90 MJ O2 sensor troubles

Typical remix low idle rough running troubles.
A weird problem I found out with my O2 sensor. Unplugged it runs the same, removing the relay it runs the same. The odd part is using a multimeter I can tell it gets good power on 30. The ground is good. And even gets a clean signal to turn off and on. The weird part is on 87 I am reading a voltage with no relay. 87 is the relay output and goes straight to the sensor. With key in off position I get 0V. With key in run it shows 10.5-11V. With truck running it goes up to 12 while the battery sits at 14.4V. I have wiggled all the wires and there is no change at all.
So guys and gals, is this a case of wire harness teardown and trace the wires?

The O2 sensor is brand new. Like just a day old. I have purchased a MT2500 so I hope I can trace down any other troubles or bad sensors if possible. Hope ya'll have any insight to my woes.

Thanks,
Lance

I'm not knowledgeable on MJ's but was an electronics tech for many years. The line that gets up to 12 volts with 14.4 on the battery sounds shaky. Could be corrosion on a connector, or a problem with that wire; just thought of something else. Used to have a Chevy van that had some weird problems and was running rough. Turned out that some corrosion from the battery had corroded two wires and a vacuum hose, was hard to find cause it was on the firewall in a hard place to access. Replaced the wires and vacuum line, worked great. Hope that helps.

The 87 pin on the O2 heater relay is a ballast resistor bypass at WOT. Some engineer got inventive and thought up a dual use for the O2 sensor relay. When the computer turns off the O2 sensor relay at WOT, it also bypasses the ballast resistor for max fuel.

87A is your O2 heater.

Like Jeepinandy mentioned the lower voltage you are reading is line loss and/or the voltage you are reading is after the ballast resistor in the fuel pump circuit or both.

Google mj1988electricalmanual nice little book.

I never did get the low idle to go away with my Renix copletely, what I did was to optimize my ignition system so it idled low but like a metronome. Mine idles low but doesn't miss a beat.

Various reasons for low idle.

A small vacuum leak and the motor tries to adjust. Could be anyplace, but the first thing is to tighten down your TB, the gasket shrinks over the years.

Your small crankcase vent tube plugged.

Weak spark, wrong ignition cables, rotor cap, low voltage to the ignition coil, bad contact between coil and ignition module. Can all make for a rough idle.

Clean out your IAC.

Or like in my case it is just the nature of the beast and I live with it.

The wiring for the O2 sensor and the knock sensor, where it runs up the front of the motor, is prone to cook on the exhaust manifold. Something to look at especially if your water pump or power steering has been messed with. The harness gets rerouted wrong during reassembly.

The gremlins you are messing with are things I dealt with many years ago, my solution may not be optimum but it works.

Mine runs really well at 500 RPM. Thump,thump, thump, thump thump, never misses a beat.

Something else to take a close look at is the ground for your TPS, It doesn't take much resistance in that ground to fool your ECU, we are talking about .8 volts here. A tip off can be a changing idle or surging at idle.

REEEAAALLLYYYY!!!!

How interesting. Had not run into that yet. I wonder if that has any real effect on WOT engine performance? I have low power at WOT, been that way for 13 years since I bought the 87 Waggy (the 89 kicks as at WOT), but it is also on the low end of the limits for compression so I just figured it was an old tired engine, like us LOL. BUT!!!! I never checked fuel pressure at WOT. It runs fine at WOT, just takes too long to get up to speed or pass, fells like it is at 1/2 of WOT.

8Mud said:

The 87 pin on the O2 heater relay is a ballast resistor bypass at WOT. Some engineer got inventive and thought up a dual use for the O2 sensor relay. When the computer turns off the O2 sensor relay at WOT, it also bypasses the ballast resistor for max fuel.

87A is your O2 heater.

Like Jeepinandy mentioned the lower voltage you are reading is line loss and/or the voltage you are reading is after the ballast resistor in the fuel pump circuit or both.

Google mj1988electricalmanual nice little book.

Click to expand...

I solved my low idle on the 87 years ago with a larger orifice. I think I finally went with no orifice about 5-6 years ago, but that was after uncle body adjusted the throttle stop a number of times, LOL. Runs at about 650 with no AC and solid 700 with AC now. Smooth as butter these days as long I keep clean or new plugs in Cyl 1 and 2.

I will make a full reply tomorrow when I can upload pics of the mt2500 outputs. Seems I have 15" of vacuum and even with no o2 relay installed it works just fine at idle. I adjusted my tips from 16 to 17% on the mark and replaced my b+ latch relay today. Seems the 1% did make a noticeable difference. Seems the relay was bad so truck starts and seems to have a IAC that is more responsive now. Will need to see what y'all think of the fuel trim numbers and such. I should either swap my map sensor to see if results are different or check with a gauge.

Adjusted Tips? TPS?

Must be autocorrect on my tablet. TPS, correct.

Here are the pictures. After a couple of trips it seems the computer is learning to lean out the fuel mixture in the LT trim. starts at 128. <128 means lean and >128 means rich. so i do noticed the ST_trim (short term) wants to be very lean at idle telling me I am running quite rich at idle and it goes up to 120 when i am driving.

O2 voltage fluctuates up and down. the exhuast cycles between roch and lean. also the fuel sync shows +/-. So seems everything is working on that end. I also have no o2 relay plugged in right now as it seems I have a short somewhere but it isnt hurting me. I am worried that at idle my vacuum seems quite weak. Perhaps a weak vacuum leads to FPR delivering too much fuel at idle but works itselx out when Im driving around.

8Mud said:

The 87 pin on the O2 heater relay is a ballast resistor bypass at WOT. Some engineer got inventive and thought up a dual use for the O2 sensor relay. When the computer turns off the O2 sensor relay at WOT, it also bypasses the ballast resistor for max fuel.

87A is your O2 heater.

Like Jeepinandy mentioned the lower voltage you are reading is line loss and/or the voltage you are reading is after the ballast resistor in the fuel pump circuit or both.

Google mj1988electricalmanual nice little book.

Click to expand...

Ballast resistor has been removed and direct connected for a while now.

8Mud said:

I never did get the low idle to go away with my Renix copletely, what I did was to optimize my ignition system so it idled low but like a metronome. Mine idles low but doesn't miss a beat.

Various reasons for low idle.

A small vacuum leak and the motor tries to adjust. Could be anyplace, but the first thing is to tighten down your TB, the gasket shrinks over the years.

Your small crankcase vent tube plugged.

Weak spark, wrong ignition cables, rotor cap, low voltage to the ignition coil, bad contact between coil and ignition module. Can all make for a rough idle.

Clean out your IAC.

The wiring for the O2 sensor and the knock sensor, where it runs up the front of the motor, is prone to cook on the exhaust manifold. Something to look at especially if your water pump or power steering has been messed with. The harness gets rerouted wrong during reassembly.

Something else to take a close look at is the ground for your TPS, It doesn't take much resistance in that ground to fool your ECU, we are talking about .8 volts here. A tip off can be a changing idle or surging at idle.

Click to expand...

-I installed a new TPS gasket.
-Crankcase vent is recently replaced with it working properly and no clog.
-Spark plugs new, entire distributor is new, brass all the things, new plug wires. also did Cruiser54 tip of advancing the timing by modifying the distributor.
-IAC has been clean for a while. i did put in a new b+ latch relay and that seems to have helped startup and general heavy roughness.
-looked at o2 and knock wiring. knock sensor is disconnected as I worry the rough idle is causing my exhaust to chatter really bad against my crossmember. Worried it would cause false readings and retard my timing. I guess with the mt2500 i can test that theory. But otherwise the wiring shows no signs of heat damage.
-testing grounds from TPS to battery show 0.2ohms.

Installed a what? "New TPS gasket?"

Why is the coolant temp so high???? says 222 F

O2 sensors are not reliable at idle with out >12 Volts power to the O2 sensor heater on Renix. Know to cause a rough idle if missing or wiring or relay is bad or heater in the sensor is bad or ground is bad. You had different Vac readings, 16 is OK I think, 13 is too low I think.

Love the MT2500 data, I recently bought one too, but have not gotten serious about getting real data with it yet and learning what all it can do.

Ecomike said:

Installed a what? "New TPS gasket?"

Why is the coolant temp so high???? says 222 F

O2 sensors are not reliable at idle with out >12 Volts power to the O2 sensor heater on Renix. Know to cause a rough idle if missing or wiring or relay is bad or heater in the sensor is bad or ground is bad. You had different Vac readings, 16 is OK I think, 13 is too low I think.

Love the MT2500 data, I recently bought one too, but have not gotten serious about getting real data with it yet and learning what all it can do.

Click to expand...

I was idling in driveway a while. Will have to check that my aux fan is working. There are 2 vac readings as combined will equal atmosphere 30.1 I believe. Will have to look at manual to see which one is manifold vacuum.

I meant new throttle body gasket.

So while poking around the Jeep I notice that the voltage that is backfeeding into my 02 heater wire happens to be the same voltage that is powering the relay coil. ~10.5volts or so. I also disconnected a large plug next to the relays and surprise the voltage goes away. i reconnect it and it is back. I am hoping someone here has some insight about this plug.