Full Throttle Hesitation

I did a quick visual inspection of the Cat while I had the down pipe removed and it looked ok to the best of my knowledge. And everything beyond the Cat has been replaced roughly a year ago according to the previous owner (visually you can tell it is new). I got everything back on tonight. I'm gonna drive it to work tomorrow and see how it does.

 

this thread was created May 4, 2016 at 9:51 AM and the OP has never replied to it since so yes my reply to the OP was 3 weeks after he posted so that is an old post

now SOMDTACO has posted once every week after the OP to keep this thread alive in hopes of solving his problem and you guys did just that as I also correctly mentioned

 

Well I got the headers and down pipe back on nice and tight with no leaks. happy about that. However my bucking issue at load and acceleration is still there. I did however manage to get a couple codes but I don't think they are related to this problem

P0441 - Evaporator Emission Control System Incorrect Purge Flow
P0446 - Evaporative Emission Control System Vent Control Circuit

It's hard for me to see how those codes relate to this problem. Possibly a vacuum leak that only occurs when the evap system is running? Something to do with fuel delivery? The Bucking seems so random, but definitely more prominent under load and during quick acceleration.

I should note the engine seems to idle just fine at right around 650 rpms. I can also rev it up in idle up and down and notice nothing out of the ordinary, seems strong all the way through. This problem only exists while driving.

 

Out of curiosity I disconnected the down pipe where it meets the CAT and basically road around straight piped. Just like before, once the car got up in temperature, the jerking and bucking came back so I think I can rule out anything on the exhaust end (other than possibly the 02 sensors).

 

holy shit that must have been loud.... like, nascar racecar loud..... but yea, that rules out exhaust clogging.

 

good. now youre on to something. the evap system only really kicks in when the vehicle is "warmed up". assuming your fuel trims will behave just the same, if your LTFT is +7%, you have an intake leak of sorts. and those codes are pointing towards the charcoal cannister area.

first lets start here, inspect all the evap vacuum hoses and make sure it's not sucking in air from outside the system AND that they're not clogged. be it with rotted rubber from inside the tubes breaking free or excessive fuel that leaked in from overfilling the gas tank too much.

 

Well a quick update. I managed to figure out what was wrong with the Evaporative system. That was pretty obvious. The straight tube that connected the VSV to the vapor sensor on the firewall was missing...it was just gone. I replaced it with some tubing and the code has gone away and not come back. It did seem to help my hesitation/miss problem a little but the problem is definitely still there. I have recently replaced my fuel filter but still no dice. I am getting no other codes so its hard to say what to look at next. I can say it seems like everything I replace/fix seems to help a little bit. Maybe the rig was just neglected a bit too much. I bought it cheap about 6 months ago with 180k on it and have been trying to nurse it back to life.

 

I'm thinking I need to fully investigate the EGR valve. Maybe once the engine is warm it is getting partially stuck open, or it is opening and closing slowly.

What is quite clear is this:
-Runs much better after cold start and gradually gets worse as engine warms up.
-After engine is warm, if I turn off the truck, it takes a lot more cranks than normal to start the car. sometimes feels like its not going to start but then it does.

just trying to correlate how a bad EGR valve can cause these symptoms.

 

i would need scanner results to further help. did you get one yet?

also, how is your braking? do you hear a hissing sound when you apply the brakes? or is it really difficult to get the pedal down to the ground when idling?

 

Yes. I have a scanner. The check engine light is not on and I have no stored codes. I think I'm going to block off the EGR and see if that changes anything. If it does I'll work back from there.

And even though I did the test with the cat disconnected I'm still not so sure that there isn't blockage on the exhaust side only because thats kinda what it feels like when driving if that makes sense.

 

Disconnected air from EGR valve to keep it closed and still had bucking problem. Also did a pressure test on the exhaust right before the cat and pressure stayed below 1 PSI through revving so that's good.

For some reason I just saw this. It is very hard to apply my brakes. I'll have to listen for hissing. Are you suggesting there is possibly a vacuum leak through the brake booster line?

 

what commonly happens is there is a tear inside the booster in the diaphragm that renders the vacuum assisted braking borderline useless.

i had this issue, let me explain so you can have something to judge off of.

when i bought the truck, it was hard to come to a hard/emergency stop. when the pedal reached half way, it felt like i was towing/stopping a friggin NASA shuttle. so, coming to a normal slow stop, it was fine.

the tear that i had in my booster diaphragm was tucked in a bit so it was only exposed when the pedal was halfway down.

so, if your truck always feels like its difficult to stop no matter where the pedal is, you probably have a tear on the outer portion of the diaphragm and it's constantly leaking air.

and the hissing usually isn't apparent. its usually drowned out by the engine. i wouldn't bother with looking for one, i would just right out change the booster. and you don't have to worry about whether it will fix anything or not. im about 99.99% certain this is your issue.

if you want, you can test for a leak while the engine is off. you just need to get a cheap vacuum pump of some sort and try to pull air from the booster's check valve or hose. if you cant hold a vacuum at the hose/check valve, you have a leak.

 

Just to update my issue,
I replaced the K&N maff with a factory unit , the truck now pulls much better, tho I can feel what seems like an injector trying to fire .
I have run Lucas Injector cleaner thru several tanks of fuel, getting better.
Yesterday I put Sea foam in the tank as per instructions, and in the crankcase just or the check of it .
I am thinking that since it had water in the center cylinders due to the cracked heads, and then sat , the Injector is gummed up , and it is in fact trying to fire.
All in all , once the Injector works as it should , everything will be as it should be .

 

I've been running the techron fuel injector cleaner through it but have only ran maybe 2 tanks of gas through it since the problem due to driving so infrequently.

Thanks for the information, I will look at the brake booster this weekend. Big question is, can a vaccum leak lead to what feels like a miss under acceleration? Also, I'm interested to find out why my problem is more evident once the engine is warm. Maybe its not so much when the engine is warm, but after I have done a descent amount of braking which just so happens to be the same time frame that it takes for the engine to warm up. Welp at least i have something else to look at.

 

yes it can! if the engine got a sudden surge of air from an unmetered source (huge vacuum leak) you can get knocking and or power loss because it gets air so erratically that the O2 sensors cant compensate quickly enough.

 

You guys are so frikken SMART! You're like engine surgeons or something..... So valuable (among other things!)

 

Thought I'd tie this thread up with my solution. Ended up being that I had a bad distributor. Replaced the distributor and the problem was solved. see this thread for more details.

https://www.tacomaworld.com/threads/2-7-stumbles-and-bucks-under-load-solved.449971/

 

Thank you very much for updating this thread. It will definitely help somebody out in the future.

 

Yes, very thoughtful of you. I HATE when people don't give us closure, that's defeating the whole purpose of the commentary when no solution is offered.