Tacoma v6 2013 manual stumbles slightly around 800 rpm (2nd gear) oddity. Found a Strange fix.

my Tacoma 4.0 2013 manual stumbles slightly around 800 rpm usually 2nd gear leaving a stoplight.. it's just a oddity.

when I first got it, the stumble was so slight that it would been impossible to bring in to the dealer. I even wondered if I was really detecting it.
it got worse over time.. so I always noticed it.. never really got upset about it..

but it's not like it interferes with driving or anything... I replaced everything as a hobby tracking this down mainly out of just intrigue,

Replaced:
plugs, Mass Flow Sensor, throttle body, replaced accelerator pedal, VVT solenoids, replaced power packs, gasket on lower intake manifold, changed all 6 injectors..

all does sound expensive but it wasn't really, and was easy to do. (plus I learned a TON about how newer engines work) which made it totally worth the effort.

Anyway At 80k now and although the stumble is worse.. pretty pronounced... (hasn't stopped the world from turning) it occurs on about 3 of ever 4 takeoff's from dead stop.
I typically ignore it.

ok now...
I tell this sordid tale to anyone who professes to be a car guru.. (which must be like 200 people by now) and finally at 85km someone suggested just trying to drive it with the fuel cap unscrewed.. I laughed.. yeah right... thats just idiotic..

and HOLY CRAP the 800rpm stumble stopped happening!!!!

I was totally dumbfounded.. I ran the entire tank of gas thru trying to make it happen and it just wouldn't happen (aside: evap warning did come on eventually). on the next tank I screwed in the gas cap and bam 800rpm stumble started happening again..

Heres where I need the tacomaWorld help... WHAT IS HAPPENING HERE? how could unscrewing the gascap fix (fix being a stretch, coverup is more likely) this issue? the gasCap seems to function normally. it allows air in when sucking on it. (yes I looked like a real winner sucking on my gascap in the walmart parking lot)

whats going on???

 

I don’t know enough about how gas caps work, other than I gotta unscrew it to put gas in, then screw it back on.

My guess is it releases vacuum when it reaches a certain point, and doesn’t release positive pressure. Maybe yours isn’t releasing vacuum like it should. If you don’t already have one, upgrade to a locking gas cap. It’s one of the first things I did when I bought mine.

A tip if you do replace with a locking gas cap, and end up with one with a key that goes in one way only, meaning it has the grooves part on one side instead of both. I took a file and put some grooves in the cap so I can look at it and know how the key goes in instead of having to try, maybe get it right or maybe have to turn the key 90 degrees.

 

Start off in 1st gear

 

lol.. yes it happens on the shift to second and clutch out..... funny.

 

Very much spitballing here and could be very wrong, but could your fuel pump be going out and it's stumbling to switch from the low pressure to high pressure modes? I know 2nd gens have the fuel pump resistor/magic that switches it between the two "speeds".

 

Does reving it out a little in 1st help. 800rpm close to lugging the engine.

 

it's interesting.. you can toy with the throttle and avoid it, most people are over 1000rpm on the shift to second.. personally the way I drive I hit 800rpm in my sequence..
this is my 4th toyota pkup/tacoma (93,97,2006,2013) and I drove them all the same way.. and this is the only one that does it.

I've noticed on these that terminal lugging is around 550rpm

 

Ive never heard of this 2-speed fuel pump thing.. I'm going to research that!

 

Like I said, it's a wild ass guess, but who knows, worth doing some more research on.

Heres an older post that describes it in more detail than I did:

 

I’m not sure how you guys are getting the engine rpms that low. When op said 800, I figured it was a typo and he meant 8000. It must idle somewhere around 700-900. So what are we doing, shifting at 1200? I already know I have a bad habit of not shifting soon enough, hitting the rev limiter too often, consequently getting horrible mileage. 550 I can’t even imagine.

 

yes 550 is about when the detonation capping hits.. but 800 isn't too bad. it's only for a fraction of a second.

 

800 rpm in 2nd gear is pretty darn slow for an engine under load.

 

What you can do is to remove the resistor from the circuit and see if that is a problem.

Unplug the resistor that is located in front of the battery.
Insert a jumper wire between the two terminals of the harness connector. This will run the fuel pump in high speed all the time. Drive it for a bit and see if it does it.

Test the resistor:

 

The gas cap should allow air to enter the fuel tank in case a vacuum would develop but not allow pressure (fumes) to escape. It’s like a check valve assembly. Excess pressure built up is released via the purge valve into the intake manifold. If your gas cap passes the test my next guess would be fuel pump. Take the cap to an inspection station or mechanic and have them do an official check on it. Does the stumbling occur at all levels in the fuel tank or just at lower levels?

 

before this week, I had never though to correlate the level of the tank with the 800rpm misfire... I will start looking for that.

I now wonder how a evap system could create a vacuum by removing air (creation of vacuum) when a tank is topped off to the top off the fill pipe... (no accessible air to remove?!)

I'm theorizing that the Evap system pauses operation until the tank has a lower level?

 

The Evap system will not run a test until the tank is below 90% level to prevent just what you described. But if your gas cap is not functioning correctly, as the fuel pump is sucking fuel out of the tank it could cause a vacuum. I had the same thing happen to my honda engine that was on my log splitter due to a defective cap vent design that caused it to sputter out after it ran for a while (attached pdf). Those caps have seals that can degrade over time, something I'd look into a little more before spending any big money. It could also be your purge VSV valve malfunctioning which is designed to prevent a vacuum in the fuel tank by allowing the stored vapors to be sent into the intake manifold at a predetermined time based on pressures in the tank. You SHOULD get a DTC if there's a malfunction there for the evap system of any kind. I know my local inspection station has a machine they hook the cap up to to test its functionality, the attached toyota document doesn't go into much detail for the inspection. Here's the flow diagram for the EVAP system for your information.