Code 51 - Lean - Need help!

Symptoms:
Stubling and hesitation when first starting up the jeep, and when stopping at traffic lights and stop signs.
Motor impossible to start for at least 15 minutes when stopping for a break on the higher mountain trails around here.
CEL light and Code 51

Possible contributing factors:
Bad cat replaced about 2 years ago but not rear O2 sensor. (Bad rear sensor?)
Found rust or dirt in fuel filter when I changed it, but it hadn't been changed since I bought the Jeep 3 years ago.

Essentially, for the past year and a half the check engine light on my '96 has been off and on. For the past 6 months or so it has stayed on. When using the on and off trick with the ignition, I get a code 51 which indicates that the engine is running so lean that the ECU cannot compensate to bring it back into stoich. I first replaced the front O2 sensor which seemed to cure the initial start up stumbling/hesitation, but not when at a light or stop sign. I next replaced the fuel filter with improvements in driveability, but not curing the CEL. I broke down and took it to a local diagnostics shop which told me it was from a vacuum leak in my brake booster and a partially clogged fuel injector in cylinder #3. I replaced the booster along with the master cylinder which was leaking with no improvement in drivability.

A couple weeks later I replaced the injector. While replacing the injector I decided to check my plugs to see if the #3 spark plug was showing signs of that particular cylinder being run lean. All 6 plugs including #3 looked fine with a slight tan coloring on half of the plug insulation. It has been a couple of years since I changed the plugs so while I was in there I replaced the Bosch platinums with some regular Champion brand plugs. I had disconnected the battery to work on the fuel system so when I reconnected it, the light had been reset.

During the first day of driving I was sure the problem was solved as the Jeep ran far smoother than ever without any of the stumbling and hesitation it had shown after starting up or when stopping at a light or stop sign. But the next day the CEL came back on.

I've sent about 4 bottles of fuel system/injector cleaner of various brands through during this whole fiasco with no noticable improvement. I even tried running premium fuel based on a suggestion I found while searching.

I'm really getting tired of throwing parts at and am seriously considering a propane conversion. Any suggestions?

my 93 had the same problem had to change the O2 and put in a new exhaust manifold. Mine was cracke just in front of the collector on the inside (closest the block side) thus sucking in air and setting the code. Id check the manifold and see what you find before throwing more parts at it.

It was a good suggestion, but the exhaust manifold looks fine. No obvious cracks or discoloration. Any other suggestions?

Your other injectors could also be clogged causing the engine to run lean. Go to http://www.fiveomotorsport.com for replacements and avoid the STEALERship (for obvious reasons).

Stubling and hesitation when first starting up the jeep, and when stopping at traffic lights and stop signs.
Motor impossible to start for at least 15 minutes when stopping for a break on the higher mountain trails around here.
CEL light and Code 51

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Israel, all those symptoms point to low fuel pressure. I'd first check the pressure at the fuel rail, if low then replace the fuel pump/regulator.

ROE

I'm digging up my old thread for anyone who performs a search and comes across it.

The fuel pressure was low as was suggested. I pulled the pump out of the tank hoping to see that it was just in need of a new in-tank filter, but found the filter in good shape. I then noticed that the ground wire from the pump itself to the pump "holder" was corroded badly. I clipped the corroded connecter off and as much wire as needed, then put a new clip on it. I also sanded the connecter on the "holder" to remove any corrosion. I reassembled the whole thing and now the jeep drives like a dream. The motor has more pep now with 35s and 4.56 gears than I did with 31s and the 4.56 gears. Its been three weeks now and the light still hasn't come on and the power is staying. Problem solved, finally!

O2 sensor, the points of the 21st century.

When using the on and off trick with the ignition,

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what on off trick is this??

maine96xj said:

what on off trick is this??

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With certain year vehicles, if you turn the ignition on-off-on-off-and then leave it on, the CEL will start to flash. There will be a pause between each series of flashes. Count the flashes and write them down. 5 flashes followed by a single flash gave me my code 51. There is more detail out there if you search on it.