Mystery misfire

So I was driving home the other day and my 04 started having a noticeable misfire when getting on the throttle. It wasn't terribly noticeable while just cruising, but it seemed like it got progressively worse on the drive home. It would stumble a little taking off and actually even pop out the exhaust some while taking off, which was new to me. I'm used to it doing it a little on coast down, but it's never done it while taking off. I limped it home and parked it until I had a better chance to look at things tonight and now I'm even more confused what might be going on.

I hooked my DRB III up and it was showing a one time trip misfire code on cylinder 7. I pulled the fuel rail and inspected the injector and even tried swapping it with cylinder 9, but that is a bit inconclusive to me. On the test drive it still drives terrible with a noticeable miss, and the DRB III still had a cylinder 7 code. I also tried swapping spark plugs and using an inline spark tester and I don't see anything obvious going on. I did a compression test on cylinder 7 and 9 as well and both made it up to 120 psi before I stopped cranking. i pulled the valve cover as well to see if maybe I had a broken spring or something else going on, but again, nothing conclusive.

What annoys me at the moment is that the car has a very obvious misfire while driving, but it's not setting a check engine light or even giving me the one time cycle misfire code it was giving me before. I'm not sure how long of a drive I have to do before it will set a one time code though, so maybe that's part of the problem. Now it's even starting to pop the exhaust while idling or on a quick rev when it's hot.

So at this point I'm still leaning toward a spark issue, but something feels a little off because I know the cylinders fire in pairs, and I'm only getting a single cylinder misfire. I would think if the coil or wire was bad that it would present itself on both cylinders, but maybe I'm misunderstanding how the system works. The car has a cam, exhaust, and a tune from Viper Specialty and has run great up to this point. I just can't figure out how it can run so obviously off without some kind of code or error setting. Any ideas before I just start throwing parts at it? I put on a set of AB Quality wires a few years ago, though I can't recall how many miles I had at the time. Maybe 50k on them since the swap? Plugs looked okay, nothing crazy like being soaking wet or black. At this point I'm thinking about just putting a fresh set of coil packs on it since I've got 140k miles, but I don't want to just throw money at the problem if there's something more obvious going on.

After your explanation, I'm leaning towards faulty coil. At 140k, (outstanding miles by the way), probably wouldn't hurt to swap those out. They aren't particularly expensive and would round out your spark diagnosis.

^^^^ I would lean in that direction too. By now my Xtool D9 would have been on it and have recorded what's going on electrically while driving.

I was trying to do some injector kill testing with my DRB III while idling in the driveway and it's kind of crazy how little effect killing a whole cylinder has on it. It's maybe a 50 rpm drop, but it's a choppy enough cam and loud enough exhaust that it's surprisingly hard to notice. I think you can occasionally hear the bad cylinder cut in as well though as the idle will sometimes pick up a bit and sound different.

It's kind of weird since the inline tester was showing consistent spark, but I guess that doesn't necessarily mean it was a good spark. I was hoping the DRB would have some more coil testing functionality, but it's a bit limited in that department. There is a misfire monitor where you can adjust the sensitivity from what the computer normally watches for, but it wasn't reporting anything when I had my dad test drive it with me in the passenger seat watching. I need to read up on it though and see exactly how it works and how they expect you to use it.

Popping from the exhaust does indicate raw fuel in the exhaust being ignited. I would lean toward the ignition coil or plug wire. If you still have cats, you should fix this soon as fuel on the cats will destroy them.

MoparMap said:

I was trying to do some injector kill testing with my DRB III while idling in the driveway and it's kind of crazy how little effect killing a whole cylinder has on it. It's maybe a 50 rpm drop, but it's a choppy enough cam and loud enough exhaust that it's surprisingly hard to notice. I think you can occasionally hear the bad cylinder cut in as well though as the idle will sometimes pick up a bit and sound different.

It's kind of weird since the inline tester was showing consistent spark, but I guess that doesn't necessarily mean it was a good spark. I was hoping the DRB would have some more coil testing functionality, but it's a bit limited in that department. There is a misfire monitor where you can adjust the sensitivity from what the computer normally watches for, but it wasn't reporting anything when I had my dad test drive it with me in the passenger seat watching. I need to read up on it though and see exactly how it works and how they expect you to use it.

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Can you record with your tool? What I do is throw my tool on the Passenger's seat and take it for a good ride and then go back home and look at it. The good thing with my tool is it connects VIA blue tooth so there are no cables draped over my foot while driving.

By any chance, did you check the resistance on plug wire #7 ?

daveg said:

Can you record with your tool? What I do is throw my tool on the Passenger's seat and take it for a good ride and then go back home and look at it. The good thing with my tool is it connects VIA blue tooth so there are no cables draped over my foot while driving.

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It has some recording capability I think, but I'm not sure I can get much for useful data out of it. Part of that is just the limitation of the JTEC controller as it doesn't exactly have a ton of fields to read out. I think I really almost need something more like an old SunScope that can hook onto the high voltage side of the coil to see what the spark energy looks like. There is very little ignition based data that you can see from the JTEC aside from advance and crank/cam sync. I can see injector pulsewidth as well, but that's just the commanded value, not necessarily what is actually coming out of the injector.

Steve-Indy said:

By any chance, did you check the resistance on plug wire #7 ?

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I haven't yet, but more just because I was trying everything I could before I pulled the intake. I'm going to have to pull it if I swap new coils in anyway, so that's likely my weekend project and what I'll check next. I thought about trying to sneak in there with a long pair of needle nose pliers, but I figured even if I got it unplugged I might not be able to plug it back in. I may still have my old original wires kicking around as well because I have a problem throwing things away, lol.

I recall tracing a misfire of one plug on our 99 only to find that the one OEM plug wire's resistance was WAY high. Changed wires, problem solved.

I would do a fresh set of plugs. Then wires. Do you know the age of the wires?

This will make it easy to change wires without removing the manifold.

I have some new OEM wires if you need a replacement.

Wires were replaced with ones from Qualitywires.com, looks like maybe 10 years ago (at least based on an old forum post). I pulled the intake over the weekend and checked resistance on them and nothing looked that different. New coils and plugs should be showing up today, so hopefully that will get it. I did notice some remnants of mouse under the intake though, but didn't see any damage on the wiring harness. One of my cats actually caught a mouse a couple months ago, so guessing it was probably the same one. I'll check the rest of the harness over a little closer, but given that I've been driving it for months without issue and it only cropped up the other day, I'm inclined to think it was still a part failure and not a rodent issue.

Rodent issues, all wires in the last 40-50 years are bio-degrade type.

Many modern car manufacturers use soy-based (not direct sugar) wire insulation, which is eco-friendly but notorious for attracting rodents. This soy-based material is used to improve biodegradability and reduce reliance on petroleum. As a result, rodents like mice and squirrels chew on the wires, causing significant damage and costly repairs.

@pokeyl they changed that a little by adding bitter stuff to the soy based installation.

@MoparMap depends on the type of spark tester you are using.
Spark is different under compression.

Goggles Pizano said:

@MoparMap depends on the type of spark tester you are using.
Spark is different under compression.

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Yeah, I had two different ones and they gave inconsistent results between them. Both are just inline neon tube type testers or whatever kind of bulb is in there. One showed a pretty consistent pattern and intensity, but the other one seemed like it would have some dim flashes on and off, so it was inconclusive to me. I did try running up the throttle a little bit and my dad tried slipping the clutch a tad to get some load on the engine while looking, but hard to really say.

Coils didn't show up yesterday like I had hoped, so no progress so far. I may give all the coil and injectors wires a good once over and continuity check back to the PCM while I wait and I've already got the intake off. It's a pretty short run though and all of the covering is still intact that I've checked so far, so it seems unlikely that I've got an issue, but it could also just be a break after years and years of heat cycles.

I never used yet but this allows you to adjust the spark gap.

I really doubt it is the coil as cylinder 7 fires the same time as 4 from the same driver and there are only 3 individual coils in one coil pack and 2 individual coils in the other pack.

Run the engine in the dark and look for wire arcing. Pull plugs 5, 7 and 9 to compare them. Look carefully for signs of ceramic cracks, or grounding paths from wire/boot to plug heat shields.

Goggles Pizano said:

I really doubt it is the coil as cylinder 7 fires the same time as 4 from the same driver and there are only 3 individual coils in one coil pack and 2 individual coils in the other pack.

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I was thinking the same thing, but I'm kind of running out of ideas at this point. Swapping injectors didn't seem to make any difference, though I guess I could still potentially have a driver in the PCM going out. You can hear it cutting in and out at idle every once in a while. I tried a fuel injector test with the DRB III, but I didn't hotwire the fuel pump, so all I could confirm was that it was clicking, not how good it was flowing. I might try again now that I don't have an intake in the way.

I went after the three typical things you do with any combustion engine of air, fuel, and spark. Air seems fine as best I can tell since the valves are moving and it's making compression. For fuel and spark I tried swapping plugs and injectors to the bad cylinder (at least as determined by the computer) and didn't see any difference, though I am beginning to wonder if it's really isolated to just one cylinder or not. The main reason I say that is because the DRB III only reported the one cylinder having a misfire after it acting up the whole way home on a 1+ hour highway drive, but it never reported it again even when the car was running poorly for subsequent test drives around my house. I'm a little skeptical of what it takes to trigger the misfire code at this point. I even tried the "misfire monitor" test on the DRB III, but I can't find any documentation explaining how exactly it works. It just shows a misfire counter for each cylinder and lets you adjust a sensitivity value, but it didn't matter what I had it set to as it never counted a misfire even when the engine was running noticeably poorly.

I figured coils were cheap enough to swap out and probably couldn't hurt at 140k miles. I can't recall if I did plugs at the last rebuild or not (spun a rod bearing one year, rebuilt the engine, then two years later it did it again and shattered a rod, so it got another rebuild), but same story there where it's cheap enough to just replace them. 7 and 9 both looked minorly darker on the insulator than 5, but both were still a nice tan color with a white portion on one side. The car has also had some startup issues from time to time where it will crank for a while and only start very weakly, almost like it's literally only lighting one cylinder at a time before it picks up. It almost sounds like a hit and miss engine when it starts like that. Other times it will start almost before I even get the button pushed all the way down.

GTS Dean said:

Run the engine in the dark and look for wire arcing. Pull plugs 5, 7 and 9 to compare them. Look carefully for signs of ceramic cracks, or grounding paths from wire/boot to plug heat shields.

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You can also spray a fine mist of water to help arcing.

why didn't I think of that? Wait - I did.

GTS Dean said:

why didn't I think of that? Wait - I did.

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I know that you know, but for the others reading that don't know.

So went to put in the new coils last night and naturally one doesn’t fit as it has a different keying on the connector, so kind of back where I started. I checked all the electrical connections I could and everything seemed consistent. All the injectors and coils had good resistance back to the PCM, all the plug wires were pretty consistent, and all the coil primaries and secondaries were consistent with each other as far as measurements, so I put it all back together with the original parts and new plugs. I did find the #9 and 10 plugs were very dark though. I’m not sure if the JTEC has individual cylinder trim capability or not, but I’m wondering if maybe they were just fouled and running inconsistent? Took it for a quick test drive around the block and it feels like it's more back to normal, so I guess maybe sometimes the simplest solution is the easy one?



I think the shiny spot on the second one was probably me hitting the threads on the way out of the cylinder.

MoparMap said:

one doesn’t fit as it has a different keying on the connector

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Had the same issue when I replaced mine, I just cut the key off the new coil.

Similar situation on the Delphi coils I installed. Little trim with the dremel made short work of any fitment issues for the connector. Glad some clean plugs sorted her out.

BoondocSaint said:

Similar situation on the Delphi coils I installed. Little trim with the dremel made short work of any fitment issues for the connector. Glad some clean plugs sorted her out.

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Delphis where what I had bought as well. I was thinking they were going to be a different Delphi though, so I was a bit disappointed when they showed up. I'm guessing they are probably still fine and the only thing that really makes the Viper ones unique are the different cylinder numbers stamped in the top. The Delphi ones were for a 4 and a 6 cylinder, which didn't surprise me at all, but I was a tiny bit disappointed since they were labeled "premium", according to RockAuto at least.

I noticed the same thing, standard vehicle cylinder order stamped on them. I thought they would connect straight up, but they did need a little massage from the dremel to clip in properly. Figured that was just a demon coil thing to fit the connector, but guess not. They do work good though, new coils, wires, and plugs made this car noticeably happy.

If I do end up replacing them down the road I'll probably try NGK next, though I'm not sure if I remember them offering both packs or not. Maybe that was one reason I picked the Delphis originally. Seems like very few brands made both coils.