XJ OBDII codes

I beleive ratmans right. It takes so many cycles ( a cold start to a full warmed up engine ) for the ECU to clear it's code out. It's still stored but the ECU doesn't see it as a vital problem so it allowed the check engine light to go away. Since it went away on its own, just drive it I think you'll be OK. But like he said keep it in mind just in case. Someone said something about the Catalytic Converter going bad. By buddy is on his 3rd CC on his 94 YJ. A CC isn't something that's normally replaced in the lifespan of a vehicle. I never thought of the leak on the exhaust manifold causing the CC to malfunction. I'll listen for the ticking sound the leaky EM makes next time I see him.
 
rbo1577186 said:
Someone said something about the Catalytic Converter going bad. By buddy is on his 3rd CC on his 94 YJ. A CC isn't something that's normally replaced in the lifespan of a vehicle. I never thought of the leak on the exhaust manifold causing the CC to malfunction. I'll listen for the ticking sound the leaky EM makes next time I see him.
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Given the way a 2.5 and the 4.0 mount the intake/exhaust manifolds, an exhaust leak at the mounting area could also produce a vacuum leak in the same area. This could cause a lean condition at first, then a rich condition later in the drive cycle. once or twice it'll have little effect, but long term, it'll eat them pretty regular.

A cracked lower exhaust manifold can suck air, causing the O2 sensor to not read the exhaust flow temp. correctly, then causing the engine to run rich as well, creating a similar issue.

Raw fuel is the mortality of a converter, they get too hot burning the raw fuel (rich mixture), melting down, cracking, what ever.

GM vehicles are very well known for eating cats, and it is usually due to a leaking intake gasket sucking air, causing the computer to read lean, then it over corrects by richening the mix, thus burning out the cat(s).

Explaining this to a customer is even more fun !!

Kevin
 
Cheap cats, the beds bust loose and rattle. Then if you keep driving them the beds explode. The chunks can~will damage the probe portion of the downstream O2. Cats do not last the life of the vehicle anymore, on average 75K seems to be the roundabout number. They are run too hot and expected to do too much these days. The older cats would never pass a "efficiency test".

The issue with the manifold leaks is mostly cold air entering the exhaust with overheats the cat. Same prinicple as using a air pump to heat a cat quicker, more o2 to work with.
 
My buddy has less than 100k on this 94 YJ. I'm sure the Ex Man is leaking. I know this a common problem with the 4.0's and my 98 TJ had it's Ex Man replaced at 38k. The ticking sound from it was SO LOUD.
 
rbo1577186 said:
Would changing to a particular header help out??
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The best set up I've seen to date for the 4.0 is the 95 HO 4.0 like the ZJ's. Mine has 195K on it, the original manifold and gasket, no issues and its second cat. The flange wall thickness and mounting hole size is better. Thicker flange and larger holes for heat walk. So far so good. I've stuck a couple of used ones on earlier 4.0's with the ZJ down pipe and it seems to work ok. The XJ needs either a lift or a flat spot beaten into the downpipe to clear the driveshaft on the HP30 front. Replacing all the studs when swapping out helps too, they get heat treated and brittle over time. :beer:
 
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