[Catalina]

On my '94 4.0l I'm getting a Code 51 - Extended Lean Condition whenever I drive about 15 minutes at highway speed. I can't remember it ever coming on if I'm only city driving.

I've changed the MAP sensor and while I seem to have better performance, the light continues to come on. (I did reset the system by doing a battery disconnect).

Next step was to try the O2 sensor but before that, I was wondering if this could be related to a crack in the stock exhaust manifold? I have a new stainless one but have not installed it yet.

My other thoughts were the fuel filter (don't know if its ever been changed) or to have a mechanic do a thorough check for vacuum leaks.

Anyone else experience this? Thanks.

Catalina

[balloo93]

I would be inclined to agree with your guess on the crack in the exhuast. That is probubly throwing of the O2 sensor which is reading a lean condition.

Have you checked the plugs? If it Truely is running lean the plugs will have a white powdery look to them:
http://www.motocross.com/motoprof/moto/mcycle/plug2/plug2.htm

If the plugs look good, then it more than likely is the exhuast.

[Catalina]

Thanks. New Splitfires last week so I'm going to replace the exhaust manifold and see if that does it.

[balloo93]

Did you happen to notice what the old plugs looked like?

[MSHarnett]

How big is the crack? It may just be easier/cheaper to repair it unless you've already got a mani. If you don't want to replace it right away, you can try to weld it together or try this stuff. http://www.por15.com/product.asp?productid=7

[SimpleXJ]

The cracked exhaust manifold is one of the most common XJ 4.0 problems. I have 3 and 2 of them are cracked. All my friends' XJ manifolds are cracked too. However, the crack would have to be REALLY big before it started to affect your O2 sensor to the point where the MIL comes on. You would notice crappy gas mileage first, I think.

But hey, if you've got a manifold to throw on there, go for it. You can test the sensor while its on the bench. Take an OHM reading between the 2 white sensor wires. It should read 5-7 OHMs.

[Catalina]

Thanks all.

The old plugs were Bosch Platinum tips and they look normal. Nothing out of the ordinary.

I have tried some metal weld stuff and it holds for a while but it seems that the crack has grown with time and use. The visible gap is about .0625 and I can sure tell its blowing through when the hoods up and engine runnng. And I do have the new one already so I'll see if it improves. If not, it was still a good replace and I'll go to the new O2 sensor if the current one looks bad after testing.


Now...as luck would have it some guy hit my son today while he was driving and I just ordered $600 worth of replacement body parts from Quadratec. Being a '94 with a 16 year old driving it, collison coverage on my insurance would have been astronomical!

I'm going to post another thread to see if anyone knows other places to find collson replacement parts.

[jandj]

if you are still having the problem try replacing the bosch plugs with champion plugs, it solved my problem. This seams to affect jeeps with the 4L from the mid 90's.