help me out fellows I checked my fuel trims today and my short term fuel trim is close to 0% but my long term fuel trim is around -18% I checked for vacuum leaks and none found. I monitored my a/f g/s and Im around 5.5 and spec is 3.3-4.7. Im unsure on how to test my a/f ratio sensor. any tips would be helpful b/c right now my truck is running very rich and drinking the fuel .
my truck is a 1993 manual trans with the 3.4 swap out of a 2000 Toyota tacoma

 

You're misinterpreting the long term fuel trim negative percentage as being indicative of more fuel being injected(by increased injection duration over the basic injection duration of 0%). When in fact the opposite is true. Meaning, your ECU is running the engine leaner(adding less fuel), not richer(adding more fuel).
http://autoshop101.com/forms/h44.pdf

Anywho, -18% isn't a whole lot. I wouldn't be worried about that...necessarily. Might just need a tune-up.

Also, the 5.5V signal from the a/f sensor suggests that the ECU is actually succeeding in running the engine leaner(likely too lean, for factory specs atleast). Since higher voltage = leaner, lower voltage = richer.
http://autoshop101.com/forms/h37.pdf

BTW, what octane fuel are you using? Just curious...

 

well I looked at my data today and my a/f ratio sensor is maxing out my fuel supply at 3.7 when I added an external fuel supply it started dropping. I checked all around for vacuum leaks and I did not find any at all. my spark plugs are new so I know those are good. does anybody have pictures of how they routed vacuum lines especially the one that goes to the fuel press. regulator. I was told by somebody to check for vacuum at the fuel press. regulator. which I did not have any. I ran my vacuum line to the after market intake tube. so according to my fuel trims the engine is trying to take away fuel b/c it is getting to much fuel just cant figure out why.

 

My dad's 98 with the 3RZ-FE had bad a bad O2 sensor. It ran lean because of that and got abysmal mileage. Took a long time to pop a code. Worth investigating the sensors before and after the CAT.

 

Starting to sound like one of those "there's a problem because I say there is, NOT because there's any actual physical evidence of there being one!".

In that case...I refer you to my brother. Who's been off his meds for about a week. So he's crazier than a ____house rat at this point. I bet he could help you figure out what's wrong...with your engine...and a bunch of other things that aren't really happening either.

 

well mud hippy I know that fuel trims are supposed to be plus or minus 10% so - 18% is not good. also I checked my fuel mileage trying to see if it increased with the new motor and right now im about 10mpg. one thing is that I haven't changed out is my original rear o2 sensor. and the exhaust shop plugged it in. not sure if that could be causing the problem but I do have a cel on for a rear 02 heater circuit fault, also I do not have a cat.

 

To do it right you need to be running all sensors/ electronics to match your new ecu. Does the ecu switch into closed loop or are you running open loop everywhere? how are you checking gas mileage? tire size and gear ratio?

 

everything is in closed loop. and I determined my fuel mileage by filling up and then drove 110 mi and then filled back up and divided my fuel that it needed to fill back up by my mileage.
gear ratio 4:56
tire size 32x11.5r15 91 octane last 2 fill ups since just got the motor running figured I'd treat the motor good since it hasn't ran in over a year.

 

your ratio will not be accurate. 4.56 came factory with 31's at least on my truck from factory. You step that up in tire size and your mileage will be off. I average 14-21 mpg depending on how heavy my foot is and the hwy to city ratio.

 

the thing is though with all the same gear ratios tires just the 3.0 motor I was getting around 13 no matter how I drove it. and Im always burying my foot in the throttle now with the 3.4 its worse. I should be getting better than that. judging my my fuel trims that's why my mpg is bad. I just need to figure out why my ltft is stuck at -18%

 

whats ect at? old filter? hows tune up? plugs? compression readings?

 

ect is right around 198 degrees, replaced fuel filter about 10000 mi ago but that was with the old motor in. plugs are good and new. unsure on compression yet haven't gotten that far but the motor only has 60k on it so im hoping its ok. the truck runs fine no bogging hesitation or missing just fuel mileage is horrible. I still have the original 3.0 fuel pump and the fuel filter is 3.0 fuel filter if any of that matters

 

If your fuel pressure vacuum is hooked to your 'intake tube' (sounds like it's ahead of the throttle plate, barely feeling manifold vacuum), then your idle & light cruise fuel pressure will be too high, which the ECU will respond to by trimming lean. This could also explain most or part of your fuel milage issue.

Fix: Plumb the fuel pressure regulator vacuum line directly to manifold vacuum.

 

I too have a weird low mpg's issue...possibry a wiring issue......

 

so I plumbed the fuel pressure regulator to the manifold vacuum fuel trim has dropped to around -11% getting closer to specs but still a little out. now that I have done that the truck has lost its power that it had before. something makes me think that something still isn't right

 

Have you done a fuel pressure check? The long term trims will take a while to zero out, if at all. I agree - u still have a basic fire-triangle issue. Next step is to closely inspect ALL of your vacuum lines.

 

what do you my mean by a fire-triangle issue. fuel press is at 46psi with out the regulator hooked up to manifold vacuum and 34psi with regulator hooked up to manifold vacuum. which puts it within specs. the only thing I still don't get is on all the 3.4 that I have seen the regulator is hooked up to a box on the intake tube.

 

You should hope for 0% in a 3.4 that will leave you Ideal fuel economy.

First take your scanner find Engine Load calculation at WOT (wide open throttle) you should exceed 80% and do it eerytime. best is to start at 20 mph cruise and floor it if 5spd 2nd gear is best. my guess is skewed MAF. I have replaced tons of these. Most common reasons for trim problems in all makes and models.

1. Vac leaks includes intake gaskets
2. MAF (if equipped) make sure your intake is seal (the boot from MAF to intake)
3. Fuel Pressure (pretty rare)
4. Low or high vacuum (timing belt or timing chain issues)

Lastly I have seen a few failures of the B1S2 O2 sensor cause trim prblems

 

I tried that with the scanner and engine load is fine was at 90% or better everytime I tried it.

 

Your rear o2 sensor has nothing to do with fuel trims, its just there to monitor the converter. that being said, what are your front o2 sensors reading at idle?