He, ahhh, Jeep that hesitates is... slow

[Mudderoy]

After changing in the right injectors, in other words I stopped running 24 lbs injectors (@ 39 psi) and now have 24 lbs injectors (@49 psi) I've had a hesitation issue.

Here's some data...

2179 2180

[4.3LXJ]

So as I look at this, do you think you are getting lean or rich. Not sure on how to interpret the data

[Mudderoy]

So as I look at this, do you think you are getting lean or rich. Not sure on how to interpret the data

Lean, very lean. Well I've been told that the higher the voltage on the O2 sensor the richer the air/fuel mix. So ZERO volts would be a lean as it gets. :D

[4.3LXJ]

That is what I was thinking. Is lift on that setup mean vacuum?

[Mudderoy]

That is what I was thinking. Is lift on that setup mean vacuum?

Long Term Fuel Trim

[4.3LXJ]

I did a little learning on this. Check this out.

Understanding Short Term and Long Term Fuel Trim - YouTube

[NW99XJ]

I wonder if the static resistance of the injectors between the old and the new is so different that the voltage applied is going to be affected.....
If you have a DMM....just for the sake of having as much information as possible....measure the resistance (OHMS) across the leads of the the two different injectors.
The resistance IS going to affect voltage at least somewhat..... and if the computer doesnt know that there are different injectors (not sure if its even capable of learning this) ...then there's a good chance that the current (old) voltage map for the injectors isnt good enough to make them perform.

https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcQw0lcv0A08p4VLL7OgcEQr77CdlEL6s jlG6X3OdJeVAZk-1bxj

So Voltage = Amps X Resistance
If the static resistance across the injector varies from new to old, and the amperage of the circuit is constant, the voltage could potentially be way off....
Now this of course does not explain the sudden drop of voltage to zero.....thats a bit harder to try and figure out and is obviously the underlying cause of the problem.
Hopefully this whole resistance thing gives you something else to look at to fix this issue.

[4.3LXJ]

Tony, after watching the whole vid, you may just be seeing the results of the long term memory. My experience with the Jeep ECM is that it is slow to learn. You might try to clear it and let it relearn from scratch.

[Mudderoy]

I did a little learning on this. Check this out.

Understanding Short Term and Long Term Fuel Trim - YouTube (http://www.youtube.com/watch?v=oRX2V6_a3dc)

Great information! Thanks Steve.

I've added short term fuel graph to my display. I'll monitor it on the way home.

[Mudderoy]

Tony, after watching the whole vid, you may just be seeing the results of the long term memory. My experience with the Jeep ECM is that it is slow to learn. You might try to clear it and let it relearn from scratch.

It's been cleared several times the latest the day before yesterday.

[Mudderoy]

This is interesting. So I think I need to reset the computer, wait for closed loop, then watch both short term and long term fuel trim to get a fresh good reading.

What I have seen for years, at least from the upstream O2 sensor, and I'm talking at least 3 different ones, is the voltage runs between .5 and .8 volts, generally, which would indicate a rich environment.

What really throws me off is why while during acceleration the O2 sensor would go to lean all of the sudden. I mean the LTFT gets to 15% then it drops to zero! So it makes sense that the air fuel that the upstream O2 sensor is reading would drop as well, but why?

[4.3LXJ]

Tony, I wonder if your ECM is a little wonky. For instance, with the higher voltage you should see the LTFT go negative, but it remains positive. Then with low voltage, it goes to zero, which according to the vid is the opposite of what should happen

[Mudderoy]

Tony, I wonder if your ECM is a little wonky. For instance, with the higher voltage you should see the LTFT go negative, but it remains positive. Then with low voltage, it goes to zero, which according to the vid is the opposite of what should happen

lol I was just pricing ECMs on ebay.

Here's a curve, or two.

I have the 99+ intake, an aftermarket cam (as big as you can put in without computer changes says CRANE) and prior to installing the injectors I installed a 63mm(?) bored throttle body. I need to do some monitoring of both fuel trims to be certain but maybe I have exceeded the ability of the computer to adjust the injectors, in other words hitting the bump stops on the max positive % :male:

[Mudderoy]

Here's a petal to the floor acceleration run up to hmmm 60 I believe.
2184

This is the 2003 TJ with the throttle held at 2000 rpm
2187

I noticed on both the 98 and the 03 (The 99 wasn't at the house when I got home) that they both run LTFT 10% and STFT (about) 0% at idle.
You can see it returning to that setting here.
2189

[Carves]

This is interesting. So I think I need to reset the computer, wait for closed loop, then watch both short term and long term fuel trim to get a fresh good reading.

What I have seen for years, at least from the upstream O2 sensor, and I'm talking at least 3 different ones, is the voltage runs between .5 and .8 volts, generally, which would indicate a rich environment.

What really throws me off is why while during acceleration the O2 sensor would go to lean all of the sudden. I mean the LTFT gets to 15% then it drops to zero! So it makes sense that the air fuel that the upstream O2 sensor is reading would drop as well, but why?


Since the XJ is presumed to be running on preset fuel trims in Open Loop and O2 sensor adjusted fuel trims when in Closed Loop .... Im wondering whether you should be factoring in open/close loop status when you are looking at the fuel trims.

Can you monitor when you are in Closed Loop ??

This is the factory info.

Open Loop - Ignition switch ON
Open Loop - Engine start-up (crank)
Open Loop - Engine warm-up

Closed Loop - Idle
Closed Loop - Cruise

Open Loop - Acceleration
Open Loop - Deceleration
Open Loop - Wide open throttle (WOT)
Open Loop - Ignition switch OFF


You wont have long to wait for the system to go Closed Loop ... a coupla minutes should do it.

As for the Open Loop status in accel / decel modes etc., ... It either happens too fast for my scangauge to pick up on ... or I'm not brutal enough with the go pedal .. :D

[Mudderoy]

Since the XJ is presumed to be running on preset fuel trims in Open Loop and O2 sensor adjusted fuel trims when in Closed Loop .... Im wondering whether you should be factoring in open/close loop status when you are looking at the fuel trims.

Can you monitor when you are in Closed Loop ??

This is the factory info.

Open Loop - Ignition switch ON
Open Loop - Engine start-up (crank)
Open Loop - Engine warm-up

Closed Loop - Idle
Closed Loop - Cruise

Open Loop - Acceleration
Open Loop - Deceleration
Open Loop - Wide open throttle (WOT)
Open Loop - Ignition switch OFF


You wont have long to wait for the system to go Closed Loop ... a coupla minutes should do it.

As for the Open Loop status in accel / decel modes etc., ... It either happens too fast for my scangauge to pick up on ... or I'm not brutal enough with the go pedal .. :D

Oh that's interesting. I thought that once it had completed all the "tests" that it went into and stayed in closed loop. I'll have to see if I can display that information.