Question on fuel trim

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Originally Posted By: Vikas
sorry but the objective here is to diagnosis here and not do blind parts swapping. tell him how he can actually test the O2 sensor. OP has live data scanner.


Look at O2 values in real time. Most cheaper scanners aren't going to react fast enough to do this so either use an O-scope or a good ($100+) high impedance multimeter. If the O2 sensor voltage is not swinging above and below 0.500v at least 10 times a second with the engine fully warmed up after a drive then the O2 sensors are getting lazy and not reporting the air/fuel ratio properly, thus leading to the ECM dumping more fuel to compensate for the inaccurate information supplied by dying O2 sensor(s); and over time changing the LTFT to a highly positive number.

A couple other possible causes of extremely high LTFT: Plugged/dirty injecters, already ruled out; low fuel pressure (being discussed RE: FPR).

O2 sensors are a very inexpensive wear/maintenance item and their failure mode is extremely slow degradation. The point at which the CEL is triggered for O2 sensor "failure" is often something like 90-120 seconds with the voltage not going over 0.500v. A new sensor will switch dozens of times per second. ECMs are WAY too lax in reporting "dead" O2 sensors because of mandatory emission warranty duration. IMO if the sensor isn't switching faster than 3-5 times/second it's dead. (It's also causing you to dump fuel and hence dollars out your tailpipe, and causing the primary symptom in this post - an extremely positive LTFT.)
 
He should be able to watch STFT changes on his scanner and introduce extra air or extra fuel. If STFT responds, then he knows primary O2 sensors are working. Since both LTFT are pegged at negative values, it is unlikely that both O2 sensors went bad exactly at the same time. Concentrate on common thing between the two banks. First thing to do would be to put the fuel pressure gauge on the rail and see what it is showing and compare it against the specification. He can also read the MAP reading from live data and see how it matches up with the factory service manual. Both fuel pressure regulator and MAP sensor would be common for the both banks.
 
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As would an exhaust leak after the primary O2 and before the rear O2 (a flange or cracked hanger weld).
This wold also show itself on this engine adding fuel on both banks as its an inline engine single cat (it may have a pre cat also) system
 
Originally Posted By: Trav
Originally Posted By: Clevy
Jeeps use a speed density type injection system. So no mass air sensor.


But it does have a MAP sensor.



Yes. A map senser,an AIT sensor and iirc there's one more though I don't recall the name of it
 
The flue line goes directly from the fuel rail along the engine and then towards the rear of the vehicle. I do not see a fuel pressure regulator near the engine.

If it were vacuum operated I assume it would be near the engine??
 
Screenshot_4.png
 
can he hear the exhaust leak?

what is the MAP reading given by the scanner when engine is off but the key is on? It should be reading atmospheric pressure. note down the units. if the MAP has drifted that too could cause engine adding too much fuel which then gets cut down by feedback mechanism resulting in negative LTFT. Yes, it is confusing or may be I am having trouble explaining it correctly :-(
 
Originally Posted By: Vikas
Since both LTFT are pegged at negative values, it is unlikely that both O2 sensors went bad exactly at the same time.


Partially correct. IF the numbers are in fact negative (I have asked for clarification on this but not received it), then the ECM is pulling out fuel... If that's the case O2 sensors are not the root cause or even part of the cure. In that scenario I would be looking towards the FPR first, either FPR leaking and dumping raw fuel into intake, or maybe a vacuum hose leaking or detached so fuel pressure isn't adjusting relative to engine vacuum (load). I don't own a Jeep and have never worked on one so I am completely unfamiliar with their FPR setups.

If the numbers are positive then the ECM is adding fuel and we're both making assumptions. I'm assuming the problem has been a long time in the making that was just discovered (like normally degrading 150,000 mile old O2 sensors that have been functionally dead for thousands of miles but not dead "enough" to trigger a CEL) and you're assuming that this is an acute problem (like last month LTFT was low single digits, this month it's pegged). In that case I may still be wrong, maybe the O2 sensors are only 10K old something else is going on. Maybe the OP just filled up with E85 trying to "save the planet", maybe the fuel filter is virtually plugged and can't flow enough to run at the proper pressure, maybe there is a FPR issue, maybe a lot of things.

Right now we don't know for sure if the number is positive or negative... Knowing that would be good because it leads us down two completely different diagnostic paths.

Just because STFT changes slightly that only indicates that O2 sensors are switching at least fast enough to not trigger the CEL.

Here you can see a quickly switching O2 sensor (narrowband gauge). https://www.youtube.com/watch?v=gJUqDEJZBUg From about 26-32 seconds in this video you can see the leds switching very fast. This is how a properly functioning O2 should switch.

Here is a cold (most likely) or old o2 sensor switching https://youtu.be/w6BgRV3YUu4 (Probably cold because old is not only slow but also looses it's ability to ever reach the extreme high end of the voltage scale). Anyway these are both at idle with maybe throttle blips, but at a steady state cruise on the highway you would still see the same activity. Fast=good, Slow=bad, greater than 90 or 120 or whatever seconds Jeep decided without switching = CEL. But if the voltage is only switching like the second video (~1 switch per second) that's 90 or 120 times better than "dead enough" to set a CEL however it is still functionally dead and wasting (metaphorically) tons of gas (as well as increasing the LTFT number).
 
Originally Posted By: Donald
I tried it again and I think I forgot that the numbers were a -21 and -23. After driving they were -16 and -19. Old eyes, old memory.

Since this was his reply, we can assume the negative trims.
 
Originally Posted By: linksep
Originally Posted By: Vikas
Since both LTFT are pegged at negative values, it is unlikely that both O2 sensors went bad exactly at the same time.


Partially correct. IF the numbers are in fact negative (I have asked for clarification on this but not received it), then the ECM is pulling out fuel... If that's the case O2 sensors are not the root cause or even part of the cure. In that scenario I would be looking towards the FPR first, either FPR leaking and dumping raw fuel into intake, or maybe a vacuum hose leaking or detached so fuel pressure isn't adjusting relative to engine vacuum (load). I don't own a Jeep and have never worked on one so I am completely unfamiliar with their FPR setups.

If the numbers are positive then the ECM is adding fuel and we're both making assumptions. I'm assuming the problem has been a long time in the making that was just discovered (like normally degrading 150,000 mile old O2 sensors that have been functionally dead for thousands of miles but not dead "enough" to trigger a CEL) and you're assuming that this is an acute problem (like last month LTFT was low single digits, this month it's pegged). In that case I may still be wrong, maybe the O2 sensors are only 10K old something else is going on. Maybe the OP just filled up with E85 trying to "save the planet", maybe the fuel filter is virtually plugged and can't flow enough to run at the proper pressure, maybe there is a FPR issue, maybe a lot of things.

Right now we don't know for sure if the number is positive or negative... Knowing that would be good because it leads us down two completely different diagnostic paths.

Just because STFT changes slightly that only indicates that O2 sensors are switching at least fast enough to not trigger the CEL.

Here you can see a quickly switching O2 sensor (narrowband gauge). https://www.youtube.com/watch?v=gJUqDEJZBUg From about 26-32 seconds in this video you can see the leds switching very fast. This is how a properly functioning O2 should switch.

Here is a cold (most likely) or old o2 sensor switching https://youtu.be/w6BgRV3YUu4 (Probably cold because old is not only slow but also looses it's ability to ever reach the extreme high end of the voltage scale). Anyway these are both at idle with maybe throttle blips, but at a steady state cruise on the highway you would still see the same activity. Fast=good, Slow=bad, greater than 90 or 120 or whatever seconds Jeep decided without switching = CEL. But if the voltage is only switching like the second video (~1 switch per second) that's 90 or 120 times better than "dead enough" to set a CEL however it is still functionally dead and wasting (metaphorically) tons of gas (as well as increasing the LTFT number).


The numbers for the LT fuel trim are definitely negative.
 
According to my repair manual, the fuel pressure regulator and filter are a combined assembly in the rear of the vehicle. There is no vacuum line involved.
 
Originally Posted By: Donald
Maybe the next step should be to measure the fuel pressure at the fuel rail?
No, measure the MAP reading from your scanner first as I mentioned before. If that comes out reasonable, then hook up the fuel pressure gauge to the rail assuming the rail has a schrader valve on it.
 
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OK, the manifold pressure was 28 in-hg with key on, engine not running. With engine running, its 15-16 at idle, 18 or so while accelerating, and 11-12 coasting down a hill.
 
OK, my fuel pressure test gauge kit that has been on my shelf for a few years does not seem to have the Schrader valve connector. I assume its a 7/16" x 20? That seems to be the common one for GM & Chrysler.
 
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