Odd stumble at highway speed, or wind turbulence?

[454cid]

Glad I have AAA. I'm 240 miles from home lol.

Oh wow, would they cover all of that?

 

[redfishsc]

Ok, got my hands on a code reader, misfire #6, was a faulty plug wire (burn through near the manifold). White hazy spot on the outside where it was cracked.

So, I haven't gotten it on the highway yet but I feel confident this was the cause of the odd stumble. Either way a cracked plug wire sure wasn't helpful.


Now that probably doesn't solve the odd rumble when coasting so I'll still do a diff service when I get home.

 

[redfishsc]

Oh wow, would they cover all of that?

Kinda. My gold membership will tow it up to 100 miles to whatever certified AAA location I want. It will at least get the truck to a somewhat secure location. Glad the cats on these Vortecs are so strange that they're rarely stolen

 

[AuroraGirl]

shorten your plug gap .010 and see if you have it still

 

[redfishsc]

Gents, after chasing this odd problem and scratching my head for months I finally seemed to have gotten rid of it.

I changed my cam retard from -1.85 to +2.9 and the stumble is gone. I'm reading the cam retard with an app and Bluetooth obd.

I tried this on a whim after reading other posts of people saying that they had similar cruising speed stumble on these Vortecs and adjusting the cam retard a few degrees positive helped.

I don't think this fixed the actual problem, but rather masking the symptom, which I suspect is an old timing chain and maybe some lash from gear wear on the dist. My dist gear looks fine to the eye, it's been a year since I've looked at it but that's only about 6k miles.


Today I made a 3 hour drive and stumbled a good bit on way, I had a few thousand misfires logged on several cylinders. Whole visiting family I adjusted the cam retard and drove it 3 hours home showing not one single misfire on the scanner.

So, not necessarily"fixed" since technically I'm out of spec on the cam retard but she runs great.

 

[Road Trip]

Gents, after chasing this odd problem and scratching my head for months I finally seemed to have gotten rid of it.

I changed my cam retard from -1.85 to +2.9 and the stumble is gone. I'm reading the cam retard with an app and Bluetooth obd.

I tried this on a whim after reading other posts of people saying that they had similar cruising speed stumble on these Vortecs and adjusting the cam retard a few degrees positive helped.

I don't think this fixed the actual problem, but rather masking the symptom, which I suspect is an old timing chain and maybe some lash from gear wear on the dist. My dist gear looks fine to the eye, it's been a year since I've looked at it but that's only about 6k miles.


Today I made a 3 hour drive and stumbled a good bit on way, I had a few thousand misfires logged on several cylinders. Whole visiting family I adjusted the cam retard and drove it 3 hours home showing not one single misfire on the scanner.

So, not necessarily"fixed" since technically I'm out of spec on the cam retard but she runs great.

redfishsc,

Thanks so much for updating your thread with your experiment with changing
the cam retard. (!) You know, when it comes to the mechanical distributor shaft alignment
vis-a-vis the front crank snout, the more culmulative wear in the
timing chain>sprockets>distributor gear = falling further behind rotationally = increasing
negative degrees electrically. (ie: more wear never = distributor sensor advancing from 0°. (!)

No doubt with brand new everything (factory tolerances) the optimal setting is exactly 0°.
But out in the real world with a little wear/lotsa miles maybe a +2° adjustment
would actually compensate/mask (as you said) the total wear between the nose of the
crankshaft and the sensor in the distributor?

The weird thing is that the relationship between the CMP & CKP signals seems to
be so fussy as to cause various members in this forum all these flaky misfire scenarios?

While I comprehend that from the high voltage delivery perspective we want the
rotor tip to be at it's closest point to the correct cylinder terminal when firing...at the
same time I wonder what is the relationship between these 2 signals in the algorithm
that the ECU uses in trying to set up the ideal spark timing for any given scenario?
Or is it only used to accuratize cylinder misfire reporting & isn't part of normal spark
advance calculations?

Anyway, I'm going to file this trick away for future troubleshooting use. If I can duplicate
the improvement over here in some upstate NY engine bay I'll be sure to update this
thread with the results.

Good stuff - thanks again!