Failed emissions, running lean code 51
 

Hi all

Back again with another (hopefully) small problem.

My Jeep has recently thrown the 51 code and just failed test on emissions. I have just replaced the O2 sensor since this just to rule it out, although I have a feeling I was just shooting the messenger.

I believe this could be numerous things such as cracked exhaust manifold (which I have visually checked and it looks good), vacuum leak, bad map sensor or map sensor line, or cat.

I have noticed a 'hissing' sound coming from the back of the engine toward the firewall, not too loud but I can hear it above engine noise and fans. I have tried just moving the vacuum lines slightly by hand to see if I can get the noise to change but it doesn't seem to. I'm wondering if this could be contributing to the lean condition.

Is there anything else I could check or an easier way of troubleshooting the hiss?

I haven't taken the Jeep to be re-tested since I fitted the O2 sensor yet, but wanted to rule out as many things as possible first.

Perhaps I should add that the Jeep seems to drive great, no problems with MPG or running conditions.

It's a 97 XJ 4.0.

Thanks

Anyone any ideas? :(