wow maybe i should change my map sensor and fuel filter then. I just got done doing a complete tune-up and i only get about 168-178miles to the tank but thats with about 4gallons left out of a 20gallon tank. What you guys think

 

before i was getting like 250+ plus on 16 gallons

 

so you have a 4cyl then

 

I just bought my Jeep a couple weeks ago. It was highly recommended to me by a former Cherokee owner. So far I've just done a few maintenance repairs: drive belt, valve cover gasket, rear wiper-motor, oil-pressure sensor, & complete tune up. For what I paid these repairs are par for the course. The truck's got about 85k on it now, so it makes sense that it'll need a little TLC. I absolutely love it so far, except for the gas mileage. I expected around 15 mpg in mostly "city" driving but right now I'm averaging about 12. Granted I used the 4wd from time to time in the snow, but I've been pretty light on the pedal so far. From previous posts I gather that a new O2 sensor might help w/ this. I've done plenty of O2 sensors in the past, but on previous vehicles I always got a check engine light. Is it possible it's bad even w/o this indicator? I also read elsewhere that a clogged cat could be part of the problem. Anyone got any experience w/ this they'd like to share? I'm planning to swap out the exhaust this spring. Any suggestions?

 

Where and what is the MAP sensor. This is an imberissing questio but for the o2 sensors are you talking about all the ones that are attatched to the tail pipe before and after the cat?

 

map sensor is on the firewall above the valve cover. it has a green plug on it.

yes, the o2 sensor is on the "tailpipe". well, actually, it's on the exhaust manifold right before the engine pipe. infront of the cat. some will have another one after the cat also.

 

Test the catalytic converter by using an infra-red temperature gun. you'll see exhaust temps before the cat somewhere in the neighborhood of like 225-275, and after the exhaust exits the cat, the pipe temp will measure 325+ degrees. It's the job of the converter to heat the exhaust to convert CO into Co2, and HC into H20. That being said, if a converter isn't operating correctly; you won't notice a rise in temperature AFTER exhaust exits the converter. "Clogged" converters are more likely seen in carburetored vehicles, and is a very rare occurrence in EFI engines.

In talking about the O2 sensor failure- Most later model 02's have a "heater element" that heats the porcelain inside the sensor. If the heater circuit fails (power/ground), or if the sensor isn't switching from rich to lean, then the check engine light will come on. Getting the best MPG from the O2 means that the sensor needs to be able to switch from rich/lean at a very fast rate. If it switches slow, efficiency is lost. If you don't know how old it is, change it- I can guarantee you'll see better mileage, no matter how significant the increase might be.

 

Cool, thanks. I'll put a new pre-cat O2 sensor in this week and post any mileage changes after I burn through another full tank.

 

ok thanks for the info

 

I could not find the map sensor you said it has a green plug? Also do regular replacement o2 sensors sold in your average auto parts store that goes for your map sensor as well?

 

Look at the valve cover man go straight back the the firewall and there should be a bracket with two bolts holding it on and then the map sensor will be bolted on under it with one vacuum hose coming off it and a green plug hooked to it

 

sorry, your 99 might be different than my 90.

try this....

 

oh ***** i forgot they changed the location after like 99.

 

In my 89 follow a vacuum coming off of the throttle body heading toward the firewall. You can pick a MAP sensor in most auto stores, but an O2 sensor might need to be ordered from a near by store. MAP sensor is about 50 bucks and I just got one thinking that was the reason for my car idling so low, but it didn't make a difference. (couldn't return the darn thing) O2 sensors run pretty high $80-100, but it sounds like its worth replacing. Get a manual for your model and it will show you where all of this stuff is.

 

I don't think the fuel filter is your problem. Its actually the fuel pressure regulator/fuel filter. You have to drop the fuel tank to change it but that is the hardest part. I've done it twice in the last couple months. As for o2 sensors- you can get a jeep universal one that you have to wire in for 20 bucks. I bought one but have not installed it in the jeep yet. To damn cold up here in Minnesota lately. I think the rest of the guys are right.. probably your map sensor..