1991 Jeep YJ Wrangler 2.5L MPI Idle Issue

[Jeep North]

1991 YJ, 2.5 I4  MPI      Starts right up, even on a cold morning. Idles smoothly at 1000 rpm. After less than a minute, revs start to drop to where she almost stalls out but she catches herself, revs back up, settles down and repeats. Does this about 3 times after which she settles into an "almost ( slight drop off but quickly returns to 700 to 800 r's)steady idle". Doesn't do this when she's warmed up. Also did not do this in warmer weather, but that just might be a coincidence. Changed temp sensor, same issue. Pulled idle air motor, very little carbon, cleaned plunger tip, didn't help.  

[Moses Ludel]

Jeep North...Check for vacuum leaks and MAP readings.  Could be the Throttle Position Sensor or the oxygen sensor (not warming quickly enough).  

Do not replace parts without a clear indication of a defect.  Testing first will isolate the problem with each of these devices.  Oxygen sensor or TPS are good bets if the temp sensor is new.  Check the voltage readings on the TPS.  If you do replace the oxygen sensor use an OE Mopar or OE-manufacturer sensor (Denso with same part number as OE sensor).

Note: Test when cold to be consistent with your symptoms.  Once warmed, even if defective, the readings may be normal.

Keep us posted on results and findings...Available for further questions.

Moses

Edited December 16, 2015 by Moses Ludel

[Jeep North]

Thanks Moses. Checked for vacuum leaks last night using the WD40 trick, nothing showed up. The check engine light hasn't come on through this, so after reading about the PCM self diagnosis codes lights I tried that and got (I think) a 33 which is, I believe, an O2 sensor issue so I'll check there next as you suggest.

Thanks for the speedy reply, great web site.

Jeep North (aka Rod)

[Moses Ludel]

Rod...A quick check has "33" code as A/C clutch relay or nonessential on a Jeep YJ Wrangler without A/C.  O2 sensor or circuit is "21" code.  This is fairly reliable fault coding, so I'd check the TPS voltage and MAP readings first.  The O2 sensor could be troublesome when cold, but try the TPS voltage readings before replacing the O2 sensor.

Try a cold reading on the O2 sensor if TPS checks okay.  In real world terms, I've found that the Mopar MPI's TPS wears out over time and sends an erratic reading at various throttle settings—maybe various temperatures.  A volt-ohmmeter is the necessary tool for testing the MAP sensor, TPS switch or O2 sensor.

Also, the crankshaft position sensor at the rear of the engine block (driver's side of bellhousing or converter housing, 11 o'clock looking forward) can get oily or defective.  This is not likely the consistent idle trouble you describe.  A CPS fault will usually cause stumbling, hard start and misfire at speed.  Check the wiring to the O2 sensor and CPS:  Headers and other issues can toast a wire and cause an intermittent short, likely not your current issue from what you describe.

Moses

[Jeep North]

Moses

Rechecked the PCM fault codes and did get a "21" which, as you said, is an O2 sensor issue (took awhile to sort out the "dots and dashes") Pulled the O2 sensor for a bench check. The heated circuit showed 6 ohm's (which passed) so I tried heating the probe w/ a torch - no rise from 0 volts so put in a new O2 sensor. She starts and idles smooth now, even after sitting out all night in the cold.

Thanks again.

[Moses Ludel]

Jeep North...Glad this worked out...You minimized the parts replacement and drilled down using the OBD code.  I like the heated test of the O2 sensor...Good work! 

Happy New Year...

Moses