Sudden drop in performance and gas mileage

[RonJon27]

I have a 2000 Jeep Grand Cherokee 4.0, 6 cylinder. My car, intermittently has a sudden drop in performance and gas mileage and the exhaust fumes have a very distinct & strong odor as if it is burning very rich. My gas mileage goes from about 20 mpg (highway) to about 4 to 6 mph. it will remain this way for a long time and then suddenly, it reverts back to running normal. Can any suggest a possible cause and fix?

Thanks

Ron

 

[Bounty__Hunter]

Does the check engine light activate?

 

[RonJon27]

No

 

[greg92jeepxj]

This sounds like the evap system . All those parts are near the gas tank filler neck . Check a manual how to diagnose this but we've had threads on this issue here on jeepz.com so try a search here on the forum. As Bounty Hunter asked , was there a check engine light , it's strange no DTC was stored . Emissions fails usually get the pcms attention quick.

 

[Bounty__Hunter]

I'd throw a scanner on it anyway, see if there are any pending or hard codes. Never know if a previous owner jacked with the CEL.

 

[JPNinPA]

How are you calculating mpg? Also in the 4-6 mpg you are not including any off road crawling?

No one is stealing gases on you overnight? Are they?

 

[cougarlasch]

check that the purge solenoid isnt sticking open which would put a constant vacuum on the evap system which may suck raw fuel into the intake. ive seen problems where when the engine is running and you unscrew the gas cap the engine would start running different because it would introduce a vacuum leak into the system. or you can take a vacuum pump to the purge solenoid and hook it up on the side where the engine vacuum goes. and it should hold vacuum because it is normally closed. if you have a way to engergize it you can do that and it should open releasing the vacuum. i know on my 99 wrangler when the engine is warmed up i can hear it clicking on and off. im thinking that yours may be doing that and then sticking open until maybe a jarring knocks it closed.

 

[J Smith]

I have a 2000 Jeep Grand Cherokee 4.0, 6 cylinder. My car, intermittently has a sudden drop in performance and gas mileage and the exhaust fumes have a very distinct & strong odor as if it is burning very rich. My gas mileage goes from about 20 mpg (highway) to about 4 to 6 mph. it will remain this way for a long time and then suddenly, it reverts back to running normal. Can any suggest a possible cause and fix?

Thanks

Ron

One thing I don't see mentioned in this thread is your exhaust. There have been times that weird things happen in Catalytic converters and mufflers that will intermittently plug the exhaust flow. This too would mean a serious drop in power and the smell you speak of. If the power drops along with the mileage, this might be something to look into.

 

[Dragon5126]

Its not all that uncommon for no error to show up via the check engine light. a "soft error" wont lock the light in the on state. and intermittent problems allow them to correct without knowing why. The only way to tell if the onboard Diagnostics are working, is to hook up a reader when its acting up. However, an evap/vac error usually throws a hard ( locked on ) error and only clearing it will shut the light off. Also remember, no light doesn't mean no DTC was stored it just means a hard error wasn't stored. Ive run into more than a few that store but never turn on the light, including for hardware I don't even have, like cruse control...