Intermittent Miss: I need advice on how to diagnose its cause

[XH29N0G]

I have an intermittent miss that I found out is recorded both at idle and during run by spikes in my a/f reading (see images below).  I think the a/f spike/drop just indicates that it does not fire.  The upper trace was at idle at 1100 rpm.  The lower trace starts at this idle, and then I adjusted it down, first to about 1000 rpm, and then to about 800 rpm where it becomes more pronounced.  I think the engine should fire all 8 plugs several times per second, so the spacing between misses suggests to me that it fires cleanly more often than not.

What are the steps that I can take to identify what is the cause of the miss.

The car has an MSD 6AL ignition, MSD distributor set at 23 degrees at idle, MSD blaster3 coil, Firecore wires, and NGK 7 plugs gapped at 0.045.  When I pulled the plug it looked very clean.  Idle A/f is just under 14, but that is what it likes (10 inches of vacuum is the maximum).

[cdr]

richen it up & see if it changes,  HP type cams that have lots of duration  wont have a smooth AFR at low speeds. what cam is in it? do you feel a miss ? or is the cam a rough idle cam?

[XH29N0G]

CDR,

Thanks,  I am virtually certain it is a miss instead of the cam.  I just tried your test of richening the mixture and did not see a change in the miss. 

I did notice that the miss was not there for the first ten minutes of warm up when I first started it this afternoon and only returned once I took it out for a ride and the car fully warmed up.  I also sometimes think I can sense a miss when it is running along on the highway after I run it up WOT one or more times.  This behavior started a few months ago, but today it was much more noticeable than before. I do not know if either of these is a clue.

I am going to pull the plugs to take another look once the engine cools down, but I am wrapping up for today.

To answer your question about the cam.  The cam is not too radical ~511/.518 lift 236/246 duration 280/290 seat to seat 112/105 hydraulic.  When there is no miss and it is running well it is pretty smooth.  I idle at 900-1000 rpm 10-12 inches of vacuum, but it will idle down to 600 or so RPM.  It is not like stock, but not radical.  The engine is a 451 ci B stroker with RPM heads.  It has been very good for me so far, and the board has really helped me solve problems when they have come up.

[MoParJW]

Just a guess: oil fouled spark plug?

EDIT: nevermind I missed the part where you already pulled the plugs  .

[XH29N0G]

What is the likelihood that the symptom is an issue with the coil?

I have heard with regular misfires that it is possible to use a timing light to identify something with a cylinder or wire.  

[BSB67]

Just a guess: oil fouled spark plug?

EDIT: nevermind I missed the part where you already pulled the plugs  .

Even if they look good, I would probably still replace them, and gap them at 0.035" and see what you get.  Pretty cheap, and you are half way there if your pulling them anyways.

[XH29N0G]

They were just replaced (I went to a cooler range plug 7 instead of 6 and then to a wider gap 0.045 instead of .038)  I had this behavior a little before changing the plugs though, but as I said above, it was mostly after doing WOT tests.  I don't know whether the gap could be the issue?

This morning I pulled another plug and it looked clean, even after all the idling I did yesterday during tests.  I also tried the timing light on every wire but did not see any evidence for missed beats, even though the intermittent miss was coming and going throughout the test.

I am going to take my phone and take it for a test drive.
 

[XH29N0G]

BSB67, CDR, MoParJW,  Thanks for your feedback.  It looks like the problem has been resolved.  I removed the plugs and reduced the gap to 0.038. I just took the car for a long spin and did not see any of the intermittent Miss.  I suppose it could have been the gap or something related to pulling off all plugs, plug wires, and then reinstalling; I did not notice anything awry with either.

[cdr]

 

[John_Kunkel]

Reducing the gap takes some load off the plug wires. A scope would show plug wire leakage.

[cdr]

a little soapy water in a spray bottle, with car running, mist all around dist cap & wires all the way to the spark plugs & listen for a spark ZAP.

[cdr]

also some valve covers a right against the dist cap & I have seen intermittent spark leakage in this area, the soapy water will show it.

[XH29N0G]

Thanks for the added information.  I don't know where I would get a scope, but knowing about it is good information.  I can find soapy water.  Interesting approach.

[c00nhunterjoe]

Run the car at night with no lights in the driveway and take a peak under the hood.

[XH29N0G]

Run the car at night with no lights in the driveway and take a peak under the hood.

Thanks,  I have not done that with this set of wires, but have in the past.  Right now it looks like I have a fix with the smaller plug gap.

I had another thought.  The person who set up the car cable tied the wires to each side into bundles of four that then separate once they get to the head.  While my intuition tells me it is not the case because the plug gap (0.045) is smaller than the thickness of the plug wires, I wonder if this could have been a factor contributing to the intermittent miss.   

[c00nhunterjoe]

If it was an ongoing problem, you can carefully seperate the wires and look for carbon tracks.