First MJ(s)

[Awesome]

Alright someone help me understand this:

 

With the TPS plugged in, I get maybe 1v at pins A-B (power and ground), pins B-C get wildly varying voltage (with zero throttle activity) from .3-5.18v.

With the TPS unplugged, I get nothing at pins A-B, and pins B-C go to 5.18v steady.

 

Verified with two multimeters.

 

Bad TPS? Or a bad power lead TO the TPS?

[eaglescout526]

Power wire could be messed up. Verifying will require the ECU to be unplugged and verify continuity to the TPS from the ECU input and ECU output. Now I’ll admit, backprobing that damn plug sucks. 

[Awesome]

The TPS was bench-tested today and tested good.

 

I am armed with some schematics and the knowledge that the MAP sensor is also tied to the TPS circuit.

 

Going to check some wires tomorrow.

 

Had a stupid thought too... one of the other things I hooked up was the coolant temp gauge that had previously not been hooked up to anything. Part of me wonders if that's grounding something out that shouldn't be grounded out. I'll have to check to see if I'm somehow getting voltage across it.

[Awesome]

Got some paperclips and very carefully made sure I correctly backprobed the TPS. It was fine. The wire I was using before was causing the bad readings.

Checked the MAP plug and it was fine too (while unplugged).

Pretty sure that rules out the ECU and the wiring.

I noticed that the open port on the valve cover was blowing a LOT of air. I covered it and the engine ran differently, but built up pressure against my finger. Not enough to push my finger off, but enough pressure that when I let go of the port, it blew a LOT of oily air out for several seconds, and the engine chugged badly.

I tried to get my friend to bring his compression tester over but he didn't show, so I'll run compression tests tomorrow.
I'm now leaning toward a bad engine or perhaps a valve that's stuck open.

[Awesome]

Ok I am stumped.

 

Did a compression test.

Dry / Wet

 

#1: 170/250

#2: 170/240

#3: 145/225

#4: 125/170

 

Numbers 3 and 4 are down enough to be out of spec, but they aren't blown and there isn't a valve stuck open.

 

Reseated the TPS and got it as close to spec as I could (4.3v at WoT, spec is 4.6v).

Fired it up and it smoked a bit from the ATF in the cylinders. Realized it's got a pretty good exhaust leak on the manifold, as it was blowing quite a lot of smoke from under the #1 tube.

 

It was idling pretty good so I decided to take it for a test drive.

 

I drove it a couple of miles and it was running OK... it was down a bit on power, didn't want to accelerate well and had a few flat spots in the acceleration. Got it up to about 57mph. 

Oh by the way, the electronic speedo works. Thanks, Boxy. Need to get a different gear though as it's off by quite a bit with the gray gear. Was reading far too slow.

 

Filled the tank with gas. Leaving the gas station, all heck broke loose again. It wouldn't accelerate at all and was burbling and popping like crazy. I ended up driving the 3 miles home on the shoulder of the road at 15mph. I had to constantly let out the clutch as it would start bucking very badly after a few seconds. With no load, the engine would rev, but as soon as I had a load on it it'd start bucking very badly and kill all the power. It backfired through the exhaust a lot.

 

Right before I got home it suddenly cleared up slightly and started running like it was when I had first left. Was able to get it to move without it bucking like crazy.

 

I have no idea what to do or what to even test for. Unless someone can give me explicit testing instructions I don't know what I can do from here.

[GonzoTheGreat]

20 minutes ago, Awesome said:

Realized it's got a pretty good exhaust leak on the manifold, as it was blowing quite a lot of smoke from under the #1 tube.

If there’s an exhaust manifold leak, there is likely an intake manifold leak, ie a vacuum leak. Both manifolds share the same mounting bolts. Have you checked all of the manifold bolts and studs to make sure they are in, and tight?

[Awesome]

28 minutes ago, GonzoTheGreat said:

If there’s an exhaust manifold leak, there is likely an intake manifold leak, ie a vacuum leak. Both manifolds share the same mounting bolts. Have you checked all of the manifold bolts and studs to make sure they are in, and tight?

 

Yes, multiple times. I believe the manifold is cracked as I can hear it ticking pretty loudly any time the engine has been turning over. 

It was even ticking after turning it over during the compression testing. No fuel, no spark, but still ticking/popping.

[eaglescout526]

This makes me think either head gasket failure or fuel pump failure or being intermittent. When was the fuel filter last changed?

[Awesome]

2 minutes ago, eaglescout526 said:

This makes me think either head gasket failure or fuel pump failure or being intermittent. When was the fuel filter last changed?

 

When the fuel tank was changed. Last year. Has a few gallons of fuel through it. Less than 30, I'd think.

 

Head gasket is possible. I checked for pressure in the cooling system and didn't get any, but it could have failed in the other direction.

[eaglescout526]

Ouch. Coolant brown?

[Awesome]

1 minute ago, eaglescout526 said:

Ouch. Coolant brown?

 

Yes. Went from green to brown pretty quickly. Been trying to flush it.

[eaglescout526]

Thats a damn good sign actually. Mine would do it, Id be driving and it would cough and sputter, press down on the pedal harder for more gas and it would clear up until the issue came back. I think you have a headgasket issue.

[Awesome]

Put the map sensor from my YJ on the MJ as technician friend was leaning toward the MAP sensor being bad. 

 

It helped but did not fix the problem. It is still not running correctly, and he noted that the IAC no longer appears to be functioning. He is going to diagnose further on Thursday. 

[knever3]

Sounds like you have a bad ground for your sensors under the hood.  This could be on and off again if moved slightly.  Have you gone through the harness and soldered every junction point and verified 0ish ohms of resistance while moving harness?  The TPS is very sensitive to ground fluctuations.  This seems to make the IAC motor do crazy stuff, the O2 sensor could be faulty as well, but I bet it is a ground problem.  Check your voltage at the TPS and move the harness around, then start moving to every sensor and do the same.  

[Awesome]

On 4/30/2025 at 7:31 AM, knever3 said:

Sounds like you have a bad ground for your sensors under the hood.  This could be on and off again if moved slightly.  Have you gone through the harness and soldered every junction point and verified 0ish ohms of resistance while moving harness?  The TPS is very sensitive to ground fluctuations.  This seems to make the IAC motor do crazy stuff, the O2 sensor could be faulty as well, but I bet it is a ground problem.  Check your voltage at the TPS and move the harness around, then start moving to every sensor and do the same.  

 

This is possible, but it seems likely now that the TPS itself went bad. 

 

After significant testing, it has been determined that the TPS sometimes outputs the correct voltage and other times does not; the input voltage doesn't change. It appears to be a random failure, indicating a cracked internal resistor. A new TPS has been ordered and will be installed once it arrives.

[eaglescout526]

3 minutes ago, Awesome said:

appears to be a random failure, indicating a cracked internal resistor.

not much inside actually…

[Awesome]

4 minutes ago, eaglescout526 said:

not much inside actually…

 

I'm not sure how to pull it apart. With the way mine has failed, it's more likely the resistor has failed than the contacts just being dirty, but it can't hurt to check.

[eaglescout526]

On the OEM ones you can just remove the arm and pull the back off. I noticed on aftermarkets that they are sealed units. 

[Awesome]

Did some more diagnostics on the truck today. Hooked up the computer and entered the VIN correctly this time and was able to read a lot more data. It does have a tach signal, so that makes me hope I could hook a tach up one of these days.

 

It's running in open loop.

It is not reading a signal from the o2 sensor.

It's reading the TPS at 20% at idle, which may be why the IAC isn't working. Tried to adjust TPS some more (got it down to 20% from 28%) but I somehow stripped the screws out.

Used a 'scope to check the TPS signal in and out. Both are dirty (not terribly, but also not solid), and it appears to be working fine but the voltage is out of range on the output side. So I still think the TPS is probably bad.

Wiggled the entire wiring harness around a bunch and had no voltage changes except for one 5v drop to 2v that was a spike and we couldn't replicate.

Injector timing is all over the place from 60ms to 1ms.

New MAP sensor is reading fine.

Manifold coolant sensor is reading fine.

Manifold air sensor is reading fine, though much hotter than the coolant temperature.

4" manifold pressure is reading OVER and 10" manifold pressure is reading UNDER. I have no idea what those are or where those sensors are. I can only find one vacuum sensor of any kind besides the 4wd switch and it's over by the ignition coil. I don't know what it is or what it does.

The vacuum lines were pulled and plugged to rule out vacuum leaks.

 

So we're going to test the o2 sensor to rule out either a bad o2 sensor or (again) a bad ECU.

We were getting a tiny voltage spike from the o2 sensor when plugging it in and unplugging it, but otherwise we were getting nothing.

[Awesome]

New TPS was installed today.

 

Oxygen sensor tested bad using a sensor tester. New oxygen sensors are very expensive.

 

I am considering my options. I'm at the point where I'm not sure it's worth it to continue throwing money at this old, tired AMC 150. It may be a better option to spend the summer engine (and controller)-swapping the truck.

[Awesome]

Options weighed, discussed with wife, wife said "take a break from working on it, but don't give up on it. It's too cool of a truck for you to get rid of."

 

New oxygen sensor should be installed tomorrow.

[Awesome]

Drove the '86 MJ home today. After replacing the MAP, TPS and finally the o2 sensor, then doing a computer reset, it now runs better than it has since I've gotten it running. Runs very smoothly with no threats of dying, and feels like it has quite a bit more power than it used to.

 

The o2 sensor connector had some corrosion so I used some electrical cleaner (WD-40) and a 12-gauge stranded copper wire and cleaned the pins.

 

Daughter pulled the rear diff cover off (it was leaking badly) and we bought a gasket and new oil. Put it all back together and it's still leaking very badly. All the bottom bolts are leaking for some reason. It's like there's oil in the thread passages. I think one of the axle tubes is leaking as well, where it is pressed into the center section. Soooooooo I might just go look for a new axle with a lower gear ratio.

 

Anyway, I need to drive it some more to really make sure it's fixed, but it's running so smoothly right now that I can't imagine it's not fixed.

 

Now we can get back to fixing up the minor things so I can fix my YJ.

[Awesome]

Worked on the MJ for the past couple of days. Figured out why the rear axle cover was leaking so badly.

 

The replacement cover from my extra D35 and the OG cover.

 

Check out the difference in thickness. Spicer must have known about this problem and fixed it on the later axles.

 

The problem in question. Almost every one of the bolt holes had multiple cracks around it.

 

Also, I got the PCV system hooked back up using the original MJ PCV. I have not tested it yet but it should work fine.

 

Never did post any pictures showing the new battery cables, so here they are. Daughter did a good job making them.

[Awesome]

I drove the truck about 50 miles yesterday. About 25 miles in, during hard acceleration going slightly uphill, it started to shudder and backfired a couple of times. Letting off the throttle slightly made it stop. It did it a couple more times on the way back, going uphill under load.

 

At one point I noticed that the temperature was elevated when this was happening, and later when I noticed the temperature had fallen, I was unable to get the engine to do it again.

 

It's probably a failing coil or perhaps just a bad battery, since I know the battery is bad anyway. Might be both. The truck was running perfectly other than that. I had no problems doing 75mph on flat ground.

 

The rear axle leak is fixed, mostly. I think one of the tubes is leaking. I'll either weld the tubes or replace the whole thing, but that's a later problem.

[89 MJ]

11 hours ago, Awesome said:

I had no problems doing 75mph on flat ground.

The 2.5L really is a great little engine.