Engine hesitation
 

Replaced 3.4 v-6 in a 1999 4runner that had a hesitation in throttle response, more so coming off idle. Replacement 3.4 came out of my 1998 4runner which had excellent throttle response in the 1998 but hesitates in the 1999. Used the wiring harness and fuel injectors from the 1999 engine, no CEL or codes being set. Almost feels like a vacumn leak! Any ideas on this, thanks Jim

common denominator says there is something wrong with 99. whyd you swap the engine in the first place?

Rolled the 1998, had a very good engine and transmission. Purchased a 1999 with bad transmission, the engine started and run but had that hesitation when applying throttle from idle so just swapped out engine-tranny. The 1999 had set for some time, maybe the gas is bad! Checked vacumn lines ,seem to be good.

fuel filter?

Could be, will try that! Thanks

bad gas is a good option too. that didnt cross my mind. maybe burn off as much as possible before swapping out the fuel filter. then new filter and fresh gas. or figure out a way to drain it.

Fiiled the tank with fresh gas, took 13.5 gallons, approx. 4.5 gallons of the old gas. Added a can of Seafoam to the tank, maybe that will help! Another thing I have noticed is when the engine is started cold it idles at 1500 rpm for a while then idles down to about 600 rpm warmed up. Thought fast idle on the 1998 was only about 1000 rpm. The exhaust smells like rotten winter blend gas!

fixed
 

I am having this same issue on my 3.4 I put in my buggy. Did you find or figure out your problem.

Its also possible that you have a vacuum leak or maybe a bad injector. Is the car running rich or lean? the timing can also affect idle as well.

Got rid of the hesitation by cleaning the maf sensor again. Had cleaned it before I posted but silly me was only cleaning the air temp sensor that is part of the maf. I did not notice that the 2 wires that measure the air flow are inside of a small tube along side of the air temp sensor. They were filthy, used maf sensor cleaner, end of hesitation. The idle situation also got worse, not from cleaning the maf but was fixed by removing the IAC and cleaning it. Start up cold idle is now 1300 rpm and drops to 650 warmed up.