It could be a bad fuel injector. I had 2 clogged with rust and it made the whole truck shake. My partner's 3.4 had one go out too and it ran pretty rough but it did throw a code for a misfire

 

That would jive with my gut saying it's dropping a cylinder but a code sure would be helpful.

 

Guessing MAF and intake plenum (especially the flappers pivot points) have been cleaned? Yeah could be an injector needing cleaning too.

 

I cleaned the MAF because it's so easy to get to on this 3.4 even though live data came back good for o2 sensors and G/PS airflow across the MAF as well as inlet temp. AAnd fwiw it's always had top tier fuel. I consider that cheap insuraance like changing my oil regularly.
I've taken it as far as I'm going too and will let everyone know what it ends up being.
In 50 years of wrenching I've never seen an engine run and vibrate in such a violent manner and still start the next day. The lack of other symtoms via a scan or my inspection is another weird twist. Just wanted to throw the info out there for observation and discussion.
This is a 1 owner, 1 driver truck so I kinda know it's idiosyncracies but not this time.

 

So intake plenum and its flapper friend got cleaned same time as MAF? Some folks will take out the IAC to clean but I just leave it where it is and while wiping the back of the flapper as best as possible some cleaner fluid will make its way down there and do that too. Has worked for me on several 3.4s when idle was crazy low.

And of course ODB2 will show none of this unless significant misfires occur. How many miles btw - my 3.4 is at 357k Luck.

 

Sounds like when I had a faulty fuel injector last year, but mine threw a code.

 

Threw a scanner on it, nothing, no codes and voltages are fine.

This is the part that is weird to me. My truck throws codes of some sort (even just pending codes) any time the idle gets rough or something is amiss.

 

I even borrowed a friends more advanced scaner and it came up clean.
Also how fast it came on, driving on the freeway with no issues and then I'm driving a cheap sex toy. UGH.
It starts,it accelerates and sorta idles but doesn't die.

 

try pulling one by one your plug wires, coil pack, maf wires etc and see if the issue changes at all. If you pull one and find the same problem, there you go.

Id also hook up a bluetooth scanner and something like the torque app to see what your different sensors are reading.

Weird you dont have a code but it could be something that needs more time before throwing a code

 

Put a scanner on it and checked live data and sensor readings with no obvious problem.
The part that has me all cross-threaded is that it just happened outta no where, no warning symptoms etc. Back in 2003 it jump a tooth on an exhaust cam and it's running sorta like that now. I was a Snap-On Dealer and the Toyota Dealership I had on my route was perplexed and they found the one cam had jumped. I swapped some tools in exchange for the repair.
They couldn't explain why it did it either. No codes back then either!
We'll see what tomorow brings when a couple Toyota Master Techs half my age take a look. I'm really too old for this, that's why I need a boat! lol

 

News Flash!
Well my gut was right, the T100 was dropping #3 with no codes!
Here's the kicker (to me anyway) the spark plug, an NGK with less than 40k miles on it, the ceramic insulator broke in half, the center core was what was keeping it together and occassionally firing.
Works for me so It'll get some extra lovin with valve cover gaskets, plugs, wires (they are 30 year old OEM's ) And it's first oil change using a synthetic, Liquid Moly.

And added bonus I found a good Independent Toyota shop in the process.
All reasoanable DIY repairs but between being an old tired mechanic and not having anyplace to work on it anymore I'll support my local small buisness.
I'm just the detailer thses days!

 

I had a similar experience after I replaced the valve cover gaskets, half moons, plug o-rings intake plenum gaskets, PCV, coils, plugs, wires, and gave the throttle body, intake plenum interior, and MAF sensor a good cleaning before putting everything back. It was a big head scratcher, as I was very meticulous about it, so I thought “shit, I must’ve angered one of the injectors somehow…”

turns out I didn’t clean the MAF sensor thoroughly, so the truck got a new set of O2 sensors out of it as well as part of the troubleshooting process. I then ran a full tank with double shot of Techron fuel injector cleaner, and now at 234k miles, it runs as smooth as butter, MPG is at 22.1 with mostly freeway driving. Mine is a 96 3.4 2WD auto.

Moral of the story: make sure you really clean the MAF sensor thoroughly.

 

I'm pleased the problem was found even without any codes but still not sure why the plug on #3 broke the insulator in half. I could have just replaced a single plug and solved the problem but I'm good knowning that getting caught up on my preventitive maintanence, that might eventually have become bigger problems have been cut off at the pass from screwing with my head in the near future.

 

It happens, probably a manufacturing defect, or it was dropped or bashed against something during shipping and handling.

When I removed my plug wires, I discovered whoever changed the wires last time didn’t coat them with dielectric grease, so two of them had to be yanked out piece by piece after I cut them apart, as they were infused onto the wall that cradled the plug wires. That was not fun.

 

A plug?
We've been conditioned to guess that it's a catastrophic situation. Usually, to the point of skipping over the basics, even by seasoned wrenches. Occasionally, when something like this solution becomes clear, it "re-teaches" us to "look at the simple stuff, first."

For the OP's situation, I was going to guess "partially failed head gasket." On it's way to total failure.

Anyway~

(Had the exact same situation, with a plug on my Husqvarna 701, a few years ago.)

Would start and idle/run fine but WFO it would fall on it's face. (Twin plug system, where 1 plug worked upto 1/2 throttle, then BOTH plugs work at WFO.)

Solution? Replace a cracked insulator/plug. Odd thing was nothing (that I know of) impacted the plug as it's relatively buried...)

I was thinking fuel injector or fuel pump or a sensor somewhere. It was a friggen PLUG!!

Talk about feeling like a dunce!

 

This whole thing reminded me of a customer who brought their car into the shop I worked in. My boss asked him what was the problem and he replied, "My car is running hot". Boss asks, what makes ya think that is it puking coolant or is the gauge reading hot, running bad? Customer says Oh no it's running fine but I was just checking things and noticed my plugs were hot!

No idea how my Boss kept a straight face but he just told the guy to have a seat in the waiting room and we'd check it out for him. 15 minutes later I gave him the keys and said everything was with in specs and not to worry. Another Happy Customer.
True Story!

 

Weird it wouldn't throw a code for a clear misfire. My hypothesis is that the misfire detection on the 5vzfe is actually just a resistance check on the injectors. Has anybody seen a misfire code that wasn't a injector?

 

I have. One of the spark plug wires on my truck went bad, resulting in a misfire code.

 

That's good to know, sounds like it can tell if there is continuity to a spark plug but it can't detect a faulty sparkplug?