Computer Question: Need explanation of what Current Missfire is?

1996 Chevy Blazer 4WD S10 - Vortec V6
Currently, on the outside, my engine appears to be running GREAT!

I can step on it hard, let it run a few KMs up a mountain with no issues.

I am not seeing any diagnostic trouble codes, and no evil flashing Check engine lights (normally associated with a missfire issue).

At idle, and through the above, the engine sounds smooth, no issues as near as I can hear.

When I connect the Autotap I have access to the missire information. A complete list of GM enhanced parameters (http://www.autotap.com/enhanced_parameters_gm.asp) is also available.

When I look at the missfire values:

Cylinder Mode Misfire Index, Number of Normals all show ever changing / rotating number. They seem to increase if there is a loose plug wire and such.

Misfire Current #1-5 = 0
Misfire History #1-5 = 0

So Cyl 1 through 5 show ZERO's. If I pull a plug wire on Cyl #1, these numbers jump up to around 200.

Misfire Current #6 = 177 usually at idle, or hard running
- This value will shift around, sometimes lower, sometimes higher
numbers. It appears to be active and working.

Ok, so we see 177, which is obviously 'pretty bad' considering pulling Cyl#1 wire shows around 200! Now here's where we get into the major contridiction...

Misfire History #6 = 0 always: Idle or hard running
Number of Misfires = 0 always: Idle or hard running

What the heck does all this mean???? :banghead: :confused:
If there is someone that could please explain to me the definition and operation of "current Missfire", I'd sure appreciate it.

Regards,
----- Robb -----

Ever done a tuneup?

Ever done a tuneup?

A fuel injected car doesn't get 'tune ups' aside from new Spark Plugs (2 days old), Wires (jacobs, boots are fried), and O2 sensors (3 weeks old).

At this point, I believe the problem is one of: the plug wires since I have seeing arcing through the boot, to the block on Cyl #6, or an injector issue. I haven't checked the compression on it yet.

Result: I am getting ceramic booted wires to deal with this issue.
I just pulled out the 3 week old NGK plugs, and installed brand new AC Delco's gapped out to 62 thousandths. It's going to take me a few weeks to get the new wires in (custom order due to jacobs ignition system).

That being said, I am asking if anyone could explain how I can have a current missfire value of 177 with no engine missfire code being set. I would really love it if someone could please explain what the "Current Missfire" is, and if possible how it relates to the other values.

Regards,
---- Robb -----

I'm far from a GM expert, I'm sure people like ricksza can answer more of this. But looking through the GM book I found some things.

First, a misfire code states that "After 100 combustion events, the VCM determines if any misfire totals have exceeded a calibrated maximum number. If the misfire is excessive, a DTC is set.

So it sounds like there is a threshold limit.

In regard to the Misfire Current Event:

"The scan tool displays the number of misfires detected during each 200 crankshaft revolution sampling period. Each cylinder has its own register to record misfires that have occurred. If the misfire current cylinder is incrementing, it will be reset to zero after 200 crankshaft revolutions. If the misfire DTC has set, the number of misfires in the register will be added to the misfire history cylinder register each time the current cylinder register is reset to zero."

This is the "Misfire History":
"The scan tool displays a record of misfires that have occurred since the misfire DTC was set. This parameter will be summed with the current cylinder register each time the 200 crankshaft revolution sample is reset to zero if the current misfire test block exceeds an allowable misfire threshold."

A fuel injected car doesn't get 'tune ups' aside from new Spark Plugs (2 days old), Wires (jacobs, boots are fried), and O2 sensors (3 weeks old).

At this point, I believe the problem is one of: the plug wires since I have seeing arcing through the boot, to the block on Cyl #6, or an injector issue. I haven't checked the compression on it yet.

Result: I am getting ceramic booted wires to deal with this issue.
I just pulled out the 3 week old NGK plugs, and installed brand new AC Delco's gapped out to 62 thousandths. It's going to take me a few weeks to get the new wires in (custom order due to jacobs ignition system).

That being said, I am asking if anyone could explain how I can have a current missfire value of 177 with no engine missfire code being set. I would really love it if someone could please explain what the "Current Missfire" is, and if possible how it relates to the other values.

Regards,
---- Robb -----

Sorry, but there is NO difference between tuning up a fuel injected vehicle than a carb fueled engine.

You still need to change the:

Cap & Rotor
Air Filter
Fuel Filter
PCV Valve

I don't know who got the fact that fuel injected vehicles only got O2 sensors and plugs and wires.

Sorry, but there is NO difference between tuning up a fuel injected vehicle than a carb fueled engine.

Curb idle, idle-air mixture set screws, choke settings, points (older engines)? Not to mention metering devices inside the carb that could easily become plugged. Float level adjustment and fast cam adjustments also come to mind. Granted, not likely some things checked and adjusted on a regular basis, but stuff I considered as a tuneup back then. Obviously, all that stuff is different with fuel injection, where computer controls adjust for any difference in weather and adjust to that engine.

I'm far from a GM expert, I'm sure people like ricksza can answer more of this. But looking through the GM book I found some things.

Well I really would like to thank you for taking the time to check it out for me! :)

Do the missfire DTC's only get set if the engine is under a load? Do you see anything like that in the book?

I've pulled the plugwire totally off my #1, and not seen any missfire codes. No missfire history being set. I have no doubt that if I drove it like this that I would see a missfire DTC on #1.

While this was being done, I did see a 'sawtooth' graph show up on the scan tool, increasing by 1 up to 200 and resetting to 0, just like you described.

Where I 'lose-it' is where I have a cylinder that shows a steady number like 177 on it's current missfire register under varied conditions? It almost seems 'too stable' to be real problem to me. Any ideas? I'll play with it a bit more when I get home.

Regards,
---- Robb ----

Curb idle, idle-air mixture set screws, choke settings, points (older engines)? Not to mention metering devices inside the carb that could easily become plugged. Float level adjustment and fast cam adjustments also come to mind. Granted, not likely some things checked and adjusted on a regular basis, but stuff I considered as a tuneup back then. Obviously, all that stuff is different with fuel injection, where computer controls adjust for any difference in weather and adjust to that engine.

I know that, but I am talking about tuneing up the engine when it comes to the normal componnents other than the fuel delivery.

The following applies to that misfire code:

If start-up ECT is below 20F, misfire detection is delayed until the ECT is greater than 70F. If start-up ECT is greater than 20F, misfire detection begins after a 5 second delay.

Here are conditions:

No active VS sensor DTCs
No active TP sensor DTCs
No active MAF sensor DTCs
No active CKP sensor DTCs
The fuel level is greater than 10%
The engine speed is between 450-5,000RPM
The system voltage is between 11-16 volts
The throttle position is steady within 2% for 100 milliseconds