[gaijin666]

96 camry V6, 1MZFE, 168,000 miles
------------

I'm a noob and recently I got a good OBD2 via Ebay:

I don't really understand FUEL TRIM but I know that it's good if the short term fuel trim and the long term fuel trim maintain low values; if the values are over 10 or so that's bad, it means there's something you would do well to dink around with.

I recently watched a vid from a car flipper in Los Angeles and he was emphatic about the value of Long and short term fuel trim metrics as an Early Indicator of developing car problems.

He said if you understand those values you can intervene early and cheaply head-off big car problems later on or at least make really sure your car passes smog

I'm running long-term fuel trim of maybe 12% and sometimes 14%. I think that's bad.

What should I do to remedy this..?

Other Questions:

1. How do Long-term fuel trim and Short-term fuel trim differ..?

2. Why is there Long-term fuel trim 1 and then 2, and short-trim 1 and 2..? (My OBD2 shows those things but I don't know about them).

(Stuff that might not matter but which you should know to give a good answer, I de-gunkified my engine recently via many short term oil changes and running fuel treatment stuff pretty liberally. I also replaced the fuel filter)

 

[mcgowaw]

2 banks = 2 short and 2 long term fuel trims.
14% is a bit high. not high enough to trip a cel for lean running but still above 10% threshold.
possible causes:
MAF dirty
O2 sensor faulty
unmetered air (intake snorkel leak, other loose fitting vacuum hoses...google cigar smoke test for more info)
clogged or dirty fuel injectors
fuel delivery issues (bad fuel filter, dirty fuel filter sock, clogged fuel filter)
and exhaust restrictions such as a clogged catalytic converter, or pinched exhaust pipe section.
tony

 

[Kingdom934]

Most times slightly high long term trim is vacuum leaks. Hoses (especially intake) tends to crack.

My dad's 2000 5SFE has a big tear on the resonator box underneath the intake. LTFT is around 17% cold idle and then goes down to 13%. Trying to find a junkyard part.

My 2004 SE I4 5spd manual had a slightly high LTFT. Crack in the PCV hose. It wasn't too bad, 13-15 but now i'm under 10 after the hose was replaced and PCv valve replaced.

http://www.toyotanation.com/forum/1...4-2-4l-2az-fe-engine-pinging-spark-knock.html

 

[gaijin666]

From the 31:45 for a couple minutes this guy's very applied video was instructive:

LT fuel trim POSITIVE = engine system is lean --> ADDING fuel to balance out the underlying LEAN condition, which can be owing to various causes (his was some intake valve being stuck open)

 

[qqzj]

Most times slightly high long term trim is vacuum leaks. Hoses (especially intake) tends to crack.

My dad's 2000 5SFE has a big tear on the resonator box underneath the intake. LTFT is around 17% cold idle and then goes down to 13%. Trying to find a junkyard part.

My 2004 SE I4 5spd manual had a slightly high LTFT. Crack in the PCV hose. It wasn't too bad, 13-15 but now i'm under 10 after the hose was replaced and PCv valve replaced.

http://www.toyotanation.com/forum/1...4-2-4l-2az-fe-engine-pinging-spark-knock.html

Can you simply get rid of that resonator box? I hope there is an after market air hose that will simply plug that big hole downward.

 

[qqzj]

2 banks = 2 short and 2 long term fuel trims.
14% is a bit high. not high enough to trip a cel for lean running but still above 10% threshold.
possible causes:
MAF dirty
O2 sensor faulty
unmetered air (intake snorkel leak, other loose fitting vacuum hoses...google cigar smoke test for more info)
clogged or dirty fuel injectors
fuel delivery issues (bad fuel filter, dirty fuel filter sock, clogged fuel filter)
and exhaust restrictions such as a clogged catalytic converter, or pinched exhaust pipe section.
tony

and exhaust restrictions such as a clogged catalytic converter, or pinched exhaust pipe section.

why would this change fuel trim? thanks.

 

[mcgowaw]

restrictions make the engine work harder and leans out, o2 tells ecu to lengthen injector pulse cause it needs more fuel, so ltft(s) go up in response to keep stft(s) near zero (~sine wave) for cat health.
tony

 

[gaijin666]

This guy from 3:30 to 11 min is also very good:

He shows that if you have determined a LEAN condition (positive LT fuel trimp values) which IMPROVE when going from low to high rpm's that points strongly to a vacuum leak as the underlying cause and mostly excludes other potential causes.

He does not use a smoke test, although I think that would be instructive.

 

[mcgowaw]

yep, a vacuum leak becomes inconsequential as the engine pulls in more air at higher rpm's so the trims normalize at higher rpm's in that case
tony

 

[Stillrunning]

I'll add this guy on

If your scanner does not include graphing software it'll be harder to understand the output as the numbers come pretty quickly.

 

[mcgowaw]

yep, schoddingers box and scanner danner do a good job of explaining this stuff too.
and yes, you gotta graph out the o2 sensor data as a voltmeter probed into the connector will give a weird rms reading or averaging of the voltages which won't make alot of sense here.
tony

 

[qqzj]

restrictions make the engine work harder and leans out, o2 tells ecu to lengthen injector pulse cause it needs more fuel, so ltft(s) go up in response to keep stft(s) near zero (~sine wave) for cat health.
tony

Why is this true "restrictions make the engine work harder and leans out"? Thanks.

 

[gaijin666]

Very cogent explanation, exp after the 15 minute point.

Great vid, thank you..!

 

[gaijin666]

Post 10 video detail, minute 27 or so:

Lessay u suspect a vacuum leak is causing a lean condition (positive value on Long term fuel trim) but lack a SMOKE TESTER:

You can also use propane or even carb solution:

Apparently there are USUAL engine zones prone to a vacuum leak:

You set your OBD2 to GRAPH short-term fuel trim over time and then you spray either the propane or the carb solution near the suspected vacuum leak point, while observing the STFT graph for a sudden and strong deviation.

The sprayed stuff changes the O2 value, right?

So the vacuum leak is wherever you elicit the suddenly altered STFT value.

So that's where you make the repair.

 

[mcgowaw]

yes, the sprayed stuff (propane) makes the engine rich momentarily, driving down the o2 sensor, which makes the stft's go negative for an instant, to counter it, when you've found the leak. this assumes you get the propane to all the possible leak spots. the cigar smoke, otoh, finds them for you.
tony

 

[qqzj]

Inspired by this post, I did an experiment today. After a long drive for about 33 miles, I looked at the LTFT at idle. It was about 10%. But once I add a bit of gas, it quickly turned into a -3%. Does this look normal and do I have a vacuum leak?

BTW, STFT is pretty much useless. LTFT changes very quickly. It is actually pretty much like short-term fuel trim.

 

[brianappj]

Can you get any of these numbers from an OBD I system?

 

[BMR]

Can you get any of these numbers from an OBD I system?

I think so. I copied this a long time ago from member JasonMZ's posts on checking fuel corrections on OBD I Camrys. I think this is for a V6. An I4 wouldn't have a second upstream O2 sensor, so checking across the "VF2-E1" & "OX2-E1" terminals wouldn't apply to those.

Test voltage at VF1-E1 and VF2-E1. This is a 5V scale of closed loop fuel trims. 2.50V is very little correction and what you're looking for. Higher is leaner, lower is richer.

Next, test OX1-E1 and OX2-E1. This is the raw O2 sensor voltage output. With a multimeter, this will be averaged, but it should stay within 0.45-0.55v. If it's above 0.55v, the O2 sensor is reading rich (ECU SUBTRACTS fuel), if it's below 0.45v, the O2 sensor is reading lean (ECU ADDS fuel).

 

[Stillrunning]

do I have a vacuum leak?

Not sure what you did. Did you momentarily punch the gas pedal and get -3% or did you increase RPMs to a steady state and get -3%? When you depress the gas pedal the TPS tells the computer you have opened the throttle, the computer will momentarily add extra fuel so you don't starve the added air, because air rushes in faster than fuel to the cylinder. On carburetors this function is done by the accelerator pump.

Vacuum leaks show more at idle than higher RPMs because the leak adds less of a % of total air volume at more open throttle. A vac leak may produce bad idle, but not bad running above idle.

 

[Stillrunning]

I think so.

I don't think OBDI was ever an industry standard. I think to get output from OBDI you needed manufacturer specific scanner with it's own proprietary connector.

I'm not sure how you'd read fast switching O2 sensor voltages.