Have checked out many things on this 1999 4runner that starts harder than it should, has a lag or hesitation off from idle and just is not running like it should. Fuel system checks out, no vacumn leaks, had one pending p0171 that went away, no other DTCs.The engine is out of a 1998 I have that run very good but is now doing the same thing the original 1999 engine did. Did some diagnostics on the maf sensor and the air temp is correct along with specs for flow gm/s. Out of desperation I took off the air box, maf and intake air to throttle bore tube and replaced with said items off from the 1998 which has a different maf and air box assembly. Set a code for air temp on start up but engine fires up as soon as it turns over and no lag at all, sounds as good as it did when in the 1998 body. Obviously I can not run it like this as the maf is different and not reading the air temp. Any ideas on why the MAF for 1999 checks out, no DTCs, but makes the engine run like crap, have cleaned it with MAF cleaner? Hate to buy new MAF if that is not it !

 

Checking Toyota parts catalog the mass air flow sensor that was on the 1998 is actually a 1997 model year part, used until July of 1998 then a different part number was used until 1999 production started with a new part number used thru 2002.