STF level spikes at times... hmmm?

[budwich]

I happen to connect up my torque android device today for a run to get my trailer.
Everything looked the same in terms of expected responses... LTF trim was basically "0" at idle and moved up a bit 2-3 ish during the drive. STF trim was "wagging" back and forth at idle and drive. Vacuum looked good, high at idle, lower at drive depending on throttling.

I was going down the highway and was ready to pull over for the exit, foot off the gas to coast in to the exit speed. I just happen to notice a "spike" in the STF trim up into the 20's. Thought this was rather strange as I thought with throttle closed, that perhaps fuel would trim lower a bit on both LTF and STF.... not that I have watched that much before. It just seems somewhat wrong to significant rich the mixture. I would expect more that the throttle motor would potentially open / crack a bit for air /fuel ratio "maintenance".

I was watching the same type of response later just going down a driveway at slow speed and taking off the gas, same spike in STF. I tried to duplicate the response a couple of times and couldn't so I can't say I know what conditions cause this but it is perplexing for me and was wondering if anyone has an understanding of what this is and if it is expected. I am almost thinking that there is a "mass air response" issue... where the system is thinking a large incoming of air showed up and thus the response is more fuel to maintain the ratio.

Anyways, comments and ideas to point me in a "don't worry" or "yes it is a problem and this is the area" direction. Thanks

 

[littleblazer]

I think it's normal and may have something to do with power enrichment? If not I still think it's normal. It spikes and swings so much it really doesn't matter. I think what is happening is fuel trim adjusted for speed then pedal off equals fuel trim not jumping down right away and thus the high level. Just a guess though. Kind of like an old carb.

 

[mrrsm]

An Exhaust Leak can cause this strange condition to occur. Check it out from around 15 Minutes into this video for more specific information:

 

[budwich]

interesting video / information. certainly a possibility. the vehicle had the exhaust manifold done under the "extended program" about 2 years ago. It is a bit noisier these days. BUT it is rather strange that the spike is very short, probably less than .5 seconds on the display (ie. one read cycle... maybe), then disappears thereafter and readings look ok for a long time thereafter.

maybe as suggested, the read cycle of trims might not be often enough in general, and its a "hit and miss" as to whether you will see the "spike" versus throttle position at a given instance... basically, you need the read cycle, the throttle closed down to align for the instance. I will take a closer look at things on the underside. Maybe the p013B code that pops up "here and there"... some days / weeks is related some how. The latest "pop" (3 days ago) indicated a voltage of 11.8 (or something like that), yet monitoring the o2 voltage for that sensor in torque (thereafter) shows it basically flat (~6v).

Maybe I need to clear the "runtime" data and let the system re-learn things since I had to take the negative cable off (replaced it and savrc module) back at the start of april.

 

[mrrsm]

I'm REALLY dating myself here from a mechanical experience perspective... but 'way back when' ...Exhaust Valves were as Big as Dinner Plates in Big Block Engines... and if the Exhaust Manifold Gasket(s) were to Fail... especially around the times of The Deep Cold of Winter, then the Exhaust Valves would often Warp and Leak due to the unintentional ingestion of ambient, Ice Cold Air. The thinned down metal edging around the perimeters of the Exhaust Valves was never capable of withstanding such a sudden, chilling change from the Red Hot conditions inside of the Inactive Combustion Chambers (during sudden periods of deceleration) to the shocking contractions caused when the Freezing Cold Air impacted upon them. This ancient memory prompted me to think that with a very slight variation... the same phenomena was taking place when you suddenly "Got Off The Gas" ...just prior to making your Exit Ramp Maneuver ...and the STFT Spike was immediately in evidence as a result.

Think about it from this standpoint... As soon as the Butterfly Baffle on the Throttle Body closes... the internal decelerating motions of the Pistons inside of the Cylinders are still performing Suction Events as they pull downwards and approach BDC (Bottom Dead Center). This action would serve to scavenge Air from wherever it can be obtained to fill those voids...and in so doing, vacuuming in the Air from around a Leaking Gasket directly into the Exhaust Manifold would become its NBO (Next Best Option) to fill up all of the Cylinders.

And so the O2 Sensor could become temporarily flooded with a dose of "Fresh Air" and its signal would cause the PCM to respond accordingly with a demand for a quick injection of Fuel to compensate for all that 'New O2' and hence ...the sudden STFT Spike. But... The REAL Culprit as the Cause and Origin of the problem in all of this drama... may eventually be traced back to One or More Broken Exhaust Manifold Bolts. If as you mentioned, your 'Exhaust is a Bit Noisy'... those EM Fasteners might deserve a very close "Look-See".

The 'Hit or Miss' nature of this artifact can also be explained by the dynamic movements of the Heavy Cast Iron Exhaust Manifold and the swaying to and fro of the Weighty CAT and Exhaust Piping during changes in the vehicle's motion and angular momentum... depending upon which direction the vehicle is "leaning towards" when turning left or right (...or say... even whenever entering the Curve of an Exit Ramp during period of hard deceleration). This... right along with any variations in the temperatures of the Aluminum Engine Head and Exhaust Manifold and MLS Exhaust Gasket could cause them all to "spread apart" momentarily, ingesting Air into the Exhaust Manifold, explaining the variability of when these STFT Spikes have been occurring. ;>)