beefmobile

Hey guys I'm driving to colorado with my brother and we just got a check engine light about 300 miles into the trip. I looked up how to read the codes and what they meant. Right now I'm pulling:

12- recently disconnected battery or computer
51- bad MAP or O2

I haven't disconnected the battery recently so that's weird....but about the 51 code, we had the symptoms described as "poor drivability, stuttering, and bucking". It's cruising ok on the highway pretty good right now but I'm concerned. Should we stop and change the O2?

ZINGGG

o2 sensor would be a good place to start. Plus, it's always good to replace it if you haven't ever done it and don't have record of the PO doing it. They can cause all sorts of problems if they are starting to go out, plus it should give you a small kick in MPGs

beefmobile

Ok. We're only about 20-30 mins from Wichita, where we'll be staying with family for the night. I guess I'll wake up early and swap out the O2. Are they a ***** to change on the 4.0?

beefmobile

I looked it up and it looks like it's not very hard to change, but----

What if it's a MAP sensor issue? What would that involve?

tjwalker

The 12 is informational only, so don't worry about that one.

The 51 code is this. Air/fuel at limit. Oxygen sensor signal input indicates a LEAN air/fuel ratio condition during engine operation.

It is possible that the oxygen sensor is the fault, but also very possible that some other problem exists that creates a lean condition that the oxygen sensor can't adjust to within spec limits.

Low fuel pressure, an exhaust leak, (exhaust manifolds commonly crack) vacuum leaks can all trigger a code 51. Map sensor is a possibility, but lower on the suspect list IMHO.

1. Check for exhaust and vacuum leaks. Check that exhaust manifold closely; they crack!. Simple stuff first.

2. Then test your fuel pressure. For a 95, should be 31 at idle, 39 with the regulator disconnected.

3. Test your MAP sensor; you'll need a meter and a manual. Be sure vacuum line leading to the MAP sensor is not cracked and is pulling good vacuum!.

4. Replace your oxygen sensor if it has over 100k on it. Use NTK brand sensor if you can find one. They were the OEM supplier to Jeep when your XJ was manufactured.

Here is a link to some information that explains code 51 pretty well.

http://autorepair.about.com/library/faqs/bl629g.htm

http://www.ehow.com/how_6918725_fix-...1997-jeep.html

beefmobile

Hey thanks for the info. I don't know how I'll test fuel pressure or the MAP right now. I'm at my aunt's house with just my tools. I don't know how to test the MAP or fuel pressure, nor do I have any gauges. What is the redneck way of going about this?

tjwalker

Sorry, but there are no "redneck" shortcuts to proper testing. LOL

You'll need a fuel pressure gauge to test your fuel pressure, and a multimeter and a manual to test your MAP.

beefmobile

I replaced the O2, repaired a vacuum leak, and replaced re valve cover gasket because it was leaking terribly. We'll see how this turns out.....

tjwalker

Be sure to keep us updated. Always learning here......

beefmobile

Ok----

Holy freakin crap. We worked on Jeep all day long. After we changed the O2 and I repaired the vac leak, I realized how bad the oil leak was. It was coming from the top side of the motor, but mostly on the passenger side. I deemed it a valve cover leak, which it was. Got everything apart, scraped and cleaned off the old POS cork gasket and replaced it with Felpro's fancy rubber one. While doing that, I managed to break the elbow to the PCV hose. Swung by autozone again to fix that, then got everything buttoned up and packed the Jeep up to press on to Denver.......

Well, we walked out to the driveway and there was about a football-sized puddle of oil on the ground. Started it up and let it idle and there was a steady stream of drips coming from the passenger side of the motor---it was the oil filter housing. It was completely shot. I looked it up real quick and it turned out to be a real ***** to change the o-rings. Remember, we were packed up and ready to leave so this brought the morale down to zero. I called the nearest dealership and they said the o-rings were only $7.50 but to have them do it for me was $115 :-( but I bit the bullet and it only took them about a half hour to do it.

On the bright side, everything is done now and we're chugging along to Denver once again. We've got a smooth ride and are pulling about 15 MPG, but we've got a lot of tools, luggage, and cross-winds so I'm not expecting much more.

Sigh. I'm just glad we had family to see on the way and that this is going semi-smooth.