-20 (negative) short term fuel trim & running rich 1996 Acura 2.5 TL - need help

  • EagleRock
  • Topic Author
  • Offline
  • 1st Gear
  • 1st Gear
6 months 1 week ago #201402 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.

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 Eric’s videos on Honda’s hunting for idle and also have read his 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 and confirmed that fuel pressure regulator lowers fuel pressure by disconnecting 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 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.

New parts installed: 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.

2nd problem:; 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 as VSS reads 0 (zero) on OBDII live data (note: my scanner ad snap on scanner will not read any transmissions data or codes. 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 idle problems 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.

Please Log in or Create an account to join the conversation.

6 months 1 week ago #201406 by jimchndlr
I would focus on your primary problem to start with and worry about the shifting issue later. It may actually be the same root cause.

Now, out of everything you've written, the readings in general seem odd. If this were my car, I would start with your current readings and compare against what is standard for your car, or in general. The other thing that bothers me is getting -20 fuel trim but having a rich condition regardless of that. It seems opposite to what would be expected.

What was your fuel pressure reading? You said you confirmed that the regulator is working, but, are the pressure readings within spec?

Please Log in or Create an account to join the conversation.

Time to create page: 0.231 seconds