I've had my '99 Sport 3.5L for about 2-1/2 mo now. Been working out the bugs little by little. Not long after I bought it I noticed a slight popping in the exhaust tone somewhere up front but figured it was just a small leak at a flange or something.
After reading that exhaust manifold cracks are common in some of the later models I decided to have a look.

The PS manifold is severely cracked (possibly a complete fracture) between the rear and middle cylinder.

I assume it's not going to cause any additional problems to drive it like this for a few weeks while I line up a replacement, but I'm curious - I've been getting TERRIBLE mileage - I've seen it below 13 a few times. Could a crack that size effect mileage noticably?
It does so-so if it's 100% highway. We got right about 17 ONCE.

I'm not certain what parts interchange. By mitsupart's description I can use any 3.5 OR 3.0 manifold '97-03. Sound right?

Yes. Extra air enters the exhaust manifold and the o2 sensor reads it as a lean condition. The ECU takes the bogus signal from the sensor and tries to correct by adding fuel.

Overly rich mixtures can damage/plug catalytic converters, so it's possible you will need to replace yours. I wouldn't automatically condemn yours, but be aware of the possibility.

Frank

That makes sense.
It has a bit of a stumble every few seconds at idle. Similar to what my old 2.3L Turbo Ford did when the FPR failed and was flowing way too much gas.

hmm, that could also explain why i've been getting horrible gas mileage, even on the highway. I did add a tad bit more air in the tires, but only cuase it was low. That helped, but I still only average 13-14mpg, and thats on the highway.

At first I thought it was a heat shield cause of quick vibrating noise from underneath. But I'm gonna be doing some work to the Monty here soon so I'll have that checked off the list soon.

Just never thought that would mess with the gas mileage.

<img src="/forums/images/graemlins/frown.gif" alt="" /> Thats bad. Even with my cargo box and ski rack on the roof I still get 18-19 highway and 14-15 city.

I don't know if it's a actual design issue but that's exactly where both mine cracked. I guess if it helps I ran mine for six or seven years with at LEAST one cracked and other than the stumble you're describing and slight decrease in gas milliage it ran. Noisy, but it ran.:-)

The dealership and a couple other people I've talked to say that our exhaust manifolds don't respond well to being dunked and having cold water splashed on them when they're hot. Guys at the shop say they replace several a year on just street driven vehicles.

Edward

That explains my running rich problem, and I though it was the carb. Well Mikuni is somewhat troublesome but I cannot complain about mine BUT on my last smog test with a new catalytic converter it read 768 when the allowed is 1,2xx. I took it to this Japanese auto repair shop thinking the cab needed a little fine tuning, I think it does, and the Japanese dude said, "I don't know anything about carburetors" <img src="/forums/images/graemlins/zombie.gif" alt="" /> He just swap motors and that's it.

Hell, I can do that.. <img src="/forums/images/graemlins/lol.gif" alt="" /> <img src="/forums/images/graemlins/lol.gif" alt="" /> <img src="/forums/images/graemlins/lol.gif" alt="" />

Javy