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Discussion Starter #1
Hi, All

Would anyone know what a code 51 means?

51 A lean air/fuel mixture has been indicated by an abnormally rich correction factor.

Thanks
 

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I just wrote an entire paragraph and then deleted it before I posted it. I know, I am a moron..

Instead of re-typing everything I am just going to say this. Without knowing anything else about the truck (year, engine and trans, mileage, recent work, other driveability issues etc.) let alone being next to it, it is very hard to diagnose an issue. Especially without knowing any other codes besides this one.

With that said, I think your problem is PROBABLY going to be in your 02 sensors. A rich condition is usually only caused by 02 sensors or fuel injectors.

Now, do not go out and spent 300 bucks on 02 sensors just because i said so :p Check your air filiter, make sure its good to go and not dirty. ALSO CHECK YOUR INTAKE TUBBING AND THROTTLE BODY. Make sure nothing is in there blocking things up.

Next, go get your truck scanned. Have it hooked up to a scan tool, and see if and which 02 sensors are giving false readings.

Let us know what you find and we can help you better. Also, double check there are no other codes. A rich/lean code by itself with nothing else is pretty rare. Again, have your truck scanned for current and STORED codes, and also the 02 sensor readings at idle, cruise, and WOT and get back to us.


-Jason
 

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Discussion Starter #4
Thanks for the replies.

It's a '97 TJ 4 cyl. 2.5L. It is throwing 2 codes: 12 and 51. The 12 probably comes from some diags work done at a local shop. They ran diags, but found nothing other than an oil pump sensor that in the end did nto fix anything. We are also having an issue witht he oil gauge dropping to 0 when idle. I am guessing that the oil pump needs to be replaced.

But this other mixture issue only happens when the car is run in 5th for a while on he highway. Does not happen around town.
 

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For the oil pressure issue, try and do an oil chane, using some thinner oil than normal, maybe some 0w30 is the lightest i would go. 5W30 will be fine if you still have 10w30 in it. Although if you issue is interrmitant, using ANY 30 wieght will produce the same result when warm.

Change your oil filter, they can and do fail. Do not use fram or k&n.

Any other issues are going to be reltivly major if the new sensor is good. Pump/blocked passages/maybe even a worn pump gear if ours arent electric.

Maybe a ground issue at the guage? just a guess.

Code P0012 is a retarded timing code if i remember right. Cant help ya there, gotta be near it for that.

Timing CAN set a rich/lean code if you are getting rich/lean misfires but its unlikley. My best advice at this point is to have it looked at in person. You could check the timing, and check the 02 sensor, but I dont know enough without being nect to the truck to give you solid advice.
 

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Code 12 is nothing more than that the battery has been disconnected in recent history. It'll eventually clear itself. A Code 12 has nothing to do with a P0012 like is used in newer TJs. Again, that 51 code is indicating your upstream 02 sensor is going/has gone bad. :)
 

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Sorry for a small hijack....

Out of curiousity, what grade oil do the 4.0's use? I seem to recall my 2.5L manual stating something about using 10W30, but I'd have to double check that.

I've been using Mobil 1 10W30 for the last 3 years in my 2.5L and it has been running flawlessly and better than when I got it at 83k.
 

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I THINK the 4.0s call for 10w30/5w30 depending on season, but I have not changed the oil in a 4.0 in 2 years, and mines not due for 2,000 more miles, so I am just guessing here..
 

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For some reason, Mobil-1 does not do well in the 4.0L engine. Aftermarket oil analyses have found Mobil-1 raises the iron content to an unusually high level. Personally I just run Valvoline but there are other equally good oils. 5W-30 in cold climates, 10W-30 in temperate climates. :)
 

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I THINK the 4.0s call for 10w30/5w30 depending on season, but I have not changed the oil in a 4.0 in 2 years, and mines not due for 2,000 more miles, so I am just guessing here..
Your engine's oil is due changing just due to the time it's been in the engine. Oil change intervals are not based solely on mileage. :)
 

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Discussion Starter #11
Thanks, Jerry

I'll try replacing the upstream sensor and see what happens. Hopefully it'll do away with the "sputtering" in 5th gear.

Thanks again for everyone's feedback
 

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OH Jerry.. .Oh god no lol.. What I meant was that I sold my 97 4.0 2 years ago, and I just bought my 05 1,000 miles ago (dealer did oil change when I bought it). Not that I have driven my truck for 2 years without one :p
 
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