Eagle

Clogged catalytic converter.

Which coolant temp sensor was the scan tool reading? There are three, if you have an auxiliary fan. The one on the back of the head only sends data to the gauge; it has nothing to do with operating the engine. The sensor in the radiator tank only controls the auxiliary fan; it also has nothing to do with controlling the engine. The temp sensor that feeds the ECU and actually affects operation of the engine is located down low on the driver's side of the block, near the knock sensor (and sometimes mistaken for the knock sensor). Yes, it is basically just temperature that controls when the Renix system switches from open loop to closed loop mode. Don't discount the possibility that you may simply have a broken wire, so the information is not getting from the temp sensor to the ECU. I would verify that the temp sensor is reading correctly and that the signal is reaching the ECU before I swapped in another ECU.

Wrong. First, you don't have a fuel bowl. Second, the O2 sensor doesn't flip into closed loop mode, the ECU does. And it does so when the operating temperature reaches the "normal" range, which is around 165 to 168 degrees, as reported by the temp sensor in the side of the block (not the one on the back of the head ... that one only feeds the gauge). If you aren't getting into closed loop mode, your ECU is simply ignoring the O2 sensor and running in "warm-up" (open loop) mode all the time. You most likely either have a bad temp sensor or a bad ECU. Start by checking the wiring between the temp sensor and the ECU to be sure you have a circuit.

Older S10. Early to mid-90s, IIRC. There was a guy who posted the exact information a couple of years ago but I don't remember what he used for a screen name here. I knew him from NAXJA, and his father used to sell caps. I'm using a used S10 utility cap on one of my shortbeds and it fits perfectly ... but I don't know what year truck it had been installed on.

You can disconnect and plug the hoses going into and out of the heater core, or just stick a short length of copper tubing in the ends to make it a continuous loop under the hood, with nothing flowing through the heater. You won't have heat, but you also won't have leaks.

In my case, the CPS is brand new. Less than a year old and probably doesn't have 2,500 miles on it. It is always possible that it's defective, but it's a Jeep factory CPS and they very rarely spit out a bad one.

I assure you, if you think swapping in a 4.0L is a lot of work, doing a Chevy 4.3 will be ten times more work. NOTHING will be bolt-in, and EVERYTHING will have to be custom adapted. A guy a few miles from me used to have an XJ with a 4.3 in it. I asked him once if he'd do it again. His answer was unprintable in a family-oriented forum, but the bottom line was "Hell [insert expletive of choice] NO!" Remember, a 4.3 is just a small-block Chebbie with two less cylinder. For the work and fabrication involved, you might as well go with e 327/350. By contrast, everything you need for a 4.0L swap has already been designed and mass-produced. All you have to do is drop it in.

^^^ What he said.

That has been suggested, and that is NOT what I wanted to hear. But -- it may well be the problem. :roll:

The ignition module is on the list of suspects. Before actually swapping it, I want to generate the fault where I can work on it so I can test to see if the missing ingredient is fuel or spark.

In my case, no. I have shifted relays around and it makes no difference. And it isn't a loose wire. It is time/temperature related. I drive for maybe 15 minutes, and suddenly the power dies off and it stops. And WILL NOT restart. Sit by the side of the road for 30 to 45 minutes, and it starts up like there was never anything wrong. Something is getting heat soaked, but I can't figure out what unless/until I can get it to die while sitting in my yard, where I can unleash the testing crew on it without risk of getting clipped by a passing 18-wheeler.

My '88 has been doing the same thing, and somebody else also reported this happening. In my case, I checked the ballast resistor and that was NOT the problem. Still baffled. Problem is, it's hard to test because I can't predict when/where it will happen. AAA only gives me four tows a year, and I don't want to use them all on the heap and have to go for eight months with no safety net for my wife, so I have been limiting use of the MJ to short trips that I'm fairly certain won't cause a stoppage. That avoids tows, but doesn't provide opportunities for trouble-shooting.

Loss of vacuum doesn't cause loss of heat or a/c. What is does is cause the flappers inside the vents to move to the default position, which is through the defrost vents on the top of the dash. So if you're cruising along on a zero degrees day with hot air warming your toes, and you go up a long hill and your feet suddenly feel cold -- put your hand up on top of the dash and you'll probably feel the hot air coming out there. As noted, running with a busted "blimp" creates a vaccum leak, and that's never a good thing.

Well, not hissing now... can't take it for a spin to warm it back up since I got RugRat duty tonight... if it's a "leak", are we most likely talking just the cap, or the whole stinkin' bottle? (Where do I get either one of those other than a JY?) You do NOT buy just a cap. Guaranteed to NOT seal on an old bottle. Quadratec used to sell the complete bottle and cap assembly for $19. Of course, that's for another plastic fantastic. I prefer the several all-metal solutions that have surfaced recently, despite their initial higher cost. Oops. It's up to $25 now. Not a surprise -- I haven't priced them for several years: http://www.quadratec.com/products/52449_1493.htm

It should not be boiling (bubbling). If it is, that means it's either a bunch hotter than it should be, or the cap isn't holding pressure. Remember, higher pressure raises the boiling point of a liquid. That's the whole reason for radiators to have pressure caps.

First, it isn't a "return" line. It's a bypass line. (Don't feel bad. I owned my MJ for at least 5 years before I realized that what everyone called the "return" line ... wasn't. Second, the port in the metering block uses a straight machine thread. I don't recall if the size is 3/8" or 7/16" but you buy the shortest FINE (SAE) thread bolt you can in that size, then cut it shorter. You probably could just put some Loc-Tite on it and screw it in, allowing the end to seat against the raised cone in the block. That looks too home-brewed for my taste, so I cut it off so that the end of the shank just barely cleared the seat in the metering block with an O-ring on the bolt. Then I put a glob of black silicone RTV on the end and allowed that to set up. When I screwed the bolt in, I got (I hope) a redundant seal -- the blob of silicone on the seat, plus the O-ring under the bolt head. And it looks a lot neater to see just the hex head rather than seeing a half inch of thread sticking out.

If you already removed the height sensing valve, you don't need the instructions in this thread. You bleed as for normal brakes. If you removed the height sensing valve, you should also have removed one of the two hard brake lines from the front of the truck to the rear. The single line you have left should run from the NOSE of the front brake metering block (not the bypass port underneath) directly to the flex hose at the rear axle. Hornbrod -- I was not aware that they changed the height sensing valve in 1991. You are correct, my FSM is for the early style.

The problem with the FSM adjustment procedure is that adjusting calls for a new part (a spring, IIRC, but I'm not at home where I can confirm that), which is no longer available. In theory, adding load to the bed pushes the bed down which in turn pushes the arm on the valve up, so zip tieing it up at about 45 degrees should increase the rear brake bias. If it doesn't, the valve is probably not working. The reason I don't trust them as I have posted previously, is that mine blew out in a panic stop situation. If one 20-year old diaphragm blew out, I have no reason to believe that another of the same device won't also blow out under panic stop conditions. I was lucky the first time -- I stopped before t-boning the old man who cut me off. I can't count on being lucky a second time. The valves aren't available from Jeep, so the one that blew out left me with no choice. I had to bypass it, because it was hemoraging brake fluid. But -- I'm old enough that I grew up and learned to drive before they had such devices. The cars I learned to drive on routinely locked the rear brakes before the fronts in a panic stop. It's not a big deal to me, it's normal. I understand that it's unnerving for you younger lads who aren't accustomed to the phenomenon. But I still don't trust those old valves. I think it's a far better solution to dump the height sensing valve, re-plumb the hard lines so you have only ONE steel line from the front directly to the rear flex hose -- and then put a Wilwood adjustable proportioning valve in that one line to adjust the rear brake bias to suit your driving habits. If you put the Wilwood valve in the driver's compartment, you can adjust it on-the-fly to compensate for carrying heavy loads. Or mount it right next the the front metering block, and you can adjust it by popping the hood for access.

If you still have the rear height-sensing valve -- this won't help. The factory setup is biased to send virtually zero braking force to the rear wheels when the bed is unloaded.

That's what I recommend, for those uncomfortable with rear brakes that really work. An alternate would be to use rear wheel cylinders with a smaller diameter pistol to reduce the rear braking somewhat. FWIW, the Mopar Performance Catalog offers (or used to) an adjustable proportioning valve that I believe is the Wilwood. BLHTAZ may be able to get it for you for less than the price shown in JT's link.

Pete, the way you rigged your brakes is the way I rigged mine -- we have full power to the rear brakes all the time, so there is no need for the bypass port. If we lose the fronts, we have the rears. The problem is plugging the bypass but leaving the normal (nose) line feeding through the rear height-sensing valve. When an MJ is empty, if the height sensing valve is adjusted correctly there is almost NO force going to the rear brakes. This is why the rear brakes never seem to wear out. The whole point of the bypass circuit is to get some braking in the rear if you lose the front brakes. Plug the bypass, and the only rear brakes you have if you lose the fronts will be the "proportioned" rears -- which effectively means virtually none.

Ya know, that truck may well be rebuildable. A half-decent body man could cut the front "frame" ears off a Cherokee and weld them onto the MJ to make the nose square again, and unless it was hit REALLY hard from the back I suspect the frame rails will be either straight, or easily straightened. Beds are available ... maybe not "commonplace," but available. By all means, grab it. But don't start chopping until you have thoroughly investigated how hard/easy it might be to repair it.

If you are keeping the rear proportioning valve, you MUST keep both lines to the rear. One is for normal (proportioned) braking. The other (the bypass) is to provide full power to the rear brakes if you lose the fronts. If you keep the rear proportioning valve, DO NOT attempt to eliminate one of the lines or you will be creating a very unsafe condition.

Yeah, the rear proportioning valve DOES require a special procedure. First, bleed all four wheels normally. After that, the FSM says to do the following:

Clayton's Off-Road makes (or used to make) a complete kit for installing the front D44 from a full-size Cherokee/Wagineer in an XJ/MJ. Don't know if they still sell it.