Looking through most of the “miss” post here I’m not seeing my exact problem. My new to me 98 k1500 is in great condition for its age other than my problematic miss. Heres the problem:
At freeway speed say 70 the engine will have a quick “ millisecond” misfire. It happens 2 or three times in say 15 to 25 miles. Sometime the miss is almost what I would call violent, as if the ignition was turned off for less than a second.Whats been done:
Engine 7000 on the clock, replaces by prior owner at 129 k due to mechanical failure.
Distributer changed as we’re all sensors.
I do not have a scanner and have no idea who could help me here in Yuma Arizona.
Anyone have any thoughts or know any qualified mechanic in Yuma who might help me?
Thanks in advance
I still have the same issue with no fix yet. Working with Blackbear to see if they can find it.
Greetings Srg1016,
I just stumbled across this thread thanks to yesterday's status update. I noticed that
you didn't mention any SES light, so I'm going to assume that you are experiencing
this millisecond misfire yet no SES light / stored DTC codes?
I'd like to first explain how this is possible, followed by a proposed strategy to figure
out the root cause, and finally point you towards a video where a pretty sharp mechanic
figures out one of these real-world intermittent 'fishbite' misfires.
****
Experiencing a misfire that does
not set the SES light or show a stored DTC in the ECU's history
is a genuine possibility, for GM (and all the other automakers) didn't want the SES (aka: Check Engine)
light to exhibit 'The Boy Who Cried Wolf' behavior. So the automakers negotiated with the EPA
various reporting rules that are designed to help prevent 'nuisance trips' of the SES light.
For example, one of the biggest exceptions is that in the land of emissions, the SES light
does not have to be illuminated until the calculated emissions from the misfire(s) would exceed
150% of what the engine calibration is designed to deliver in normal operation. (!)
And for me, the biggest area of confusion is where some 'Type A' DTCs get the light on the very 1st
occurrence of a failure...but on the other hand, there's a group of 'Type B' failures where the
failure must occur on *2* consecutive driving cycles
before the SES light is kicked on. (!)
And how about test failures that can occur but they
never kick the SES light, but do get written
into the ECU memory? Well those mystery DTCs are the 'Type C' jobs. And finally, there are some
codes that are Type A in California, Type B in the Federal emissions category, but Type X for export.
(X = Test not run, will never set the SES light.)
Doesn't this sound like I just made it all up? Well, fortunately I was able to find all this in the '98 FSM,
and would like to share it here in order to boost the credibility of this 'misfire w/no SES light explanation:
You must be registered for see images attach
I find that this chart in the '98 FSM (Vol. 3, p. 1678 in the .pdf) gives me a little insight as to what errors the design engineers consider a
higher priority (or more intermittent prone) than the others. For example, look at how the P0101 for the MAF sensor P0101 is Type A, but
the P0106 for the MAP sensor is a Type B? But if you step up to the beloved 8-lugger (>8600 GVWR) trucks, both of these DTC codes are
a Type C, which means they won't kick the SES light but
will still get stored as a failure code in memory?
But let's make this discussion more relevant to your misfire issue. One of the more famous DTCs in the forum is the one for 'Intermittent Misfires', the P0300.
Check out what kind of DTC Type it is:
You must be registered for see images attach
Guess what? This snippet from the FSM cleared up a lot of confusion for yours truly. Marginal fueling issues? It better be bad enough in order to repeat
on 2 trips or the ECU gives you no hint that it's might be dealing with a edge case. Same thing for the Knock Sensor...or a Misfire condition. (!)
Furthermore, it has to misfire enough on 2 consecutive trips to exceed that engine's emissions spec by at least 50% over the limit. And would you believe
that if your fuel level is below ~ 1/8 tank then misfire reporting is disabled? How about the fact that the VCM (ECU) monitors the ABS speed sensors and won't report a
misfire is it thinks that the roads are too rough? And are there any other DTCs that, if flagged, will also inhibit the DTC P0300 error reporting?
(See attachments #1 & #2 for specifics.)
****
Apologies for the length, but if you ever wondered why so many of these misfire threads end up with the OP ghosting it's partly
because we now know that the misfire DTCs are very conservative in what they report. It's easy to inadvertantly completely block
misfire reporting, and since those 2-3 total misfires while on the highway for 15 minutes won't appreciably impact the overall emissions,
they won't generate a SES light -or- a DTC code. Definitely blocked on the 1st trip. And won't cross the emissions threshold on the
2nd trip. In English, there's a whole class of intermittent misfires that you will feel behind the wheel, but our GMT400s simply won't set
the SES light or kick a DTC.
Q: So if a scan tool that only shares the codes with me won't help, what will?
A: An sharp old-school c r a c k e r - jack mechanic
might find it with conventional methods.
But where do you find one of those that hasn't already slipped the leash?
Enter the scan tool with
live data capability. If you can find a mechanic who gets animated
when you ask if there's anything in the "Mode $06" area about these fishbite misfires, then
you just found the guy who can fix your misfire. (!)
Remember all those rules that were written/coded so that your VCM (ECU) didn't report any
misfires unless all the stars aligned just so? Well guess what, by using the right scan tool we
can look at the raw misfire data (logically organized on a cylinder by cylinder basis) and *we*
can decide whether or not the misfire data is to followed or ignored.
But since I'm tired of typing and you are tired of reading, I'm instead going to have you watch
a pretty savvy pro mechanic walk us through the troubleshooting and repair of a real-world
intermittent fishbite misfire. He is going to look at the car's misfire history in the computer,
test drive the car, stimulate the fault, and with a small number of misfires he is going to
diagnose the issue and positively fix the misfire:
You must be registered for see images attach
(Follow this link to watch this video:
South Main Auto Repair Avoca NY troublesniper)
****
That's all I got for now. I know that it was a long read, and I'm also recommending that you watch a
30+ minute hands-on video...but seeing as how you've had this problem since March and it's now May,
I figured a few more minutes to do some careful problem definition wouldn't offend you guys.
The tighter we can define the problem, the less real estate that we will have to employ spendy
triagnostics and/or swaptronics. Who knows? If we can collect the right data to follow, we might
enjoy the same success as Eric did. And then leave behind a record of all that transpired so that other
GMT400 enthusiasts will no longer have to live with mystery misfires that are below the VCM's stingy
reporting thresholds. :0)
Food for thought. My hope is that we can gather enough data that we can follow that to the source of the misfire.
Or catch it in the act. And if there aren't solid diagnostic dudes in your neck of the woods, then we'll see if we
can look over your shoulder and help you push this fix over the finish line.
Hope you found this helpful.
Cheers -- 
