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Code 51 : 94 XJ

J

jeepsrock

NAXJA Forum User
Location
LA California
A few days ago my jeep went thru CA smog and passed. THe day before i disconnected all connectors and cleaned them and changed my oil and plug wires. Today as i was driving the check engine light came on. After i arrived home i pulled the codes using the key trick and recieved code 12, 51, 55. I know 12 is battery disconnected, 51 i think is O2 lean/rich sensor related and 55 is end of msg.

Amazingly this morning i felt the jeep driving a bit weak and unresponsive. The o2 and map sensor are practically brand new (bosch unit). I just finished cleaning the connectos again today and the code came up after 15 min of drivng.

Is this an o2 code ? What can be the problem ?

thanks
Pete
 
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Not sure about the code, but PLEASE tell me you didn't use WD40 (or other silicon based lube) anywhere near the throttle body or O2 sensor? If so, you may have shot yourself in the foot!
 
I reset the codes by disconnecting the battery today and drove the jeep around locally. After driving 10 min the check engine light came back on. I checked the codes again and they were 12,51,55. THe same ones, i got before.

After searching on the web the code is listed as:
51: Oxygen sensor output indicates lean too long.

THe jeep feels like it has losta bit of power but i am not sure waht i can do. I have triple checked all sensor connectors and vac lines. Like i said before the map and o2 sensors are brand new. What else can cause a lean condition ?

pete
 
Pete, two things can cause a lean condition: Vacuum leak, and low fuel pressue. Double check all the vacuum hoses, including the brake booster. If everything looks OK, then I'd take a fuel pressure reading from the schrader valve on the fuel rail. IIRC, the pressure should be about 35-40 psi.
 
I just tested the fuel pressure and here they are.

Key in on position: 31
Idle with Vac Line: 31
Idle w/o Vac line: 39

This makes it more confusing. Any other ideas ?

Pete
 
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I am not sure if this can be associated or not but for the past yr or so when i cold start the jeep the exhaust reaks raw fuel smell for maybe almost a min. Some ppl have suggested that it cant be sensor related since the car doesnt monitor the sensors till it gets warmer. If the injectors were leaking and not closing off completly can this be of any help to this situation. Maybe its whats been causing my idle issues too a bit.

pete
 
Well, the fuel pressures are all OK Pete.

You're right, one of the injectors could be sticking open and causing the problem. You mentioned removing the connectors and cleaning them? Maybe one of the pins got bent and isn't making contact anymore? You could try pulling the plugs and see if any are obviously running rich. That would narrow it down to a specific cylinder......
 
I just had the same problem with my 96. I finally broke down and took it to the local diagnostics shop and they told me it was a partially clogged injector. I didn't have the additional 125.00 they wanted to flush the fuel system so I can't attest to the validity of their diagnosis, but it sure makes sense.
 
Well i must say that the injector theory makes sense.

The funny thing is the last few times i drove the jeep the check engine has come on and off. This has even confused me even more as its not consistant. Checking the codes will bring up the same previous codes.

I have noticed that my jeep has been running a bit more warm. I am not sure if thats consistant with me running ruch/lean as the ecu has been saying.

Pete
 
My Check Engine Light did that too for awhile until it stayed on. Maybe that was as the injector was clogged to the point that sometimes it could provide enough fuel as long as the engine load wasn't too high, and sometimes it couldn't triggering the CEL?
 
Someone told me to check to see if my injectors are leaking down by immediatly emptying the fuel rail (using the schrader valve) after i drive it and wait overnight and start it and see if the fuel exhaust smell exists.

This would only prove if the gassy exhaust is caused by a leaking injector ? If their was a leaking internal injector technically wouldnt that cause a rich running condition ? I understand that a leaky injector is bad in general buti dont see how that would cause a lean condition.

Pete
 
i never beleive it, until i tried it, but run a tank of premium through the jeep, my 88 runs so much better, and got abou 6 mpg better on a tank of 93 than it did with 87, maybe itll work for you to, if not BG injector cleaner is the only one that works, and its expensive
 
My friend, a week before all this happened i ran a can of bg44 in the tank and seafoam thru the vac lines. SO i am not sure if it had anything to do with it or not.

pete
 
Pete, sounds to me like you're getting symptoms that are opposed to each other. First the rich startup, then the lean O2 codes. Maybe an injector is clogged and sticking open after shut-down, releasing fuel into the intake and giving the rich startup condition........then when it's running, the injector (being clogged), dosen't flow enough fuel and gives the lean condition......?

I'd still pull the plugs and take a look, since they're so easy to get to. Sometimes they'll tell you things about rich/lean conditions. If one looks different than the others, then you've isolated the problem cylinder.
 
This is what i am thinking of doing, you tell me your opinion.

Get new injectors, and replace a few important sensors. THe o2 sensor i can get replaced on warranty (bosch) and i dont mind getting a new idle motor valve, intake temp and possibly coolant temp sensor.

Many of these sensor have been replaced before but i am getting annoyed with all these problems. The jeep only has 114k.

If i was to get injectors from this site www.fiveomotorsport.com they list the standard injector as 19lb# but i know the 94-95 HO had 21lb#, so thats confusing me.

Anyway let me know what u think or your opinion.

Thanks
pete
 
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