gsu0990

Okay, so i am having some problems with my gas mileage. It just started doing this. I have taken it to a few mechanics and without a bunch of really expensive tests they wouldn't even put it on the rack. One even told me that o2 sensors DON'T affect gas mileage. I asked him where he learned about vehicles lol. Well the problem is all of a sudden I am using WAY more gas than normal. I usually can go roughly 300 miles on my tank now i am lucky to get to 200 roughly. I don't have any check engine lights. I did, but it was an evap. emissions leak. but i had that one cleared and no lights have come back.

So far i have changed my plugs, checked my air filter, checked all vacuum lines to make sure i didn't have any undone or broken. Got an oil change. That is when it really started. The week that I got that is when i noticed the drastic decrease in gas mileage. I am wondering if it is o2 sensor or a MAP sensor or even my EGR valve.

The next cheapest thing that i am going to try is a new fuel filter before i go to the sensors because that can get pricey really quickly. To answer any questions, it is a 1999 Jeep Grand Cherokee WJ 4x4 inline 6. 4.0L. The motor has roughly 150,xxx miles. the transmission has a little less than 200,xxx

But to make it even more complicated as if it wasn't already, it does this really ONLY on the top 1/2 of the tank. I went like 75 miles on the top 1/2 tank than like 175 on the bottom half, until today when i have been averaging 35 per 5 gallons. I do NOT have it lifted right now. So something is screaming that there is something wrong. Any ideas??? ANYONE???? so by 190 miles, my gas light came on today. Idk what to really think about this. I know i want to start cheap and hope that is all that it is. i have been trying to monitor it and regulate it. I don't floor it or burn out or any of that stuff. i don't try to drive 90 mph either. i usually drive about 65.

Any help is greatly appreciated and thanks in advance

Keep Calm and JEEP on!!!

Bustedback

You don't have an EGR valve, so you can rule that out. Even if you did, it would not effect the fuel mileage. The MAP sensor makes about 60% of the fuel mapping decisions. It is possible for the MAP to send bad information to the ECM and not set a check engine light. If the MAP is reading too low the ECM thinks you're accelerating and adds fuel. The same can be said for the TPS, it makes about 30% of the fuel decisions. The MAP and TPS will always overrule the O2 sensor.
You really need to hook a scanner to the jeep and look at the data and see if there are any pending codes. You also need to look at the long term and short term fuel numbers.

gsu0990

The only codes that have ever been pulled were an evap. emissions leak (small), and Leak detection pump. i can't remember the numbers right now but have had them read at several places and that was all that was there. So i just can't afford to be dropping a ton of money into this thing as i am a college kid who gets no financial help and am struggling with money at the moment. So if i could get this fixed for little to nothing, that would thrill me. So i just wanna make sure, you don't think o2 sensors? i am just trying to find a really good starting place. I know that MAP sensor is a little easier to replace than the TPS just based on location. The only other thing i have noticed is that i have black soot around my tail pipe. Oh and thanks for the advice. I really appreciate all the info. cuz i really don't know too much of what i am doing lol. I just pretend!

Bustedback

You don't want to spend a ton of money, that is why a proper diagnoses needs to be done. Shot gunning parts at it will cost you more in the long run. Is there an automotive program at your school?

gsu0990

i am not sure if there is an auto program at my school. i know there is one at the technical school around here but not sure if they will do anything unless i am a student there. but i am going to try this week. oh and my check engine light just came back on. went to advanced auto and they said the only code was Evap Emissions Leak (small). he said there was no other codes but it was a cheaper small scanner. i will go and get it read on a big scanner. well what in your opinion would you assume it was? again thank you for the help i had no clue where to even start.

Bustedback

Nine times out of ten the gas cap is the cause of evap codes.

dave1123

My 97 gave me an evap code because of a vacuum leak at the canister.

Bustedback

That was the one time out of ten.

gsu0990

i am not too worried about the evap code because where i live, there are no emissions regulations. i am more worried about the rapid use of gas.

odgreen

Evap leaks can cause mpg issues. If it has a leak on the vac side.

gsu0990

More than likely it is the gas cap. i have checked all the vac. lines and no leaks that i found. Used carb cleaner and sprayed all the lines and listened to the engine with no change in rpm. I just am stumped beyond end and don't have any ideas on what to do. It has even stumped several mechanics. Cuz no other codes other than P0442 Evap Emission Leak Detected (small) thats the only code that has been read. no pending codes that i am aware of either.

Bustedback

Smoke checking the evap system is the best way to find leaks, but not everybody has a smoke machine at their finger tips.

gsu0990

So update, had a diagnostics run and looked deeper into the issue, no pending codes, nothing out of the ordinary. Checked the long term fuel trims as well as the o2 sensors and MAP sensors. Everything is running and firing fine. No misfires or any of that. It all started when i got my oil changed. I had been running 10w-30 synthetic in my jeep, and i am wondering if they put regular conventional oil in my jeep. So what i am going to do is redo the oil change and check it all again with a full tank of gas. I am going to fill up before i do the oil change and check the overall mileage per tank and judge it, if it is still down in the 270s i will change the oil again and then check out the overall gas and will continue to give updates as i see results.

thanks for all the ideas guys

Bustedback

What were the fuel trim numbers?

gsu0990

The long term numbers were right at 0.8% on the long term fuel trim. My o2 sensors were bouncing normally and no codes except p0442 (Evap emission leak detected (small)) So at least no big codes but at the same time NO codes. ugghhh makes me wanna cuss. so i am gonna try another oil change and make sure i put the correct Synthetic oil in there.