Eagle

My '88 XJ 4.0L 5-speed is stock with 3.07 gears. I get around 21 to 23 MPG highway. My '88 MJ 4.0L 5-speed has been swapped to 3.73 gears and I am currently running stock tires on it. Highway mileage is typically 19 to 21 MPG, with the tailgate up and no tonneau cover. The 3.07 gears actually leave the 5-speeds cruising well below the engine's torque peak at legal highway speeds, which is not ideal for efficiency/economy. 3.55s or even 3.73s will put you closer to the "fat" part of the torque curve at highway speeds, and that's really where you want to be running.

I don't think the FSM specifies a pressure. Never saw one. If the system is operating properly, it shouldn't matter. The normal operating temperature is 210 degrees, which is below the boiling point for unamended water at sea level. With either the second or the current (third) radiator in my XJ with the closed system, the temperature with a 195 degree t-stat never goes over 200. Ditto the '88 MJ, with the original radiator.

I am certain I can't dissuade you from that opinion, but what you are proposing is physically impossible.

You don't mean "improving" the closed system, you mean "restoring" the closed system. Changing to an open system doesn't "improve" anything -- the radiator has exactly the same frontal area and cools exactly the same as a "closed" system radiator of the smae type from the same manufacturer. The issue is that, after 20 years, the original radiator is probably clogged almost solid. IMHO it's a miracle it has lasted this long. I am now on the third radiator in my '88 XJ -- the original lasted 12 years, the replacement lasted 7 years. Just put in a decent new radiator and you should have no other problems.

What year did you have them check? The '86 used a smaller exhaust on the 4-cylinder. In '87 or '88 they standardized on the 2-1/4" for both engines.

It only helps at higher RPM's as it is an air flow thing. It helps get a more directed & consistent air flow to the engine so it is not really noticeable all the time on these engines. This brings to mind one other difference between the Renix and HO versiuons of the 4.0L engine -- the HO has a larger throttle body bore. That means it flows more air at higher RPMs. I don't know how the original poster drives or what RPM band he spends his time in when looking for "pep," but the smaller bore is noticeable for those who like to live in the upper end of the power band. You can but bored-out throttle bodies, or you can grab a TB from a junk yard and hone it out yourself. It might make a difference, and it's an easy, bolt-on change. (Once you get it bored out, that is.)

Sounds like someone put a 4-cylinder exhaust on your 6-cylinder truck. As commented above, unless you spend a lot of time at wide-open throttle, it won't hurt and it might help.

Nothing. There is no "Renix vacuum mess." What you may be referring to is the fact that the HVAC controls are all vacuum operated. That has nothing to do with the engine, and will remain after you swap to an HO engine. Aside from creating a lot of work for yourself for questionable gains, all you'll really be eliminating is the EGR valve.

You really can't compare the two engines. Let me repeat: You REALLY can't compare the two engines. The Renix 4.0L is a low-RPM, torque engine. The '87 was the first year for it, and it actually produced less torque and horsepower than the '88 - 90 but I still haven't found out what they tweaked in '88. However ... ... The '91 and up HO engine raised the torque peak from around 2200 RPM to well over 4000 RPM. It also changed the cam and the manifolds. The result is that the HO breathes freer and revs higher, so it feels like it has more "pep" -- because most Americans equate revving through the gears with "pep." The factory gave us what we wanted. Except for me. I prefer torque. The natural shift point for a Renix 4.0L engine is about 2,000 to 2,200 RPM. The natural shift point for an HO is around 3,500 RPM. If you want your '87 to run like an HO, you'll have to cam it.

That includes the mounting flange and fuel guage sender, so it would work. Except that your signature indicates that you own an '88, and this part is listed for the '91 & '92. I don't know if they work at the same pressure, and I don't know if the polarity is the same on the guage wiring. (Of course, the wires could be easily reversed -- they're just push-in connectors.)

Nope. It's because I've heard too many stories of the platinum electrode tip breaking off and taking out the piston. The stock plugs with the Jeep high energy ignition easily last 50,000 miles or better, so why mess around with a hand grenade in your engine? I also tried a set of SplitFires in the XJ once. Didn't run any better than standard NGKs, and gas mileage was worse. Sometimes, newer and more expensive doesn't mean "better," it just means newer and more expensive.

Go to the Monroe shocks on-line catalog. At the back they have a spec section that shows the mounting configurations, lengths, and travel dimensions for every shock they make. You can look up the stock shocks for a TJ and an MJ in the application listings and then compare dimensions. I don't think TJ shocks translate very well into the MJ, but that's based on fuzzy memory.

There is a resistance, but I don't recall what the spec is. The old FSMs used to give a number, but the new ones probably just tell you to hook it up to a scan tool. I think it was something like 1,000 ohms per inch or something like that. (Or maybe it was 1,000 ohms per foot?) Resistance is needed to reduce radio interference. Most factory wires, in fact, aren't "wires" at all. They are fibers, impregnanted with a conductive carbon compound. Over time, as the wires get hot and cold, and maybe man-handled from working around the engine, the carbon gets broken and there are gaps that the spark can't jump. That's when you start getting skipping and missing under load. The best replacement wires have a solid (or stranded) wire core, surrounded by a radio supprssion winding.

All platinums.

Have you seen this little chart for NGKs? http://www.spark-plugs.co.uk/pages/tech ... l_code.htm No. It's not an application listing, it's a chart that you use after you see what plugs fit your application, to see what type of plug the part number represents. I never had a need for that. I get them at Advanced Auto (same company as Shucks and Checker in other parts of the country) and just ask for the standard plug for a 4.0L Jeep.

Especially platinum plugs. Stay with standard plugs. NGKs are best, Champion truck plugs are a distant second, all else is tied for fourth.

You should have front brakes ... but you'll have a VERY low pedal. And the front metering block on an MJ is not a proportioning valve. All it does is provide the brake system warning light if you lose front or rear brakes. All proportioning is handled by the rear valve. If you really have NO front brakes with the rears disconnected, you probably need a master cylinder.

The pinion snout on a D44 (if you should be so lucky as to be getting a D44) is about 3/4" to 1" longer than the D35, which means that the driveshaft should be cut down by a commensurate dimension if upgrading from a D35 to a D44.

NGK. The standard ones, NOT platinum.

Do you have a manual for the '90? The engine is the same. They made a few very minor tweaks between 1987 and 1988 to get a few extra horsepower out of it, but mechanically it all goes together exactly the same. In fact, for the engine (not the fuel injection) you could just as well use a manual from any AMC 6-cylinder all the way back to 1965. The 199/232/258/4.0L are all the same basic engine. What do you think is missing and what do you have that doesn't belong? There's virtually no difference between the 87 and the 90 -- are you sure you're not comparing to a 91 or newer engine setup?

Is the chip replaceable? The Renix is not replaceable. I know the ZJ could be replaced with a Jet chip, so the Chrysler chip in the 91 and 92 MJ might be replaceable -- if anybody makes on for it.

That's not the casting number, it's the build date code. My '88 FSM says: 1st character = year 2nd & 3rd characters = month 4th character is engine type (M=4.0L) 5th & 6th character = day 701M20 sounds a lot like 4.0L, January 20th, 1987

I only know one guy who did this swap, a friend from the early NAXJA days. I should mention that at the time he was a student in electronics at a technical university, and basically an electronics genius. He said he had to swap the ENTIRE harness, and I have no reason to doubt him. You can't just plug into the bulkhead connector for the simple reason that the ECU for the Renix years is mounted inside, under the dash, and the wires to it come from the bulkhead connector. And if you get a FACTORY wiring diagram, you'll see that the pinouts for the 4.0L ECU are dramatically different from the 2.5L ECU. Go ahead and try it if you think it'll work. I'm curious to see how you'll make out, but I've been researching the same thing and I am fully anticipating that I'll have to swap the entire harness from the donor vehicle.

No, the rear license plate light for the MJ is completely different from the XJ.

I got a pair a couple of years ago from Auto Zone. They weren't in store stock, I had to order them. Took about three days to come in.