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Poor gas mileage '88XJ 4.0L

Don Becker

NAXJA Forum User
Good morning!

I'm ready to start hunting-down a poor mileage condition I've been putting-up with for awhile. I went from an ordinary 17 mpg to 13 mpg. I just thought if I gave a list of what's right with it, and maybe you folks would give me some ideas/starting points:
189000 miles
generally runs great
it might be taking slightly more cranks to start lately . . . 'not much, if so
regular lube, oil and filter changes
plugs, rotor and cap about 14,000 miles ago
it seems to have a subtle miss at idle
everything works, this leads me to think no vacume leaks
recently passed DEQ with flying colors: THC 1.71, NOX 1.41, CO 27.2, CO2 816
no overheating issues
all original


I'll probably start with some vacume checks and maybe replace the plug wires since I don't know the history on them. Other ideas before I take it to my local Jeep shop (NOT the stealership)?

Thanks! Don
 
Don Becker said:
Good morning!

I'm ready to start hunting-down a poor mileage condition I've been putting-up with for awhile. I went from an ordinary 17 mpg to 13 mpg. I just thought if I gave a list of what's right with it, and maybe you folks would give me some ideas/starting points:



189000 miles
generally runs great
it might be taking slightly more cranks to start lately . . . 'not much, if so
regular lube, oil and filter changes
plugs, rotor and cap about 14,000 miles ago
it seems to have a subtle miss at idle
everything works, this leads me to think no vacume leaks
recently passed DEQ with flying colors: THC 1.71, NOX 1.41, CO 27.2, CO2 816
no overheating issues
all original


I'll probably start with some vacume checks and maybe replace the plug wires since I don't know the history on them. Other ideas before I take it to my local Jeep shop (NOT the stealership)?

Thanks! Don


Just so you know, 14-16mpg (stock tires, automatic tranny) for city driving is about what I get and have always got with an '89 Renix. Highway, it shoots right up to around 20mpg but city mpg has always been lacking.


However, when it starts to get worse, I begin looking and other than qualifying elec/vac connections and basic sensor functions, 02 sensor has the most impact for me with regard to mpg.


I'd have the shop clean TB/IAC, check TPS adjustment, MAP sensor vacuum connection and operation, temperature sensors, 02 switching, etc. Do a basic once-over through the Renix MPI and you should be as good as it gets but don't expect anything much better than 15mpg city. Big tires and lifts will make that much worse.
 
Thanks, XJXJ. Yes, it is AT and stock tires. I've generally (with both of my 88XJ Limiteds) gotten 17 mpg all around and some better on the highway.
 
caspergotaz28 said:
O2 sensor comes to mind as a mpg killer. Usually sets a check engine light though. Might wanna go ahead and replace it if you have over 75K miles on it.

Renix XJ's don't have a check engine light and codes aren't easy to pull since it's not OBD. Only way for owner to tell is look for strong voltage switching at 02 signal and/OR signs of poor (hunting) idle. Poor mileage is another indication as well as total years/miles on that sensor.
 
I wiggled connectors, cleaned sensors, did a vacuum check (a little low and not quite as steady as I'd like to see). Plugs showed evidence of running rich. Could not detect any vacuum leaks. I had an O2 sensor with <2000 miles on it in my parts XJ so I changed that out and also swapped-out the spark plug wires. I've added system cleaner to the gas. Now, I'll see what happens . . . . Don
 
I went through a sensor renewal program on my 88 XJ and could not get the 21 highway the window sticker said it would get on the highway. I even changed the cat converter when I changed the oxygen sensor. I have more varoom and the feel of power with the new cat.

You may want to make sure your EGR valve is operating correctly. Mine stuck open and dropped me to 15 mpg on the highway.

Also after I changed all the sensor I had the muffler fall off one day. I got a dealer muffler because I did not want some cheap/loud muffler. I figured it was easier to pay the dealer guy to install it. I will say I gained 1 mpg city and 1 mpg highway just because of the new muffler.

Also you may want to replace the CCV harness, expect to pay 100 bucks for all 3 pieces from the dealer. It ia a 52K mile item, you may still have the factory one.

Also take you XJ to a dealer which has the scan tool and have them clean your throttle body. As part of the cleaning they will hook up the test set and make sure all the sensor and injectors are working within spec. It worth the 1.0 hour of labor for the cleaning the the check in my book, do that before you start doing wholesale sensor replacement. Also insist on talking ot the tech to make sure he understands Renix XJs and then ask if you can watch him do the work. My dealer guy now expects me to be there when he works if for nothing else to just shoot the breeze with him.
 
Thanks, Martin. 'Some GREAT additional suggestions there. I have already solved the miss at idle. As I work through this, I'll also check-out these items. Don
 
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