Facelift - Chasing LTFT Trims

I think you have that backward. The downstream sensor monitors the effectiveness of the catalytic converter, while the upstream sensor is reading exhaust O2 and sends the voltage signal to the ECU, which then regulates the air/fuel mix via changes in injector pulse width. SD Faircloth

I thought the same at the time when Iā€™ve read Veekeyā€™s post, but as I donā€™t know these engines, I doubdet my selfā€¦
And at 60.000 miles they must be way past their life span limit.

From a training manual, ā€œFour oxygen sensors are installed on the exhaust system, two upstream and two downstream of the primary catalysts. The two downstream sensors are used by the ECM for closed loop fuel metering correction. The upstream sensors are used for OBD catalyst monitoring.ā€

I know itā€™s weird. But thatā€™s how the AJ16 engine reads the oxygen sensors.

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Are the upstream sensors before or after the the catalysts?
They canā€™t be monitoring the catalysts if they are before them.

From the same manual, "The efficiency of the primary catalytic converters is monitored and any deterioration in efficiency is flagged as a fault by the ECM. Monitoring for catalyst efficiency is achieved by sampling both the incoming and outgoing exhaust at the primary catalysts. Two oxygen sensors are positioned in each exhaust downpipe assembly ā€” one upstream of the primary catalyst and one downstream of the primary catalyst.

By comparing the voltage swings of each set of sensors, the ECM can detect when catalyst efficiency drops off. An ā€œarea differenceā€ technique is used to compare successive oxygen sensor swing measurements."

I got down to 0.8% on the trims, and then something popped me back up to 2.4%

Not sure what may have caused that. I was running low, but not out of fuel. Monitoring to see if I come back down again. I donā€™t know why low fuel would have affected anything.

Any dirt/restriction before the fuel pump would have a more severe effect when the fuel level is low as you loose the additional pressure from the fuel in the tank and the pump could start cavitating.
Fill it up and see if it changes.

If not, I would be considering changing the upstream 0Ā² sensors, still not convinced that they are not the ones that dictate fuel trimming.

I drive the car everyday, so yes, the tank has been refilled. Iā€™m at 3.1% LTFT, currently.

I reread my notes and all 4 oxygen sensors were in fact replaced at the same time.

I did seal the crack in the exhaust manifold Thermosteel.

I did notice some evidence of oil getting past the camcover seal. I cleaned the area up and will monitor.

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Veekay, Iā€™m not sure if this was addressed but the AJ6/16 engines use conventional spark plug numbering.

yesā€¦I even have the little sticker on my air filter indicating which one is which. That post was just clarifying that I indeed do have two separate (exhaust) banks.

OK. After patching the hairline crack in my exhaust manifold AND replacing my air filter AND unplugging an replugging my TPS while engine was running, has now brought my LTFT to:

negative, -0.8

Thatā€™s right, I went from a +3.1 LTFT to a -0.8 LTFT on both banks!

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Persistance always wins Veekay!

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Not sure whether it was sealing up the hairline crack on one of the two exhaust manifoldsā€¦or something much more interesting.

Was it because I unplugged, then replugged in the TPS, while engine running. The idle went higher when I unplugged it, no doubt because it went into open loop. When I plugged it back in seconds later, idle returned to the lower speed. I wonder if that has any bearing on my situation.

I will wait and see if idle drifts away in the futureā€¦

As always, thanks for the help and advice here.

My LTFTs crept back up to 10 again.

But Iā€™m back to a LTFT of zero!

I believe one of two things helped the car get back on track.

  1. I disconnected the camcover hose from the engine entirely. The hose from the camcover goes straight to an oil catch can. There is a baffle in the catch can, and the outlet hose simply points down in the engine bay. Before there was an egg separator that drained oil back to the engine via the dipstick tube, that has been capped off and abandoned. The vapor was also sent into the intake manifold. That path has also been capped and abandoned. The hose no longer connects to the intake elbow either. Basically that camcover hose basically vents to the outside air now. (through a filter)

  2. I unplugged the TPS, when the engine was hot and running, and plugged it back in before the engine died.

One of those two activities seemed to have driven the LTFT for my car back to zero.

Thought I would share that here.

More to share.

My LTFTs started to creep up again. It remained at about 4.8% for awhile.

This past weekend, I pulled the TPS sensor harness off after a drive, with the engine still running, waited a few seconds and plugged it back in. When I checked the LTFTs on Tuesday (after about 30 miles of driving over two days), it was at -2.6%! I think thereā€™s something to thisā€¦

The only other thing that I did between the harness pull and an eventual LTFT reading was I changed the oil and oil filter. Iā€™m not sure if an oil change can affect the trims of an engine?

Old thinner oil, more bypass maybe?
Or a small air leak somewhere that stopped?
Or different gas?

+/- 5% is nothing to worry about Veekey, especially with a 25 year old car and 90ā€™s tech.

Thanks for the response. I stick with 0w40 Castrol Synthetic all year round. I keep it in there for 7,500 miles, which ends up being about six months. Gas is almost always the same, Shell Premium, but I did fill up on Exxon Premium just recentlyā€¦

I do think I lose oil over time through a failing camcover gasketā€¦which I will change, once again. This time Iā€™m going to use some Permatex Anaerobic Gasket Maker (51813) in the groove to help even out any uneven-ness in the cover at this point. I read about this in one of Kirbyā€™s old posts, and Iā€™m going to give it a try.

First, I need to remove the rest of the failing powder coat from the inside of my spare camcover. Itā€™s in better condition overall, and Iā€™d like to put in back into service, but I need someone to help me sandblast the areas I cant get to, and I think I found someone.

Second, Iā€™m going to add the gasket maker, then add the new camcover gaskets and let it sit, face down on a flat glass table for at least 24 hours to make sure everything sets up nice.

Lastly, Iā€™m going to do this work when I dump the engine oil in so I donā€™t get any oil on the gaskets while I reset the camcover, which I probably have done in the past which may help oil sneak by the new, expensive gaskets.

Iā€™m not concerned about the +/-5%, but it does seem to want to get to 10% eventually. Iā€™ve never seen it go higher than that. Resetting that TPS connection when the engine is in closed loop seems to have an effect. Iā€™m going to continue to monitor (while enjoying the car) to see if I can figure anything out.

Thanks for the response!

Please donā€™t assume that everyone knows what you mean with an acronym. Such as WTFDYMBLTFT or what the F%&k do you mean by LTFT and STFT.

Long Term Fuel Trim - this is a ā€œlong termā€ average of the STFT below.

Short Term Fuel Trim - these should be changing all the time. As long as they are, youā€™re fine.

Also keep in mind that wideband OĀ² sensors are not perfect machines either and they might drift with time.

Good luck with the timing cover, it also could have an effect on the FTā€™s as it might have been leaking air in as well as oil out.
Also make sure that the oil dipstick and oil fill plug have a total seal.