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4.0 running rough

R

RamAford

NAXJA Forum User
Location
N.E TN
Stock 4.0 in my 88 XJ. Not sure what caused it, but one night on my way home from work it was running rough. It acted like it wasnt getting enough fuel, and it backfired twice pretty good! Almost like it had a bad spark plug and or it wasnt getting enough fuel. It also sounded different coming out of the exhaust. Like it had a mild cam. So changed the oil, plugs, wires. It seems to have helped somewhat, but still running rough. Also i noticed my mpg went from about 15 to 7. Now when you drive it, there is no smoke. If you rev it hard, or get on it hard, it blows black smoke. Smells like fuel. the best i can tell. What would be causing it to get to much fuel?

Not a bad convertor, as it was hollowed out when I bought it. It also has a new muffler, air filter, and fuel pump with filter. Also has 217,000 miles on her.

Any help and pics would be appreciated!


 
Is it constant or intermittant? Could be MAP sensor or O2 sensor. Did all the plugs look similar when you changed them?
 
Try checking your MAP sensor. It'll make it run really rough with the vac line disconnected or any other problem with it. Makes it sound neat though.
 
Im thinking MAP sensor or O2 but not sure. Dumb question, but where is the MAP sensor at on these things? anyone have a picture? or i can take one of my engine bay.

Im a dodge diesel guy, so gas motors are a whole new ballgame for me. :laugh2:
 
mattbred said:
Try checking your MAP sensor. It'll make it run really rough with the vac line disconnected or any other problem with it. Makes it sound neat though.
Click to expand...

Now that you mention that, it almost sounds like it has a mild cam in it while idleing.

Plus my fuel mileage is getting worse. i went from a steady 15-16 to around 7-8 mpg.
 
fuel mileage getting worse/running rich to me sounds like bad O2 sensor...
 
RamAford said:
Im thinking MAP sensor or O2 but not sure. Dumb question, but where is the MAP sensor at on these things? anyone have a picture? or i can take one of my engine bay.

Im a dodge diesel guy, so gas motors are a whole new ballgame for me. :laugh2:
Click to expand...
MAP sensor is on the top of the firewall above and behind the valve cover. It has one connector and a vacuum line connected to it.
 
RamAford said:
Now that you mention that, it almost sounds like it has a mild cam in it while idleing.

Plus my fuel mileage is getting worse. i went from a steady 15-16 to around 7-8 mpg.
Click to expand...

As said, it's mounted on the firewall in about the middle, and the vac line goes from the throttle body to it. When I accidently didn't reconnect the vac line to it, it sounded like it had a good cam on it. Bwuh rump bwuh rump bruh rump. Very defined exhaust notes.
 
Well its dark now, but i will check on gettin a new O2 sensor. im off work the next few days so that will be my project if it isnt raining again!

I will also look for the MAP sensor. I will see if i can get some pics also.
 
Well took a chance and turned out it was my fuel regulator. My mileage has almost doubled. And it is idling just like normal. I am also back to around 12-14 mpg. so im happy about that. but for some reason, i think i might change out the O2 sensor anyways. its still not getting as good as it was before!
 
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