Jeep North

Moses Teed a vacuum gage into the "engage" side of the front axle vacuum motor. Select 4H while parked and the gage indicated 16in. Hg. so I took it on the road. Good vacuum readings pulling through 1st and 2nd gear. Good reading when going into 3rd and putting engine under load and that's when the clunk and jerking began. When pulling in 3rd the gage needle was steady and when the clunking occurred the needle only quivered, +/- 2 to 3 in. Hg from it's steady reading. I'm fairly sure not it's not the vacuum switch on the transfer case. I'm thinking you called it your last message regarding damage or wear in the fork/clutch area, although I haven't investigated it yet. I'm hoping it's only the vacuum motor not actuating far enough to allow the fork to completely engage the axle halves. Stay tuned.

SW 1940, I have a 1991 YJ 2.5 MPI. When I started it in the AM it would fire right up and idle fine for about a minute then idle down till it would almost stall. Traced it to a bad O2 sensor in the exhaust. Seems the ECM does not look for a signal from the O2 sensor at start up but shortly that after it does, and no signal means bad/no idle.

Moses Had some nice weather so did some vacuum testing using my vacuum brake bleeder. Applied 15in vacuum to the engage side of the vacuum motor and rocked the jeep a few inches, the front drive shaft would move. Applied same 15in to the release side of the v motor, rocked the jeep and the front shaft didn't move. Also checked for bleed down on both sides but they held steady > so the motor doesn't have an internal leak and it drives the engaging fork. Next checked the v motor connector fittings. With the engine running, a steady vacuum (no needle flutter) reading of 17in. HG on both the engage and disengage sides was correctly indicated when either 4h or 2h was selected on the transfer case. I guess I was surprised to see a steady vacuum reading on the disengage side so obviously the v motor is not spring loaded to the off position. Next applied 17 in of vacuum to the ports on the v motor connector. Both held fairly well with just a very slow bleed down which I assume was coming in through the vacuum check valve in the supply line between gauge and the manifold (engine off). So much for static tests. Next I'm going to T a vacuum gauge into the v motor engage line and take her out on the road to see if the vacuum on the engage side is really dropping out and then suddenly re-engaging. If so, i'll assume the switch is either bad OR perhaps the transfer case linkage is enough out of adjustment that 4h will engage in the case but the v switch button is not being fully depressed. Not sure what the button rides on in the case so just guessing. Any thoughts.

'91 YJ . Move the transfer case lever to 4h, the front axle engages and the indicator light comes on. All works fine at slow speeds, but as speed increases (toward 40 mph) the indicator light will flick off and then back on accompanied by "clunk" noise from the front and slight jerk felt in the chassis. Since the shift motor only has 2 vacuum lines, I assume that one applies vacuum for engagement and the other vacuum for disengagement. Both lines lead to the control switch on the transfer case. I suspect the control switch is failing. Any thoughts.

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.

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)

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.