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Discussion Starter · #1 ·
Hey guys; I have some symptom data about a puzzling problem that has affected my car recently. If anyone knows anything about this, even a few words of guidance would be appreciated.

My car's air/fuel ratio, as measured by an LM-1 wideband and supported by listening to the sound of the engine and judging available torque, has been acting peculiarly at various highway speeds. Here are some "datapoints" of what it consistently does at speed in 5th gear:

60MPH : Totally stochiometric (~14.7 with normal fluctuation on both sides)
70MPH : Overly rich at perfectly steady-state cruise; about 13.8, without fluctuation. With slightly increased throttle opening, either to keep up with traffic or go up small inclines, this number has little hesitation to fall into the 12s around this speed.
80MPH : Slightly lean, but great for cruise. Steady at 15.4 or so, with little to no fluctuation as the computer would enforce around stoich. Closer to full stoich right below 80, but high enough above 70.
85MPH : Incredibly lean--missing occurs every few seconds. The severity of the leanness depends on outside temperatures, but usually hits at least 17.8, or as much as 19+.
90MPH : similar to 80, very comfortable yet decidedly stuck on the lean side, rather than stoich.
95+MPH : full stoich @ ~14.7.

My fuel pressure is close to where it has always needed to be for this car, and adjusting it does nothing aside from change the severity of the bad points (70 and 85). If I reduce the pressure a bit, 85 is undrivable; if the fuel pressure is boosted, 85 improves, at the expense of 70.

It nearly seems that for an entire range of RPMs from the high 2000s to about 3500, the ECU does not inject any more fuel regardless of the crank RPM. At least, that's a way to describe what's happening--it scales across engine speeds.

This has gotten worse recently--to my knowledge, I had never noticed the rich @ 70MPH issue, and I watch the wideband often.

What could this be? An ailing narrowband O2 sensor? It goes without saying that the ECU is not throwing a code at this point.

Thanks for any comments guys; this is a long post, but it's a pretty specific issue I've been messing with for a while now, so I wanted to include a good amount of detail in my first mention of it.
 

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Discussion Starter · #3 ·
MK3Brent said:
How is the TPS working?
Very good question; I forgot to mention anything about that. As far as I know, it's working fine--last time I checked it out, it was well within spec. It seems to operate fine, as well. However, now that you mention it, I will check it out again tomorrow when there is more light available.

Anything else?
 

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ƒ=1/2π(√k/m)
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It's just weird going from lean to rich. (back and forth, back and forth)

I'm thinking, TPS, boost, vacuum leaks.
 

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Discussion Starter · #7 ·
Yeah, the strangest thing is that it does not stick to at least sometimes being rich or lean, but under different circumstances, will go to one or the other.

I'm not so confident vacuum leaks are the ticket, as my boost gauge has always read -20 mmHg at idle. Under decel it's even lower, basically perfect vacuum if I made the unit transformation correctly.

I think the car has an exhaust leak that enters the engine compartment, but it should not have this much of an effect--especially with the sort of airflow experienced at highway speeds.

Any boost leak would be minor, as all of the visible intake tract is intact (the 3000 pipe and angled pipe over the turbo were removed recently, checked out), and it holds 14PSI without issue.

That said, the I will definitely check out the TPS tomorrow, and go over the intake in detail.

Does anyone know what a worn narrowband oxygen sensor can do? I'm suspecting that, for sure...but don't know if any of these symptoms are inline with it wearing out.
 

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ƒ=1/2π(√k/m)
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Well, the o2 sensor is weird for me.

With it unplugged I'd go way rich while driving... but earlier last week I noticed my CEL on, sure enough it was o2.

However my AFR was the same.

I'd think if your o2 was causing it... just throw in another one and see what happens.
Try to single out the problem.
 

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Discussion Starter · #9 ·
MK3Brent said:
I'd think if your o2 was causing it... just throw in another one and see what happens.
Try to single out the problem.
I do have another one in my "Repository O' Supra Shit," but right now I'm doing a summer internship, and all of that stuff is quite far from me. So, without the ability to easily test it, I was wondering if AFRs sometimes start getting out of control before the ECU notices...but apparently not. I was just hoping this would be the case, because it would explain the problem.
 

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I was told by one of the tuning places that I use, that recently, the gasoline quality in the Bay Area has dwindled. He said it has been preventing them from doing any aggressive tuning and have noticed with most cars they put on the dyno.

It MAY be what is causing your car to act up.
 

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Get someone with the right equipment to test your o2 sensor. Ask them to do the propane test and the throttle snap, make sure switching time is no less then 100Ms. If it is any slower replace it. If it doesnt switch more then 8-10 times in a 10 second span, replace it. But yeah check for Vac leaks, you might have a manifold leak, not IC pipes.-20 is kinda high should be around 17/18.

With a little more thought it does seem like your Running in Open loop. You run hella rich when the throttle moves open, then hella Lean when the throttle moves any closed. Like the computer is just looking at it like, + throttle, Imma dump X amount of fuel, then when the throttle goes - it just takes away all that fuel. Like it isnt looking at the o2 at all for adjustments
 

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Discussion Starter · #14 ·
skaterjunky8 said:
Whats your A/f like then you come to a stop and sit for a minute
Idle AFR can vary, depending on the conditions, such as how warm the motor is (coming off the highway perhaps), and the ambient temperature, but is always lean. Sometimes it's about 15 at good times, but others it can be around 16.5. 650RPM in any case.

I've noticed recently that the rich condition is not too severe. It does run rich around 70, and with any throttle it is very easy to make it move more rich, but in the past couple days at totally steady cruise it will eventually creep to the rich side of stoich. The lean condition, however, does not improve at all.
 

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when your motors all warmed up ride your gears all the way to a stop (Downshift all the way to 2nd, then neutral it) watch what your A/f does. It should go from super lean (like 17s) to about 12, then slowly lean out to around 14.7-15, these should all be smooth transitions
 
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