new motor = bad MPG

[afjeep2082]

Hey I put a new remanufactured 4.0L in my jeep about 2,000 miles ago and my MPG are just horable IE I drive to work and back roughly 30-35 miles round trip and I burn a quarter tank! that's like 6-7 MPG!! what gives? is this common for a new engine and will it get better?


93 cherokee 4.0L automatic

[MaXJohnson]

[Daffy Duck voice]I'd expect a 5 - 10% drop in mileage for the first few thousand miles, but nothing like what you're seeing. You may have mis-wired something and could be running in open loop mode. [/Daffy Duck voice]

[afjeep2082]

I don't see how, I labeled every wire connection...I did add new injectors they are not leaking and according to Bosh they are direct replacements so they shouldn't be pumping out gas any more then normal...I'm lost here...here is a run down of stuff I have, just in case it helps

62mm throttle body
K&N FIPK
62mm throttlebody spacer
new 4.0L roughly 2,000 miles on it
Automatic Transmission

4 1/2 lift...32's....roofrack with spare on top
(weight and drag)

so should this be normal or do I have a problem somewhere? where should I look?

[afjeep2082]

oh yeah what is open loop? I'm not too mechnicly inclined when it comes to electrics

[RichP]

oh yeah what is open loop? I'm not too mechnicly inclined when it comes to electrics

Has to do with engine temp, the engine stays in cold loop [runs rich] till it hits about 180F or so, then the computer takes over emissions control, fuel injection, timeing, in other words all the engine managment stuff. Untill it comes out of cold loop the computer takes a 'hands off' postion :D HTH

[afjeep2082]

hummm would the motor running pretty hot have anything to do with this then?

[afjeep2082]

oh duh, you said it runs rich when it's cool...nope that's not the problem it atchually runs hot...I know that's normal

[Ramsey]

well 4.5" lift and 32's might have quite a bit to do with it. you heavy on the throttle?

[woody431]

are you running stock gears?

[afjeep2082]

stock gears...yeah sometimes I'm heavy on the throttle....but I try not to be

[BajaXJ92]

Did you replace the 02 sensor?

[MaXJohnson]

hummm would the motor running pretty hot have anything to do with this then?

How hot is pretty hot? As mentioned around here a zilllion times, 210° is perfectly normal. You can do some serious damage up around 240° - 250°, like blowing head gaskets, warping the head casting, valve damage, on and on. This kind of damage could have a serious impact on gas mileage.

Assuming that's not the case, I'd still guess something to do with engine management.

Under normal running conditions, various sensors provide the computer with information to determine fuel and spark timing parameters. The oxygen sensor reads the results and lets the computer know if the engine is running too rich or too lean on fuel. Based on this info, the computer makes further adjustments, the oxygen sensors checks again and on and on it goes. This process of adjusting and checking repeats over and over iin a continuous loop, many times a second.

Like RichP said, when the engine is cold, the oxygen sensor input is ignored. The computer depends on stored maps to provide fuel and spark timing based on such things as RPM, throttle position, air temp, etc. The results are not checked by the oxygen sensor, so there is no feed-back loop. There are other conditions where the oxygen sensor is ignored, such as wide open throttle or closed throttle (coasting or decelerating).

Without the feedback from the oxygen sensor, the computer settings tend to be on the fuel-rich side which results in poor mileage. Your roof rack with spare tire, lift, larger tires, poor choice of gears and who knows what else will hurt your mileage, but that's the norm around here. Considering this, you might expect anywhere from 12 to 16 MPG. Your's if half that. Somethings not right in the engine compartment.

[willysnut]

Check the "intake air temp sensor" located on the intake manifold. I think it's the back sensor on the top side. We were having the same problem and replaced it. It feeds info to the computer about whether your motor is warmed up so the computer can lean the mixture out. $17 bucks at vatozone.

[afjeep2082]

is THIS (http://www.partsamerica.com/PartDetails.asp?SourceArea=&SourcePage=SEARCHRESULTS&MfrCode=NIE&MfrPartNumber=TS81251&PartType=756&PTSet=A) what your talking about? did you get your MPG back? also what would cause this to read bad?

[Spanky414]

I've known about open and closed loop mode, but what I want to know while you guys are talking about it is, can you tell when you are in open or closed loop mode?

[yuppiexj]

start cheap and simple.
check the MAP sensor vacuum line. knocking it off causes the computer to think you are WOT all the time it will, run super rich, burn out out the cat converter and idle a bit rough but it will run.
I actually broke the plastic line on both 88's and my 94 (the 94 was nice enough to turn on the check engine light though).
my mileage was also in the single digits.
I replaced the brittle plastic hose with bent to fit brake line.
runs like a champ now.

[bugman1964]

only if ya have a scannerthat can hook into the car and read data

[yuppiexj]

only if ya have a scannerthat can hook into the car and read data

88 yes true enough.

94 no the OBD seems to get ****ed if you knock the vac line off and turn on the check engine light.

OBDII will let you do the key off on off on stuff to pull the code(s)