Newby questions about AWD, GM HEI swap, gas mileage.

I’ve got some questions about my Cherokee. It’s my first Jeep so I’m on a learning curve. I've been reading posts but there are so many I apologize if I'm repeating earlier questions.

I’ve only getting about 13 mpg. From what I’ve read, 17 mpg is more typical.

1.) Does anyone have any suggestions on improving the mileage? I may try the O2 sensor because of several other posts.

2.) Would mileage benefit from an improved ignition system, 4 prong plugs, etc… or should I attack the Air intake and exhaust path first.

3.) I’ve seen write-ups for the GM HEI distributor swap for the 258ci-I6. Is this swap do-able on the 4.0L I6?

4.) I’m wondering if the fact that my jeep doesn’t have the front axle disconnect is why the mileage is low. Can any other “Limited” rigs out there send me mileage figures?

5.) I think it doesn’t have the disconnect axle up front so that the transfer case can be shifted into Full Time “on the fly”. Again, do any “Limited” rigs out there have both the Full Time 4x4 and front disconnect axle?


Thanks in advance.
B.

Background info for those that read this far. It’s a 1989 Cherokee Limited with 165k on it. It has the “Select-Trac” transfer-case, AWD, so it has both the Part Time and Full Time 4x4. It does not appear to have the front axe disconnect. It’s a 4 door with automatic transmission.

When I bought it, it was in ruff shape mechanically and had been used as a plow truck for three years. ( Un-registered, no sticker, never left the parking lot.) It had 4 different tires on it. It was only getting about 9 mpg and only the drivers door and window opened. ( I bought it for the plow it had. It was a big help this past New England winter.)

I’ve done all the usual stuff like a full tune up (plugs, cap, rotor), changed all the fluids, (engine oil & filter, transmission fluid and filter, transfer-case, and both differentials).

It also needed a new radiator, hoses, and belts. ( I changed it to an open system.) Also a bunch of vacuum leaks are fixed. All parts are normal stock parts except the radiator is a three core. It’s beginning to resemble a real vehicle. 8^)

In order...

1) Check everything to do with fuel metering - O2 Sensor (HEGO,) MAP sensor, IAT sensor, TPS adjustment - you can find info on all of those here. When fuel metering starts to slack, it usually goes rich and that's what kills your mileage.

2) Not really - the stock ignitions are more than adequate for a stocker. I'd suggest some changes if you were going with a stroker or a supercharged/turbocharged engine, but it's not necessary for what you've got. Check the plug wires tho - they may be getting on...

3) Not that I know of, but I've not dug into it so much. While it is probably theoretically possible, the OEMR distributor carries a SYNC sensor that works in tandem with the CPS on the back of the engine to deliver timing information for fuel injection and igntion. That sensor would have to be approximated on the replacement HEI - and I haven't figured out how to do that yet.

4) That should not be a big problem - my 89 Limited hovers right around 17-18mpg with the 242 tcase and non-Disco D30 front.

5) Both the NP231/D30 Disco and NP242/D30 Non-Disco setups will shift on the fly - the difference is the engagement collar in the front axle. When the NP231 is shifted into a 4WD range, there is a vacuum switch that allows a vacuum actuator on the front axle to engage, and lock the shaft together. This actuator may be eliminated with the shift forked shimmed over for permanent engagment, Ox-Trax does a cable replacement setup for manual engagement, and you can do something similar yourself with a decent control cable and a little time. While there is some small impact to fuel mileage in keeping the front axle engaged (due to additional rotating mass at the front wheels, turning the axle internals, and creating a slight drag,) it is negligible.

Vacuum leaks are a common problem for us - the vacuum harness is Nylon (which gets brittle over time) and the manifold sealing surface begins to work loose over time - I retorque mine about once a year, for insurance (with three XJ's in service, this becomes about a half-day job.) I often find the centre bolts have loosened slightly, so this allows me to catch it before it becomes a problem.

Given the previous operation of the vehicle (as you describe it,) I'd start out with the following:
Change HEGO sensor
Change IAT sensor
Change and adjust TPS - it's probably gotten a "flat spot" at part throttle
Change MAP sensor (standard GM part - you can get these pretty cheap.)
Clean throttle body
Clean and inspect (or replace outright) fuel injectors and seals. Search here for relevant posts on using Ford or Chevvy 302/305 injectors - which you can get cheaply as inspected pulls.
Get matched tyres. The four different tyres could give you some trouble, with having slightly different diameters. Solving this problem may have a startling effect on your fuel economy!
Change ALL lubricating fluids - 75-90 in the axles, Dexron II/III in the transfer case, Dexron II/III non-synthetic in the transmission. If the AW4 is run on synthetic from early on it works well, but I'm sure it's got this much mileage with dino oil - which means that switching to synthetic is likely to cause a lot of slippage.

Report on your results, and we'll see what else we can come up with. Considering I'm running with an 87, an 88, and two 89's, I've finally gotten a pretty good handle on these things... :wave:

5-90

thanks for all the great info..... now to digest it.

All tires are now BFG Long Trails 225/75-15
All fluids have been changed.
I've fixed multiple vacuum leaks so far. Still looking.....

Welcome to life with a limited. You've got some great info here already, I think I can add few tips and cautions as well:

First I'm quite certian there were NO drivetrian options on the limited (maybe D44 rear, anyone?), and very few options at all. 4.0/aw4/242/D30 non-disco/D35.

A little lube will go a long way with the power locks and windows, cuz changing either is a pain so look after 'em if they still work. There are actual wiring deffiencies which further degrade rear window performance, there are fixes, look it up. If your seats don't work, look under the carpet right under your feet, that's where the wires are.

Few more things: 242's don't like mismatched tires. XJ's are only rated for plows weighing less than 300lbs.

We have an '89 Limited and an '89 MJ with a plow on it. The chain wore out somewhat prematurely in our 242, but otherwise it's been fine. The plow is probably much heavier than reccomended and I plan on removing it, but I can't find any signs of the "structure damage" so strongly cautioned of in the manual.

Thanks for the additional info.

Before I replaced the tires I could only get 4WD, AWD, or 4WD LO. Now with matching tires everything works right. Even got all the indicator lights working. Last check was 13.5 mpg mixed driving in 2WD.

I've worried a bit about the plow weight too but it seems to track fine with and without the plow. It's a Meyer 6.5'. I have no idea of the weight. How big is your plow?

There are air assist shocks on all four corners. I think that has prevented any significant sag. After I removed the plow, power unit and lights this spring, I dropped the shock pressure to 15lbs and measured the wheel well lip to wheel center both front and back. It's about 17" all around. I saw a post saying 17" front/ 17.5" back was normal. I aired the shocks back up to 40lbs on advice from a RV center near me. They said keep some pressure in the system. Measurments are about 18" now. Hey, an instant 1" lift.

My priority this weekend is to install a new hitch. The old one was rusted out. I what to go camping in about a month. Right now getting there is more important than the mileage.

I should be able to start on some of the mileage suggestions this weekend though. I'll start with the low cost stuff first. Clean throttle body, replace plug wires, retorque manifold bolts and continue to look for vacuum leaks. I still beleive there are more because the heater vents open/close very slowly. I've been told to remove the battery & battery tray then inspect the lines to the vacuum bottle in the front fender. Another low cost item. :repair:

thanks again guys. :yelclap:

bjlinma said:

3.) I’ve seen write-ups for the GM HEI distributor swap for the 258ci-I6. Is this swap do-able on the 4.0L I6?

Click to expand...

The 258 was a carbureted engine with a conventional ignition. The 4.0L in your XJ already has a high-energy ignition that's at least as good as the GM HEI you're looking at. Save your pennies.

bjlinma said:

After I removed the plow, power unit and lights this spring, I dropped the shock pressure to 15lbs and measured the wheel well lip to wheel center both front and back. It's about 17" all around. I saw a post saying 17" front/ 17.5" back was normal.

Click to expand...

You have it backward. It should be 17-1/2" front and 17" rear.

02 sensor is probably the mpg killer. This system will hunt for idle when HOT with a bad 02 and it's not hard to see the voltage switching with a DVM either. But whenever mine starts getting much below 15mpg city, it's the 02 that brings things around.

Yes, clean the TB/IAC because there's never any gasoline in that area of the intake to wash things down.

Sounds like we've got the same plow, Meyer 6.5' electric/hydraulic. I don't know what it weighs but I'm pretty sure it's more than 350lbs. Mine also has airbags but it's not actually the suspension that's the concern. The front uni-body structure simply wasn't designed for it, check carefully for cracks especially at the plow mounting points.

Just an update. I got the new hitch mounted. Had to remove the read bumper to bolt it up. Now I can't put the bumper back on because the two rear bumper mounting brackets are rotted. Ordered two new ones online.

I took out the old O2 sensor and it was cross threaded. I have borrowed an 18mm thread chase and will reassemble Saturday morning. Then I'll get a mileage check this coming week and post it. I want to get a reading before I change anything else.

Well I changed the O2 sensor and put 120 miles of mixed driving on the jeep. Mileage was 16.2 mpg. Pretty good improvement over the 13 mpg I was getting.

Can't wait to see what I get on a long highway trip I have planned for the end of the month.

Still have a few suggestions to try but it's very close to the 17 mpg that seems typical given the age and mileage.

Thanks for all your help guys.