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Old 04-01-2008, 03:28 PM   #1
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2001 Jeep Cherokee Timing

My 2001 Jeep cherokee has recently lost fuel mileage (about 5 mpg). Not good. 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. What could be the culprit?
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Old 04-01-2008, 04:56 PM   #2
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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.
2006 Mazdaspeed6, 2002 Pontiac Trans AM Ram Air WS-6, 1996 Jeep Cherokee Sport 4x4, 2002 Durango 5.9 R/T, 1995 Jeep Cherokee Sport 4x4, 1994 Ford Taurus SHO 3.2L, 1976 Buick 225 Limited, 1992 Miata, 2008 Miata.

Other toys: 2003 Honda Superhawk 996 sportbike, 2007 Suzuki M109R cruiser, 2006 Kawasaki KLR 650, 1996 and 1997 Sea Doo GSX limited watercraft.
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Old 04-18-2008, 05:47 AM   #3
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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.....
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