Yep, you're guessing.

Misfires on lots of modern trucks are most noticeable when "lugging it" in OD. As you start to tip into the throttle (gently) on the highway it'll present as fishbiting (Ford modulars) or horrendous shaking (2nd Gen Rams)

I was finally able to diagnose a '98 by driving it long enough under the conditions which caused the symptoms until I FINALLY got a PENDING code for a misfire on #8. Checking resistance of that plug wire revealed it was bad.

 

can you explain how a misfire, which happens at any rpm, shakes only the transmission in OD and goes away by speeding up or slowing down so the transmission shifts out of overdrive BUT the engine runs perfectly the entire time on 7 cylinders?

 

It's an automatic trans, so, if you AREN'T in overdrive, chances are good the TCC isn't locked either, so there is a fluid coupling between engine and trans. That dampens 99% of potential vibrations. TCC and O/D happen rather close together, and then you have a direct mechanical connection between engine, and the rest of the driveline, and in overdrive, the gear reduction acts as an amplifier...... So, once you hit lockup, and O/D, any vibration from misfires becomes patently obvious.....

Have a look around the tech section here. There are NUMEROUS threads about vibrations in o/d, that get traced back to engine misfires. Mostly due to blown plenum gaskets.

 

and I'd add it also has to do with RPM and engine load. Ever have an engine that idles kinda lopey but if you rev it you can't really tell? As the speed of something intermittent increases it "blurs" and blends -- at least to human perception.

As for engine load the engine's essentially working its hardest "lugging" in OD -- think trying to climb a hill on a bike in your big ring. Downshift and/or add torque converter slip and while RPM increases, engine load drops because of the miracle of gear reduction....not unlike granny gear on your bike

 

Hey folks, I have a 96 4x4 short bed with the 318 auto trans. I'm having a heckuva time figuring out what's going on. So initially I developed a terrible vibration when I hit OD, I mean like shake you off the road bad. Went away when I turned OD off so I did a spark test, had a bad plug on #6 and changed it. Smoothed right out for about 3 days. Now when I hit OD at low RPM it'll vibrate but not as bad, but still irritatingly bad. Interestingly enough, if I lock out OD it will smooth up a little from about 50 to 60 mph, like it did with the bad plug, but at 60 it'll jump right back into the bad shaking no matter the RPM. That didn't occur when the plug was bad. It would smooth out basically at 2k RPM when OD was locked out...but now that's not even doing the trick. I've heard talk of a plenum gasket? Also, I'm having some fuel leak down issues, it takes a bit to start (engine actually starting) after sitting for a few hours, but not overnight, and not immediately after shutting off and recranking. This seems like a fuel/air issue? And if it is could that be the plenum and causing my "miss" or wobble? Sorry for the book, just kinda at a loss.
for reference, it's in need of a tuneup but spark is good and wires are not arcing. I use mid grade gas religiously, 89+, mobile 1 full synthetic since it's first oil changed (truck was bought new)
check engine light is NOT on. (Did come on when plug went bad but has since gone off after swap)If anyone can help I'm happy to answer questions. Ideas? I really don't want to start throwing money at it.

 

Drive it a bunch while it's doing it then keep checking for pending codes. You may find another misfire.

You are NOT experiencing DW

 

appreciate the response.
That's good to know on the DW front. I think it would throw a code faster if I drove it in a way that forced the miss/vibration to be frequent, so I may get it on a long flat stretch and keep it around 55 in OD for a few minutes...if I can tolerate the vibration...ugh.
If it absolutely doesn't throw one, any suggestions on the next thing I should check? I'm about to go check my CC vacuum and maybe look down into my TB for oil. I just have a funny feeling it's my plenum gasket. It's never been changed on 200k miles.

 

Remember, if there are no stored codes check for pending codes.

I have not personally experienced a plenum gasket cause your symptoms but if others say it can, ok.

 

Yeah, that's a misfire on one or more cylinders. Did you change ALL the plugs? What did you put in??