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My 2001 Jeep cherokee has recently lost fuel mileage (about 5 mpg). Not
good. :2cents: I placed an OBDII chip on my jeep to monitor the timing
advance, fuel system status, speed, and intake manifold pressure. The data
showed that my timing and intake manifold pressure was all over the place
during constant velocity (70mph). This does not seem right. :cussing:
:banghead: What could be the culprit?
DustyBub
Right now, there is not enough information provided to know what might be
causing your loss of fuel mileage and/or "efficiency." I would assume that
you have a 2001 Cherokee with the 4.0 Litre I-6? That would be the most
common engine in that truck. Do you have the manual or the automatic and
is it four wheel drive? I own two of them, a 1995 and 1996 Cherokee
Sport, both 4x4's and both manuals. Excellent vehicles!
You're loss of fuel efficiency could have MANY (litterally 10's of 100's)
explanations. I am curious what probed you to think ignition timing or a
problem with the computer? You make no mention of your CEL being on or any
other MILs? Are there any history codes in the computer?
A 5 mpg loss in fuel economy might just be as simple as 5psi of air per
tire too low if you have no signs of a computer fault of O2 sensor in an
error condition. Also, how do you know you're getting 5mpg less? Have you
run the math to determine this or you're just looking at your gas receipts
and mileage and going, AHHHHHH looks a little weird to me? Gas mileage is
going to change a little. Perhaps you took a trip and noticed a certain
mileage and then you've been driving around town and of course, you won't
get near the same thing. I can tell you now the Cherokees are highly
inefficient. You've got a fairly heavy four wheel drive train to push
around that is NOT aerodynamic and has a high rolling resistance to factor
in. My mileage changes all the time depending on the type of driving I am
doing. I don't really get worried about anything like that unless I have
a light on or an obvious issue. From all of this, it appears your only
indicator of a potential problem you are looking for is that you think
you're 5 mpg in the hole?
Your ignition timing and your observations of the data on the OBDII scanner
while in a drive mode would be perfectly normal. Ignition timing is
always changing even driving at a static speed. The reason is that the
engine is using sensory data from a multitude of electronic sensors to
constantly make adjustments to the timing/ignition and fuel delivery to
meet certain pre-programmed algorhythms that are in the control module and
that meet certain efficiency(fuel)/emmisions standards set in the industry
for that vehicle.
Even while driving at a set 70mph, the vehicle is in a constant state of
change with regards to the load on the vehicle, say if you start an incline
or if you encounter a change in the altitude and therefore you have to
compensate for that, etc. etc. etc. It is one big web of events going on
that have the ability to impact everything else. Maintaining the vehicle
at a speed of 70 mph on a flat road with nothing else going on would
require LESS constant change than city stop and go driving, but for the
sake of the arguement, a typical road is far from flat and the work the
vehicle has to do is ever-changing. You can further demostrate this by
sitting in the driveway with your OBDII scanner hooked up and in monitoring
mode while watching the ignition timing as you step on the gas pedal in
neutral and you'll see the ignition degrees of timing change dynamically in
the tool.
The change in the throttle plate position calling for more or less power to
be commanded of the engine causes changes in the vacuum and intake
pressures. This would cause the emissions to change during that time as
well as the load value on the vehicle. Sequentially, fuel injection
demands are changing, etc. For these reasons on a high level, you will
notice that your ignition timing is changing very frequently to meet these
different requirements and to make the engine alway run in the most
efficient mannor that it can given the circumstance.
This is why they say that the vehicles of today learn you and your driving
style. A sudden change in driving habbits can cause the vehicle to respond
differently and also gain or lose fuel economy. While I've provided you
with a little high level overview of how this all works, I see nothing from
the data you've given that would indicate why you've lost 5 mpg. It might
be situational and created by the driving environment. There could be an
air pressure problem in the tires. With no MIL's on, that would be the
first thing I'd check. You could have a stuck brake caliper or one
dragging. Generally a tuned in driver will pick this up, but I've seen it
all. Good Luck.
cmeseadoin
Thank's again for your help. I appreciate you taking the time to respond
to the messages I sent.
If anyone's interested, I'd be more than happy to provide my responses.
But really it's nothing more than just blah blah blah..... :laughing:
DustyBub
