Mitch XJ

I have a 1999 Cherokee that is having starting issues and terrible mpg. I am currently getting like 6 mpg which is killing me. It started happening after i replaced the camshaft position sensor which seemed pretty straight forward. Any ideas what this could be?

JPXJMOAB

something catastrophic had to of gone wrong if you're getting 6mpg!

JamromXJ

Wow.. Brilliant deduction Homes. For a 1st post, you hit the nail on the head. Amazing.

1st, check everything you touched to change that sensor. Make sure nothing is missing, or disconnected. That sensor helps with the engine timing, which could be causing the issue. Keep in mind, it could be a bad sensor out of the box as well.

Do you have an OBDII reader? If so, you could hook it up, run the engine and see what your actual readings are. But to me, it sounds like it's a bad sensor.

Anything else wrong? Is it idling ok? Poor acceleration? Need more info.

JR

Kyle00XJ

With torque (an OBD-II reader)...my readings are 6.5 mpg with a 6.5ich lift, I'm not sure why it's so low but I really don't have any issues

JPXJMOAB

If it were me, I would start with a tune up, and there is a product called auto rx that you can buy online. Get the basics covered first so you can work at it slowly without jumping into 10 things at once. I know for a fact this won't help completely if you're only getting 6mpg, but it's a place to start.

tjwalker

1. Calculate gas mileage the old fashioned way. Miles driven divided by gallons consumed. You must get at least 3 samples before even starting to come to any conclusions.

2. Good old fashioned tuneup. Champion copper spark plugs, quality spark plug wires, quality distributor cap and rotor (with brass contacts)

3. If you've ever seen the check engine light, retrieve codes for clues and guidance on what circuit to concentrate on.

JPXJMOAB

x2
However I have a question.. Anyone know how many gallons the jeep XJ tanks are?

salad

20. But for the purposes of mileage math you always use amouny of fuel dispensed, not "oh that looks LIKE half a tank on the gauge so it mus be 10 gallons".

JPXJMOAB

how do you calculate fuel dispensed? Stupid question I know but I'd like to figure this myself

salad

Meter on the fuel pump? Gallons are the part that isn't in dollars.

JPXJMOAB

so say I drive 250 miles, I used 11 gallons and I put in 9 gallons. I'd just divide 250 by 9? Correct?

mschi772

No. How would you even know you used 11 gallons if you only put 9 in? The gas pump is the only practical way to know how much gas you consumed, and to do that you have to fill the tank each time.

1. You fill your tank til the pump clicks off. DO NOT top it off beyond this point.
   
2. Drive.
   
3. Fill your tank til the pump clicks off again. DO not top off beyond this point.
   
4. Divide the miles you traveled between fill 1 and fill 2 by the number of gallons pumped during fill 2.

This is even more accurate if you use the same gas pump for both/all fill-ups as there can be some variation between pumps regarding when they click off.


Your profile says you're on 30" tires. If you haven't changed your speedometer gear (how to: http://go.jeep-xj.info/HowtoSpeedoGears.htm), then your speedometer/odometer/trip are not accurate. You're traveling something like 5-7% farther/faster than your instruments are telling you. Either change your speedometer gear accordingly or compensate mathematically for the change in tire circumference from stock every time you calculate your MPG.


This thread has the most thorough coverage of technical MPG repairs and mods regarding the XJ that I'm aware of. It's a long thread, but it's worth it to page through the whole thing. http://www.jeepforum.com/forum/f11/m...5-ats-1551782/

JPXJMOAB

alright. However, why fill up twice? You should get the same mileage if you drive 200 miles and top it off with 8 gallons vs if you 400 miles and you top it off with 16 gallons?.... Same thing. Why not just do a separate test each time, like 3 times to get an average? Not trying to jack the thread or argue.. Just a little lost.

mschi772

You have to start with a full tank. If you don't, how will you ever know how much gas you consume? The gas gauge? HA! The gas gauge is tremendously unreliable, inaccurate, and imprecise.

The gas pump is the only way you're ever going to know how much gas you consumed during a period of driving, and the only way for it to tell you that is if you fill your tank to the exact same point every single time. The only point where the pump will automatically stop pumping every single time is when the tank is full.

Right.

JPXJMOAB

alright, thanks man