HELP! My stock XJ is only getting 7 MPG!

[BCParker]

I've had the same problems - '97 4.0 auto-

summer highway mileage around 18-22mpg,

current mileage = 12 MPG ~about 5 miles city then 30 hwy and back again = 12mpg

I already threw a bunch of parts at it (plugs, wires, cap, rotor, upstream o2, regular oil changes with M1

hooked it up to an ignition scope and a snap on red brick.

ignition all good. exhaust gas readings good- cat is working, no miss, low HCs, no CO, plenty of CO2


scan tool shows it does go into closed loop, and stay there
unless I load the engine, good o2 fluctuations, LTFT= -1,

I think I might throw a TPS and an ECT at it just for the hell of it.

i'm clueless.

B

 

[Blaine B.]

Strange, yours seems to be 100% but the mileage sucks! If you figure it out, we all will......12 mpg with mostly highway is rediculous, I think you get the all time low for highway mileage award.

 

[acrid]

when my mileage suffered i had a leak in my vacuum caused at the tb not being tight enough on the block, bolts had turned out, replaced the burnt out gasket and recranked to spec, wallah, prob solved, could be a nice free fix for you.

what's your idle sitting at, mine got up to 1200 at idle when i had the leak. now it's right around 800 - 900 depending on the time of year.

 

[97 Green Machine]

I've had the same problems - '97 4.0 auto-

summer highway mileage around 18-22mpg,

current mileage = 12 MPG ~about 5 miles city then 30 hwy and back again = 12mpg

I already threw a bunch of parts at it (plugs, wires, cap, rotor, upstream o2, regular oil changes with M1

hooked it up to an ignition scope and a snap on red brick.

ignition all good. exhaust gas readings good- cat is working, no miss, low HCs, no CO, plenty of CO2


scan tool shows it does go into closed loop, and stay there
unless I load the engine, good o2 fluctuations, LTFT= -1,

I think I might throw a TPS and an ECT at it just for the hell of it.

i'm clueless.

B

That sounds EXACTLY like the problem that I'm having. I haven't had a chance to check as many things yet, but it does run perfectly except for the horrible mileage. This seems to be a pretty common problem for the newer XJ's

when my mileage suffered i had a leak in my vacuum caused at the tb not being tight enough on the block, bolts had turned out, replaced the burnt out gasket and recranked to spec, wallah, prob solved, could be a nice free fix for you.

what's your idle sitting at, mine got up to 1200 at idle when i had the leak. now it's right around 800 - 900 depending on the time of year.

I'll have to go over the engine with a can of carb cleaner to check for that. I wish I knew what the engine was idling at, but unfortunately my Jeep has the idiot light gauge cluster. Maybe I can borrow a tach from someone.

 

[Ecomike]

Strange, yours seems to be 100% but the mileage sucks! If you figure it out, we all will......12 mpg with mostly highway is rediculous, I think you get the all time low for highway mileage award.

I think I had that beat until last week when I found the wiring harness connector attached backwards on the three pin connectors to my TPS and 02 sensors. The amasing part is that it still ran at all. Go Renix!arty:

My mileage is back up from about 7-8 city, 11 hyw, to about 11 city & 14 hyw right now. I have not had a chance to get a new full tank hyw only mpg reading since fixing the crosswired sensors. It is an 87 Wagoneer Ltd ed. 4 x 4, 4.0 liter OEM Renix.

Also, I just changed the rotor and cap which only had about 6,000 miles on them after inspecting them and finding that they looked like they had 50,000 miles on them. :gag: I suspect they wore out prematurely due to the other bad parts, and cross connected sensors that were replaced periodically over the last 5,000 miles (12 months) as I worked on restoring this jeep.

I too am running out of things to fix and replace to reach the oh so ellusive +20 MPG highway mileage so many of us seem to be missing out on!!!!

 

[Blaine B.]

Did you disconnect your battery after correcting the plug problem?

 

[needsrepair]

Check the compression for the heck of it. Dont know for sure if there is a spot on the xj's exhaust to check backpressure but there may be a spec for this to indicate cat/exhaust restriction. When my 86 cj cat colapsed it wouldnt accelerate well and the heat build up in the cab was unbearable. When the left bank cat on my 4.3 chebby colapsed it wouldnt accelerate and had no power. It dosent sound like your conv. but as others have said check everything. It could be a combination of things. I had a dodge slant 6 with a burned exhaust valve. It didnt seem to run that bad but it just seemed sluggish and had poor mileage. It would seem like it could check out ok on most of the tests but the vehicle with the compression on the high end of the specs would probably perform better then a vehicle with min. or below spec compression. Short trips and not getting the engine up to temp are very bad. I can get to work before Im finished with my coffee so I use the remote start and like to always get it hot. The engines today are FAR better then the old carburated stuff but Its still not good to have to many short trip cycles. A lot of condensation occurs in the heating cooling cycles so its best to take it down the eway to keep it burnt off. I dont worry about the extra fuel but I can feel your pain when getting 7 mpg. Good luck.

 

[Ecomike]

Did you disconnect your battery after correcting the plug problem?

NO I did not disconnect and recconect the battery (to clear any memeory) and the change in performance was immediate! Keep it mind it is a 1987 Renix. I highly recomend checking those connectors on the early Renix jeeps. It is way too easy to accidently reconect them backwards. Make sure that A-A and C-C are lined up parallel on the connectors. There is an A-B-C indication near the rear of each connector. You can also use the manual FSM drawings and check wire colors.

Mike McGinness
South Houston, Tx

 

[Ecomike]

Check the compression for the heck of it. Dont know for sure if there is a spot on the xj's exhaust to check backpressure but there may be a spec for this to indicate cat/exhaust restriction. When my 86 cj cat colapsed it wouldnt accelerate well and the heat build up in the cab was unbearable. When the left bank cat on my 4.3 chebby colapsed it wouldnt accelerate and had no power. It dosent sound like your conv. but as others have said check everything. It could be a combination of things. I had a dodge slant 6 with a burned exhaust valve. It didnt seem to run that bad but it just seemed sluggish and had poor mileage. It would seem like it could check out ok on most of the tests but the vehicle with the compression on the high end of the specs would probably perform better then a vehicle with min. or below spec compression. Short trips and not getting the engine up to temp are very bad. I can get to work before Im finished with my coffee so I use the remote start and like to always get it hot. The engines today are FAR better then the old carburated stuff but Its still not good to have to many short trip cycles. A lot of condensation occurs in the heating cooling cycles so its best to take it down the eway to keep it burnt off. I dont worry about the extra fuel but I can feel your pain when getting 7 mpg. Good luck.

Exhaust pipe, cat, or muffler restrictions usually lead to overheating as well as loss of power, I think. I had an exhaust pipe that delaminated on the inside of the pipe. The muffler guys cut small holes in the pipe several feet apart and checked each hole for pressure by hand and were able to isolate the restriction very quickly. Works on bad mufflers and bad cats too.

Mike McGinness
South Houston, TX

 

[BS87]

So, i know that there are a million variables to MPG, but what are some basic main things that i can make sure are working 100% to keep the milage up? I replaced the exhaust manifold, new o2 sensor, new cap, rotor, plugs, and wires, new fuel+air filters, even ran some techron & seafoam through for the heck of it. What else can affect it? TPS? CPS?

 

[Blaine B.]

You say they cut holes to check out what was blocking your exhaust....

What's the purpose if you were just going to replace the exhaust anyway?

 

[Flash]

If you are going to replace the hole ex.....nothing.

Some time the cat. can deteriorate to the point that, instead of plugging up it fills the muffler up with the materiel of the cat and end up in the muffler, causing the restriction not to be be in the cat even though you have replaced the known bad cat.


Good though on the Renix o2 sensors ability to be plugged in wrong........will be checking that on my self.............I think that you can only plug it in wrong if the clip is broken off!

With over 300,000 on it, it's vary possible!

 

[yikes]

another thing to check is your spedometer. if it is off, the miles driven on your odometer could be incorrect. my speedometer is off by 7 mph on stock tires, so that is a possibility for you. if this is the case, your milage probably isnt as bad as you think it is

X 2

did you change the speedo gear ?

- your speedo / odometer is probamly off because of the tire size.

 

[Ecomike]

You say they cut holes to check out what was blocking your exhaust....

What's the purpose if you were just going to replace the exhaust anyway?

The holes allowed us to isolate and replace just the bad part or section, be it a piece of exhaust pipe, muffler, Cat or the old style thermostatic exhaust valve on the exhaust manifold I had on my old 78 Dodge. Don't know if anyone uses the thermostatic exhaust valves any more but they use to hang up and cause overheating and or cold start/run issues back on then.

The holes also allowed us to verify an abnormal pressure drop before spending money.

Mike McGinness
South Houston, TX

 

[88XJSport]

I would like to know what mileage others are getting with their OEM stock 4X4 1987 Renix, 4.0 liter Jeeps? Mine is a Wagoneer Ltd. Ed. (small one the like the Cherokee), and just 1987 4.0 liter Renix systems?

My OEM 88 Renix 4.0 4x4 Sport (same guts as yours) gets 19 city. (166,000 miles)

 

[parkeruph]

2 things for a laugh
#1 take it out of low range
#2 Check your math

Everything else has pretty well been covered.
Good luck on catching the syphoning guy.

 

[Ecomike]

My OEM 88 Renix 4.0 4x4 Sport (same guts as yours) gets 19 city. (166,000 miles)

OK for the rest of us who are RUNNIG ON EMPTY, what are we doing wrong?

What is city driving for you? Are you getting like 24 to 27 highway?

 

[mailtruck]

This is a little off track but would having a 180 temp thermostat vs. a 195 temp make any difference on gas mileage?

 

[Blaine B.]

Yes, it would make your mileage lower, and would make your heat a littke sucky in the wintertime.

 

[streetpirate]

This is a little off track but would having a 180 temp thermostat vs. a 195 temp make any difference on gas mileage?

mmmaky


i put a 180 in mine when i replaced the water pump in september, and as soon as the weather started getting colder my mileage dropped from 19-21 down to its current 14-16. the funny thing is, i knew about it and put the 195 back in, and the only difference was the heater works a little better.

the 180 would run the motor at 160, and the 195 gets me 210 in traffic and 195 or so on the highway-like back to stock.

havent noticed a big drop in performance, other than the normal of going from my 26" summer tires to the 30" winter ones.

my ideas have been a cracked header, bad sensors, maybe the idea of a burnt exhaust valve, i dunno. im not getting any detonation, that i know of. my jeeps so effin loud and makes enough engine noises i cant really hear anything different since i put the new water pump in.

i would says its just the 30" tires and the winter gas, but ive run this combo before in the winter and didnt have this much of a mileage problem.