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4.0 - just got CEL + code 51

516 views 7 replies 4 participants last post by  LouC 
#1 ·
" A lean air/fuel mixture has been indicated by an abnormally rich correction factor. (O2 sensor)"

If it were the O2 sensor would I also assume I'd have code 21?

The only other symptoms are a (new) occasional stumble at idle, and my mileage has gone from 16-17 average down to 13 and 14.2MPG for the last two tanks.

A vacuum leak seems like a possibility, but (a) I haven't really dug into this engine yet but I don't recall seeing a MAF sensor and (b) seems like it'd have to be a pretty big vacuum leak to throw a code?

The engine runs great, without hesitation at revs above idle; if it were fuel pressure I'd expect to see it have more trouble under load. Any other thoughts/suggestions? Thanks!
 
#3 ·
Hm. Worth a shot. I went over the engine FAIRLY thoroughly to check for vacuum leaks and found none.

I could only find one O2 sensor though, just below where the exhaust manifold meets the pre-cat pipe.
 
#4 ·
Not until OBDII 96+ did the ZJ's have 2 oxygen sensors and your 95 4.0 will have only 1 upstream oxygen sensor in the downpipe you found.

You may want to pop the hood and on a cold start listen to the driver's side engine closely as it warms up. The 4.0 is notorious for cracks in the manifold tube welds and if so you should be able to hear an exhaust leak which quiets down as the metal expands.

Air can be sucked into these tube weld cracks then the o2 sensor reads too much oxygen which in turn signals the PCM. The PCM will dump more fuel in thinking there's a lean condition when often there's not.

Then you end up with a rich fuel condition which can eventually destroy or burn up the inside of the cat. But it doesn't sound like you have a clogged cat or you'd notice a drop in power at higher rpm's usually.

I'd still be checking some other things and your emissions canister should be behind the front driver's side bumper. Make sure those vacuum hoses are in good shape and the charcoal canister isn't cracked. Good luck.
 
#5 ·
Good call. I'll try to double-check the manifold; which I intend to replace with a header when I do the head gasket in a few months. The drop in mileage is just in the last two tanks though. I WILL check for some changes in audio. Thanks for the additional thoughts! :)
 
#6 ·
You're welcome and good luck.

PS. you may want to disconnect your o2 sensor and see if it quits hesitating at idle. You can also drive it a little while like that to see if it makes a difference. Just make sure the connector/wires are taped or out of the way. You'll need to disconnect the neg terminal on the battery to remove the code and turn the CEL off when done.

Not saying you couldn't just have a bad upstream o2 sensor as was originally mentioned. The OE NTK 23119 sensor for your 95 4.0 isn't very pricey at $33 from Rockauto w/a little shipping charges.

http://www.rockauto.com/en/moreinfo.php?pk=1644016&cc=1182103&jsn=391&jsn=391

Some say the replacement NTK o2 sensor won't have the same spec as the Mopar NTK but I haven't had any problems with them.
 
#7 ·
OK installed new NTK sensor. WHAT A PAIN. Much more difficult than I anticipated.

In the process of moving so I couldn't find my O2 sensor socket. No problem, I have a big crescent wrench... that barely fits between the UCA and LCA. Couldn't get the sensor to budge. Sprayed it with WD-40 and waited. Still no movement. Tried putting the torch on it. Nothing. I even brought out the jack and put it under the wrench... the motor started to twist on its mount and still the sensor didn't turn.

Finally I cut the wires off and put a deep socket over it with my breaker bar. Much tugging later it twisted out. 90% done, right?!

New sensor says "finger tight, then 3/4 turn." Well, "finger tight" was about one thread. I suspect the threads weren't in good condition; I'm PRETTY sure it's not cross-threaded since I started it several times and got the same result each time.
Anyway. I grabbed some channel locks and managed to turn it all the way in and down to the little crush gasket-washer thing. Still not certain it's tight enough, but it sure as heck ain't falling out. I think my neighbor might have an O2 sensor socket if you guys thing I should make it tighter.

Hopefully it fixes the mileage anyway!!
 
#8 ·
Next time (if there is one lol) try this:
Spray the area where the sensor threads in with Kano Kroil for a week each day before hand.
Then before you try removing:
Warm the engine up to about 1/4 up on the the temp gauge (about 180* on my scan tool)
Then try turning it out with a wrench that really fits well. Sometimes those auto parts store O2 sensor sockets do not fit well.
Lastly, get a thread chaser for the threads in the bung on the down pipe. Lube this with grease and carefully thread it in. The threads on these can get clogged with rust...

I warmed ours up like above and it came right out and it was in there 19 years or 175,000 miles!
 
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