LTFT anxiety

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circuitsmith

circuitsmith

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I hooked up my OBD-II scanner partly out of boredom after not using it for over a year.
The LTFT was +10.x. I was hoping it was just dirty fuel injectors so I put a bottle of Techron in the tank and filled up.
After 35 miles of running errands 50/50 city/highway then the LTFT was +19.
When I rev the engine up to ~2500 rpm in neutral the STFT swings negative and LTFT drops to single digits.

I replaced the intake manifold gasket with updated orange silicone type 19 months and 13k miles ago.
I was careful to clean the gasket surfaces when I installed the new gasket, but I vaguely remember it being folded up when it came in the mail.
I also replaced the throttle body gasket at that time.
I didn't check fuel trims before or after changing the gasket.
So it might have had this issue from the day I changed the gasket.

I tried spraying carb cleaner: no effect on idle or fuel trims.
I sprayed around the manifold, throttle body and vacuum hoses.
I pinched off the brake booster and PCV hoses: no change.
Pinching off the hose to the EVAP purge had no effect when the solenoid wasn't active.
STFT dropped ~5 counts when hose pinched with the purge solenoid was clicking.
Cleaned the MAF sensor: no change.
LTFT now back to 10-11 and I'll just live with that as long as there are no codes.
 
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Do you have any data on the misfire count for the individual cylinders? Plugs, weak coil, bad wire?
Fuel pressure test? Fuel pressure regulator making a difference in pressure?
Fuel filter changed?
Air leak between the throttle body and MAF? A hairline crack in the accordion hose?
Crack in any vacuum hose.

There is more to check these are just a few. You could have one really dirty or defective injector, the O2 measures average from all 4, it doesn't see individuals.
Defective doesn't necessarily mean dead, one could have significantly different current draw as it heats up causing it to perform poorly, bench testing is the only way to know for sure.
That being said the injectors used in the Toyota are a good units and one of the last things i would suspect.
 
Trav, I might expect a dead or weak coil would show up as negative LTFT, right, due to too much unburned fuel in the stream, and the computer trying to compensate by removing fuel from the engine over time? This one seems to be the opposite -- it's adding fuel to compensate for a lean mixture.

OP, I would first disconnect your battery overnight or however long it's supposed to take for your car to reset its spark and fuel mapping to the default. Let it completely reset itself, then go for a long drive and monitor LTFT.

I'm not sure that a moderately high number like that is necessarily a problem, as long as it's consistent. If it runs up to 10-11 and stays there for years, that may just be where that particular engine runs compared with the "baseline" that has been set by Toyota for that computer logic.
 
I sprayed the carb cleaner around while watching the fuel trims to look for a vacuum leak.
Sucking in the flammable cleaner instead of air would have an effect.
There are no misfires reported. Engine runs smooth.
Computer was reset: no difference.
O2 sensor waveform looks OK, and I hope it wouldn't fail at 60k miles, there is no oil consumption.

Revise the 4th line in my OP to say:

"When I rev the engine up to ~2500 rpm in neutral or driving under load the STFT swings negative and LTFT drops to single digits."
 
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Originally Posted By: circuitsmith
O2 sensor waveform looks OK, and I hope it wouldn't fail at 60k miles, there is no oil consumption.


I've seen the waveform look fine on a scope, but the voltage was off. Looking at it without data it looked perfectly normal, but the voltage wasn't right.

Not saying it's the O2's, just somewhere else to look is all.
 
+ is adding fuel. The reason a weak coil can cause a + or ecm adding fuel is the O2 sensor senses O2 not raw fuel.
The misfire will allow excess oxygen from the non firing cyl to be pumped into the exhaust stream and with it the unburden fuel of course which can overheat and destroy the cat eventually.

+/-16 is the CEL limit, anything +/-10 indicates a problem that should be addressed ASAP.
The reason i asked about misfire count not just misfire codes set, there is no such thing as a pending misfire code.
A higher count on one or more cyl may not be enough to trip a code but could be enough to cause a + trim reading.
This engine uses a COP design, maybe there is oil or moisture in the tube, its worth a look.
 
I know you didn't have any joy with the carb cleaner looking for a vacuum leak, but there was a bulletin specifically for the 1zz a while back for intake manifold gaskets becoming brittle and causing a p0171. As soon as the first cold snap hit, we'd start seeing the corollas and matrices (sp?) come in with lean issues. Just a thought.
 
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Maybe the car's baselined for E0 and you have E10 gas in? Chasing ghosts?
 
I worked on late 95 Toyota that was OBD II and FT was around zero (+/- 1 or 2) on E10.
Something obscure like a hairline crack in an exhaust weld even after the front O2 but before the rear one can cause these symptoms on some models.
 
Originally Posted By: Cyclicrate
I know you didn't have any joy with the carb cleaner looking for a vacuum leak, but there was a bulletin specifically for the 1zz a while back for intake manifold gaskets becoming brittle and causing a p0171. As soon as the first cold snap hit, we'd start seeing the corollas and matrices (sp?) come in with lean issues. Just a thought.


As I said in my OP the gasket was replaced about a year and a half ago with the revised orange gasket.

Misfire count is 0.
There's no sounds suggesting an exhaust leak.
 
Raised ltft typically describes a lean condition (leak) detected between maf/map and the head and #1 o2 sensor is causing pcm to compensate or maf/map and/or o2 is faulty. Is it running rough or otherwise unusual?
 
What Is your maf reading at idle? Does your scan tool tell you? When replacing the intake gasket you didn't happen to remove any ground straps?
 
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Originally Posted By: circuitsmith
Running smooth. Exhaust note even. MPG's OK.

As posted you MAF sensors reading, also when looking for air or vacuum leaks starting fluid (ether) is much better than carb cleaner.
Better yet a plumbers propane torch with a hose, turn it on and let the gas flow unlit and check the STFT as you hold it under the hose or intake tube.

Any propane drawn in will spike the STFT and is the source of a leak. You might have a dirty injector, in tank cleaners cant fix most truly dirty units, off the car ultrasonic cleaning is the only way to clean and test them for flow, spray pattern and electrical soundness properly.
 
Originally Posted By: Trav
Any propane drawn in will spike the STFT and is the source of a leak.


Trav -- If the propane makes it into the engine, would the STFT numbers go up, or down ?
 
Originally Posted By: Colt45ws
I imagine it will richen the mixture so the PCM will pull fuel trim down.

Exactly, the spot it does it is the source of a leak.
 
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