Hi there, I'm looking for some guidance in diagnosing a misfire that I'm getting. At cold start, the car starts easily and runs perfect. Driving around, everything feels fine. However, once the car is hot and idling, The RPM jumps around a bit and it runs a little rough. Nothing crazy, but enough to be slightly noticeable with vibrations and such. I took some logs using Gary's GD-Scan and it's showing a cylinder 6 misfire when the car is hot and idling. I've attached the log below. I don't have a check engine light for a misfire though. I also notice that the only other time the misfire happens is when I'm at a low RPM in a high gear and give it throttle; "lugging" the engine I guess.

The car had a new crate engine put in a few months ago which now has about 3000 miles on it. The spark plugs, wires, cap and rotor were all replaced at that time. I figured that the number 6 injector could be the culprit, because the injectors appear to be factory which would mean they have a little under 180k miles on them. Today I just swapped the number 6 injector and the number 2 injector around to see if the misfire would follow. It stayed on cylinder 6. I assume my next step would be to pull a couple spark plugs and see what they look like, and possibly try the number 6 plug wire.

Just posting this to see if anyone has any other suggestions or advice. The biggest thing that confuses me is the fact that it is only happening at low RPM (mostly idle), and only happens when the engine is hot. Thanks in advance.

 

After replacing parts because of age, I found one spark plug wire had burnt on the header. I also found a crack in the porcelain of the plug and one plug not tightened all the way. Just things I was able to check easily and for free.

 

I downloaded the file. I haven't looked at the misfire counts in the OBD-2 data, so I need to poke around a little. Appears Cyl #5 has a sympathetic misfire, at a lower rate, that appears when #6 misfires. I'll need to get into it in more detail

 

Can’t stress this enough especially on a Gen I or II SBC.

 

Yeah, that's definitely the next step. Haven't gotten around to it yet just been super busy with work and wasn't my first thought because they're all only a few months and a few thousand miles old. Also put the headers in at the same time so very possible something got burnt under there. Headers are wrapped and the wires have boot protectors on them but they're hard to see down there in the stock location so have to check.

You're right, and I didn't really notice it because I was really just looking at the larger numbers. Just checked and noticed the same thing but even more pronounced in a log that I took after swapping the injectors. I don't really know how misfire detection works, but is it possible the computer is just picking it up on cylinder 6 the most even if it's happening on more than that?

I attached the additional log below. I also just made a quick chart in excel for a visual. Not sure if it's useful but I found it easier to see what's going on at least with the misfire counts.



I'm pretty busy with final exams this week so if I'm slow to respond that's why. I appreciate all the responses though and I will respond eventually as I get time!

 

There are cases in the first log where you have as many as 4 cyls misfiring, and they start about the same time and stop about the same time. Yes, #6 is higher misfire count in any of these events. But what could happen to #6 to spread the misfires to other cylinders. One possibility might be carbon tracking in the Opti cap. Just an example that comes to mind. A single faulty wire on #6 might affect the other wires on that bank of the engine (cross-firing) but they aren't going to affect the cyls on the other bank.

Do you actually feel the misfires?

Did moving the injectors change the high count on #6? (I haven't looked at the new log. Sort of limited on available time right now)

To detect a misfire, the PCM compares the rotational speed of crank position sensor (added in OBD-2, 1996 specifically and only for misfire detection) to the rotational speed of the cam sensor in the Opti. A misfire causes tiny variations in the indicated positions and causes a tiny slow down. It is possible to have misfires falsely reported as a result of mechanical problems with either sensor.

I actually color coded the cells misfires in the spreadsheet of the first log. Very visual indication of the relationships. But it's time consuming.

 

Ok that makes sense. Obviously hoping it's not the Opti but that's always a possibility. Cap and rotor were replaced at the same time as the plugs and wires a few months ago but not going to rule that out.

I can definitely feel the misfires. At idle, once the car gets hot and the misfires start, the vibrations increase a noticeable amount and there is an audible misfire when listening closely to the exhaust. Also at low RPM cruising if I give it throttle it stutters/hesitates pretty aggressively.

Moving the injectors didn't seem to change which cylinders were most problematic. However one thing I noticed is that in the log that I took after moving the injectors, the counts were overall higher. In the first log I sent in the original post (before moving injectors) it seemed like Cyl #6 was topping out at around 40-ish counts before it reset back to 0 and started counting again. In the last log I sent (after moving injectors) the values were climbing much higher, topping out at 80-100. I'm not really sure what that represents or if it actually had anything to do with moving the injectors but it is a notable difference. And I totally understand, no need to explain yourself. Always amazed at how much time you put in helping everyone on here for free!

Ok that makes sense, and is quite interesting! I doubt there's anything wrong with those sensors since I can feel the misfires for sure. Any chance it's the coil or ICM? Just crossed my mind considering it's clearly heat related. Originally I had kind of ruled those out because it seems to be centered around a particular couple of cylinders and I figured they would cause a misfire on all cylinders pretty evenly.

 

Ok so today was probably one of the last somewhat nice days out for the year so I just decided to take the car out for a while, even though I haven't done anything to it since the last time. Neither of my other logs had much high throttle so I figured I'd take one with more varied driving. I also deleted a bunch of the columns that were not supported or redundant info just to clean up the file a bit. Big thing that I noticed that I didn't before was that there's A LOT of knock retard in some spots. Like more than I thought was possible. I don't know much about knock and what causes it but I'm probably going to avoid driving the car until I try some more stuff out because I do know that knock could do some damage. I attached the log below.

Don't worry, I'm not expecting that someone will magically be able to diagnose it in one go just by looking at logs, I just figured I'd throw all the info I have out there so if someone sees this and has any experience with a similar problem they could chime in. I'll update when I get some time to finally start actually pulling stuff apart and looking at the potential issues. Thanks again to anyone that took time out of their day to read this! Really love the community on here.

 

I'll try and take a look at the new file. I'm a bit busy with personal issues, and as you may have noticed, there aren’t a lot of other people here helping out any more.

Just a suggestion..... leave the file alone. If there are redundant columns, or some that may seem irrelevant,I hide them. I don’t delete because you never know when you may need them. I always save an unmolested copy of the file before I go about reformatting, moving columns around, etc.

Did you replace the plugs, wires, cap/rotor in an attempt to eliminate the misfire, or did the misfire develop after the replacements?

If you see knock retard, it means the PCM is doing its job md eliminating knock. But I'll take a look. Typical max limit is 15-degrees.

 

Thanks, I appreciate it. Please, no rush on your end, take as much time as you want. I'm not going to be driving the car much - if at all - in the winter and I certainly don't expect you to go out of your way to try to help some stranger.

Good to know for the next time I log, I won't do that again. I deleted the rows straight in the GD-Scan software so it never even scanned for them so can't add them back into the file, however the software doesn't save those changes so it'll log them next time.

I replaced the plugs, wires, and cap/rotor because a brand new longblock was going in the car and I wasn't going to reuse those old parts from the old engine. It wasn't misfiring before the new engine (that I noticed) and I'm nearly positive it started fairly recently, and that it didn't start right after the new engine was put in. The engine was put in mid-August and as far as I can tell I don't think the misfire started until about a month ago.

The only parts on the engine that aren't new are: The actual Opti part of the Optispark, the coil and ICM, the intake manifold and fuel rails/injectors, the harness and any sensors.

Some of the knock retard that I'm seeing in the log seems reasonable, but a couple times it is upwards of 30 degrees, and even spiked to 109 once, which is why I was confused.

Thanks again for your time and help.

 

I saw those retard #’s with a quick scroll through. They are incorrect/corrupt data, not unusual. This can be caused by electronic interference on the data wiring. In the case of the 100°+ knock retard readings, there are impossible readings on other columns on the line of data, confirming the corrupt data. Additionally, when you see a number in the knock retard column, that value should also reduce the advance in the spark timing column, and that is not happening in the data lines with the spikes of faulty data.

In my analysis, I use Excel functions to identify max and min values in the data range of each column, and that helps me find and eliminate these incorrect values. For example, you might find an injector pulse width of 255 miliseconds.... impossible. Often the entire line of data is similarly corrupt and needs to be removed.

While you have a bit of (small value) knock retard, it is being handled by the PCM reducing spark advance. That will reduce performance, but is not going to damage the engine. When I get to look at the log in detail, I’ll see if I can find the cause of the knock retard. What octane fuel are you running, and what is the elevation above sea level where you live? The latter affects the engine's octane requirement.

 

Ok, that's good to know about the corrupt lines of data.

I'm at only about 250 ft, and I always run 93 octane.

My friend and I have been looking over the data and we noticed some trends with the fuel trims. It appears that a lot of times the bank 2 fuel trim (both long and short term depending on throttle position and load) is adding a lot of fuel during the major misfire events, or during high throttle. The bank 1 trims are fairly steady only really going +/- 5% and never steady in one direction for a long time. We're no mechanics or tuners, but that's making us think a problem with fuel delivery in bank 2. Would we be correct in thinking this?

Is it possible that the PCM is not correctly identifying the specific cylinder that is misfiring? Just asking because we were thinking if that was the case, it could still be possible that there's a suspect injector on 1 or more cylinders on bank 2. And if the PCM isn't correctly determining the cylinder of the misfire, then swapping injectors 6 and 2 would have made no difference because they are both on bank 2.

When I get a chance this weekend I'm going to go grab a stethoscope and just take a listen to the injectors before the misfire starts, and once it starts, to see if there's an audible difference. Not super confident it's that, but it's just a trend I noticed in the data and an quick/easy thing to check while I'm still busy with final exams. Then once I have some more time I'll start digging into the ignition side of things.

 

When I had the same problem your having I put the old plug wire that was on before and all the issues went away. The new MSD wire looked good so I cut the insulation back to check the inside and it was damaged. Not sure if you have checked, but it's very common on these. I always keep the old parts around for troubleshooting.

 

Hi guys, I'm back. Been busy with some stuff and haven't had much time to work on the car. Mrmint, I've been trying to follow your advice but I'm struggling. I've found a spot on my #6 plug wire that there's a possibility it could be close to/touching the header, however I'm at a total loss as to how to get in there. The headers are ceramic coated and heat wrapped, and there's titanium boot protectors on the plug wires. Here's a picture from the bottom showing where it's possible under the boot protector the wire is touching the primary.

I decided I might as well start by pulling the wire off the plug and pulling the boot protector off to see if there's any visible marks on the wire. I can't even get the boot off the plug! I can't fit my hand between the dipstick and the #6 primary to pull it off. Here's how cramped it is from the top.

I've considered going to over the valve cover wires to make them easier to keep an eye on with these headers, but obviously being able to remove these wires is the first step! I'm going to keep trying and see if I find a way to get enough leverage in there to pull the wire off.

 

Do you have a plug wire removal tool? On my other vehicle years ago I ruined the wire pulling it off without any tools. It's crazy how tight that area is and how close the wire is to the header. When I bought the wire shields and installed them I was shocked that they touched the header because they are so close to each other. I used a couple zip ties to pull the wires away from the headers as much as possible in a couple places. It sure takes some patiences to work in such a tight area. I always turn/twist the plug wires on the spark plug a little to help break them loose before pulling on them.