Funky engine performance and P*** poor mpg
 

So i been dealing with some funky engine performance the last few months. One major one is a rev hang issue. This randomly happened after i started my jeep up and backed out of my sisters driveway and it kept going a little instead of the rpms just dropping back down like they should. I have a video on here somewhere i can link it if you want to see. and i deal with random engine peppiness. Some startups the engine feels very choked down and very doggy and slow like i hit my gas pedal half way it slowly climbs up in mph and other startups its very quick and get up and go like i hit it a quarter way and itll climb up quickly in mph and generally feels so much better. Another issue is idle. Now i will admit my idle air controller may need a cleaning again last time it was dirty in 1200 miles. But on a cold start itll run kinda poorly like rough the rpms slightly jump up and down and you feel it vibrating alot in the cabin until you start driving a little then everything will smooth out. Then i have an occasional stumble at idle. I have done a tune up recently. All napa ordered copper spark plugs, cap and wires with new air filter. It ran better after but these issues still persisted. Then we get down to the major issue the gas mileage i have been averaging 75 to 115 miles on a full tank of gas each time filled up to 17 gallons. Now my next suspected items are o2 sensors and fuel injectors. But i want some other input so im not throwing cash at it. Im trying to save up for another vehicle atm as well. Could there be an ecu issue or something as well? Im starting to get annoyed with how my jeep is performing lately. I do plan on replacing my exhaust system as well but that will be after my newer car so my jeep isnt my sole daily driver. Any other questions on parts i will answer upon questioning. But mostly alot of things on here are still factory. Its a 99 jeep cherokee with 209k miles. I driven a few other jeeps as well with the 4.0 such as wranglers, grand cherokees and regular cherokees and those engines feel way better, smoother and faster than mine. I do have compression tests laying around here when i did my tune up and one cylinder was low. all were either 156 to 154 but cylinder 3 was 124 so idk if that would have anything to do with it. Some said it was a head gasket and i havent got around to do that test. I did do a 100 mile trip to orlando and 100 back and that time i got 122 miles out of a full tank of gas going 75 mph. It made me cry when i borrowed my grandmas car to make a trip and hers went 60 miles and basically didnt budge the gas gauge lol. So any help would be greatly appreciated. i feel like im that member that rants about his jeep not working right but never fixes it on time or never lol. Work makes sure im nice and tired in my days off. Some members said their jeep ran alot better with non ethanol gas i may try this some day. I dont know what else could contribute to these mpgs I really want to hook up those obd2 scanner things and see what my o2 sensors are doing along with many other engine data like fuel trims and such and gpm ratings because this is making a tahoe and many other big giant suvs look good on mpgs

With the stumble at idle and feeling a little sluggish, I would check the fuel pressure and clean the IAC

Yes, cylinder #3 is weak and out of spec for compression.

The compression spec calls for no more than a 30 psi variation between cylinders.

You could do further testing (dry/wet compression) or leakdown to further isolate what the root cause is.

Fuel pressure? Its a newer pump but then again it was cheaper. It isnt an airtex btw and yeah i been trying to find a tester near me i dont want to buy one but no one near me has one available to loan at the auto parts store

holy cow EEVEE.... 122 at 75?.... and here I was a bit grouchy about getting 14-15 mpg on this crap winter blend stuff and I'm at 7,000 ft. above sea level.

hope you find the root cause quick!

really starts chewing up the wallet fast tbh i want it fixed soon but i cant really afford to throw money at it in sensors and that. But idk what or how to diagnose whats wrong. Theres no cel or anything and doesnt smell to rich or maybe im to used to it by now that it smells normal....

When i first got it on the road after my new fuel pump i got it up to 300 miles before i filled up and still had a little bit of gas left to maybe squeeze 30 more miles. So idk what is going on! it took a steep decline with being on the road for 1 year and 10k miles

Hey if you want a new set of injectors I will give you a set of stockers. I swapped out the set on my 99 (went 4 hole) and the old ones worked perfectly fine. Pm me if interested.