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-20 short term fuel trim & hunting idle 1996 Acura 2.5 TL - need help

  • EagleRock
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11 Feb 2019 21:15 #26986 by EagleRock
I have a 1996 Acura 2.5 TL ( 5 Cyl.) automatic with 90,000 miles. The car runs rich, has really bad gas mileage, black sooty plugs.

When idling and driving:

short term fuel trim stays around -20 (negative 20)
long term fuel trim is 0.
Upstream O2 sensor primarily reads around 0.94 to 0.98, ,
Downstream O2 sensor will read similar to upstream

When the engine gets warmed up, the engine will hunt for idle and idle speed will drop and almost stall.
Engine idle will oscillate/hunt between 300 rpms and 800mrpms.

If a controlled vacuum leak is introduced, the car will idle correctly around 800mrpms (ie: remove PVC valve from valve cover and turn so it is facing up (valve open) provides adequate vacuum leak for engine to idle correctly. Doing this does Not change high upstream O2 reading or short term fuel trim readings. SHRT fuel stays at -20.

The car will throw 1 or 2 of the following Misfire DTC (either Random Misfire or Cyl. 1 or Cyl. 2) if RPMs go above 4000 by giving Wide open throttle. . it does not get any other DTC codes for the engine.
Freeze frame data when giving W.O.T. & Misfire DTC shows -- Short term and Long term fuel trim 0.0, MAF 0.17 lbs/sec, Absolute throttle position 45.9% and fuel system at Open Loop - DRIVE.

FYI - About 8 months ago the previous owner had a new radiator installed by the Acura dealer. the owner said the car overheated and afterwards the transmission would not shift properly. Unknown how hot the car got or how far they drove it.

Using engine scanner to obtain live data:
ECU goes into closed loop.
When idling and driving:
Upstream O2 sensor primarily reads around 0.94 to 0.98, short term fuel trim stays around -20 (negative 20), long term fuel trim is 0. Downstream O2 sensor will read similar to upstream.

When decelerating,: upstream O2 will drop to 0.2 and short term fuel trim will go to 0. That’s the only time upstream O2 sensor will drop below .85 This drop is caused by ECU turning off injectors when decelerating).

Note: If upstream O2 sensor is unplugged, voltage shows at 0.1 or .2 and short term fuel, trim goes to +20 (positive 20). This fuel increase negatively impacts idle with all that extra fuel.

ECT engine coolant temp Sensor is new and reads correctly.

VSS (vehicle speed sensor) - Speedometer and cruise control both work but scanner shows 0 MPH for speed on VSS while driving. The scanner does not show any DTC’s for,the VSS.
Note: Sometimes the scanner would show Scanner speeds for VSS but that was sporadic and then stopped all together.
I Installed new VSS but still get same same readings — 0 MPH when driving but speedometer works fine.

I have watched various videos on Honda’s hunting for idle ( including Scanner Dan and Eric the Car Guy) and also have read Eric the Car Guy’s detailed website post/page on this topic.

My IAC valve opens and closes and gets signals. (Valve was taken off and cleaner and energized).
Fast Idle Valve closes all the way.
Cooling system has been bled to make sure no air in system.

I have checked fuel pressure with pressure gage 42 psi with vacuum line disconnected from press regulator and confirmed that fuel pressure regulator lowers fuel pressure to 32-34 psi with vacuum line.

I have confirmed there are no vacuum leaks by using commercial smoke machine connected at power brake booster vacuum line.

I have removed all 5 fuel injectors and removed screens from each and cleaned them and Re-assembled, switching cyl. 1 and 2 injectors to see if it affected which cyl. Got Misfire DTC. After doing this, mainly get a Random Misfire DTC now instead of Cyl 1 or2 Misfire. I did not have injectors tested off the motor for spray, volume or leaks.

I have used LED signal tester on each injector connection and all injectors are getting signal. I have disconnected injector connectors 1 at a time while engine running to confirm that doing so creates a cylinder miss and it does. I have not viewed live scanner to see if disconnecting injectors in order changes short term fuel trim.

New parts installed: MAF, cap, rotor, ignition wires and wire from coil, platinum NGK spark plugs, ECT sensor (engine coolant sensor), radiator fan switch at radiator (due to small leak at o-ring), fuel filter, VSS.

I have not changes any O2 sensors. They do not routinely cycle above and below 0.45 v when viewed in graph on scanner. Upstream O2 only dips significantly when decelerating., otherwise stays high 0.9 v or higher.

Need help in getting car to idle smoothly and getting fuel trim in order as gas mileage is horrible.

2nd problem if anyone has auto trans experience: auto trans does not shift correctly at times requiring you to let off the gas to initiate up shifting to next gear.
New trans. Filter installed along with factory Honda A/T trans fluid.

This has helped but trans shifting is still not correct. Do you think the VSS is causing this shifting problem? VSS operates speedometer which works fine but VSS reads 0 (zero) on OBDII live data (note: my scanner and snap on scanner will not read any transmissions data or codes on this car. Jumping diagnostic plug per manual does not show any OBDI diagnostic problems either - ie: DS4 light does not flash any codes.),?

Any help of what to test and try to resolve the running rich and hunting idle problems that almost cause it to stall is appreciated as well as thoughts on the trans shifting in-consistently. I have not done any testing on EGR system nor have I tested vacuum operated fuel pressure regulator diaphragm for vacuum leak down. I have not done a compression test on the cylinders either.

Thanks in advance for your help and suggestions.

Eagle Rock.

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12 Feb 2019 03:29 - 12 Feb 2019 03:30 #26992 by Andy.MacFadyen
With the STF maxed out negative but the LTF fixed on zero is the one that sticks out for me, something very odd there.

" Welcome to the 21st"

Last edit: 12 Feb 2019 03:30 by Andy.MacFadyen.

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12 Feb 2019 08:50 #26996 by juergen.scholl
The LTFT not moving while STFT maxes out might be a pcm strategy........

On certain Nissan vehicles for example with a given problem the short term fuel trim will rise up to 20% and if the there is no change in O2 readings the long term fuel trim will stay at zero. Then the pcm will enter into open loop , then closed loop again, then STFT will climb again up to 20% and so on for ever....

You might want to read on the operating principles...

An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
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  • Tyler
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12 Feb 2019 08:59 #26998 by Tyler
Was this fuel trim problem present when you changed the spark plugs? If so, do you recall what they looked like? Black, sooty? Mostly trying to confirm if you have an actual rich condition, or a false rich condition. Unlikely, since both O2's say rich, but you never know. :silly:

The misfiring at around 4000 RPM may also suggest a false rich condition. Do you feel a noticeable lack of power when accelerating?

If you were to force Open Loop, does the engine idle and run well? Sorry if you answered that. :blush:

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12 Feb 2019 16:23 #27007 by fudgie
Can't remember- does this car have an egr valve?

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14 Feb 2019 07:44 #27035 by Rev Scott
Have the valves been adjusted?
Tight exhaust valves can cause a rich condition like that.

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  • EagleRock
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18 Feb 2019 15:29 - 18 Feb 2019 15:32 #27148 by EagleRock
Thank you for your input. To answer your questions:

The Acura does have a vacuum operated EGR valve.

The plugs were black and sooty when changed them. I recently drove the car on the Hwy and then parked it and checked the plugs. They were starting to burn off clean (light brown/tan) but still had some soot remaining. I believe it runs rich at idle while RPM hunts up and down causing black sooty plugs.

I have installed a new MAF sensor, and air filter & cleaned throttle body with carb cleaner (without removing Throttle body) but it did not change anything. I also ran without air filter in place an no change.

Using scanner and viewing live data:
Timing does not change when the car is driven or rev’d up. it stays ast 7 degrees.
Vehicle speed sensor VSS on ECU - OBDII - reads 0 zero most times. I do not get any VSS error/DTC code though.
While driving. VSS will,occasionally read correct speed or a lesser speed very intermittently for short time periods. When the VSS does read a speed other than 0 (zero), the timing increases and the cars RPM’s increase 200-300 rpms. This only lasts for a minute or so and then the VSS returns to 0 zero and then the timing returns to 7 degrees too, regardless of actual speed of Engine rpm’s.

Please see my post today “VSS not registering signal at ECU” for more details.

I still need your help and input/suggestions.

thank you for taking your time to read and provide assistance. I appreciate it.


I have done compressionhtest son all 5 cylinders when warm. Compression readings are 170 psi +/- 5 psi.
Last edit: 18 Feb 2019 15:32 by EagleRock.

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