Topic: CEL

[SKI BOY]

Background -  97 TJ; CAT went bad a couple of years ago, so I replaced with straight pipe, no problems other than running a little louder. Last month developed exhaust leak at the straight pipe & noise increased, so decided to install replacement CAT. Install went fine & exhaust noise was considerably less.

Current issue - Within 4-5 days have CEL code 51-"A lean air/fuel mixture has been indicated by an abnormally rich correction factor." Engine runs OK until warm up and then stumbles when stopping @ idle & MPG has fallen thru the floor! Research indicates possible fuel pressure loss or bad O2 sensor.

Question - Would best approach be to test fuel pressure 1st or replace O2 sensor? If O2 sensor, would upstream or downstream sensor be the most likely to cause the CEL?


Any advise is appreciated & thanks in advance!

[chardrc]

upstream O2 sensor is the one that effects how the engine runs, the downstream sensor just tells the computer how well the cat is working (will through a code if not working) have you checked for any exhaust leaks before the upstream sensor ( such as a cracked exhaust manifold)

[sharpxmen]

like he ^ said on the O2, could be fuel pressure so your plan is sound, could also be a plugged or bad injector, might want to also check how much each cyl contributes (unplug the wires on the spark plugs one at a time but careful not to get shocked, you can also unplug the injectors).

[SKI BOY]

Thanks for the info...I do have a slight manifold leak because of a broken stud in the head, but it's been that way since I bought it 7 years ago, however while troubleshooting last weekend I did discover a small pinhole leak between the 1st - 2nd runner of the exhaust manifold, so will need to check that further. How does an exhaust leak cause the engine to run lean?

Sharp... what am I looking for while disconnecting the plug wire or injector?? I'm not a mechanic, but have capable maintenance skills, so just the basics, thanks!

[sharpxmen]

you're looking at how much the engine rpm drops (just by the sound of it running, you can check the tach too but not really needed) - if one of the cyl is contributing less then will be obvious as all other 3 would be noticeably more significant.

another thing, you could get that CEL by running close to empty and doing long turns (line an exit off the highway), so if you suspect that you might want to reset the computer and see if the code reappears.

[chardrc]

How does an exhaust leak cause the engine to run lean?

It allows air into the exhaust which makes the O2 meter read lean because of the excess oxygen. so the computer thinks you are running lean and tries to correct and overall makes the engine run rich. 

[SKI BOY]

Chard / Sharp - Thanks for the advise, much appreciated.

Have already reset computer a couple times & CEL comes back with #51 each time ,
so it's not an issue with low fuel.

Is there a way to test the O2 sensor while it's on the vehicle? I hate replacing parts by trial & error and the sensors are a PITA to remove!

Thanks, again!

[sharpxmen]

if you never replaced it then it's time as it will go sooner or later, there's not really a way to test it but if it's dead you'd get a code, if it's just sending the wrong info hard to tell (they usually start like that and will eventually die, if you never replaced it you should as it will go anyway, they don't last forever).

[dwtaylorpdx]

If you have access to a oscilloscope you can see the signal level off the sensor, but about 80% of the time when a cat dies the O2 is bad as well, in my experience anyway....

Dave