything because at the time, I was still chasing the misfire. Process of elimination and we are scot-free. Found a buyer for the Blazer today. I'm sad to see it go but luckily I actually know the guy so I'll at least know where it lives from here on out. I have a feeling he won't get rid of it.

Thanks all for chiming in, I hope the lengthy diagnosis and thought process in word form may help some others with tricky issues.