Cracked manifold = rough idle?

[khrispy]

The mechanic at the shop where my 1G is currently being serviced says that my exhaust manifold is cracked. I believe him, he showed me. However, he says that the reason it's not idling high enough and stalls at stoplights is because the O2 sensor isn't getting a good reading because of exhaust leaks, which doesn't sound right to me. I'm guessing the real problem is a vacuum leak or bad ISC.

Of course I'll replace the exhaust manifold anyway, but does his diagnosis about the O2 sensor make any sense?

Thanks for your help as always, you guys are awesome

 

[93LaserRSBham]

Well, I just discovered that my manifold is cracked up pretty good as well, and my car does idle kind of rough. I don't think that the manifold is the sole reason though. I think I have a bent valve or two, but even with all of that, my car never stalls out at red lights or anything like that. Sometimes if the engine is cold and I'm backing up it will stall out, but that's it.

 

[khrispy]

I thought that didn't sound right

 

[Talesin]

Sounds pretty logical to me, actually. If the O2 sensor isn't reading right, your ECU will get 'confused', altering timing and fuel settings to compensate for a nonexistent incorrect burn.

 

[maynard]

Sounds pretty logical to me, actually. If the O2 sensor isn't reading right, your ECU will get 'confused', altering timing and fuel settings to compensate for a nonexistent incorrect burn.

Talesin is absolutly right. I know this from first hand experience. I had idle issues with my car from day one. I used this link to eliminate most of my problems. http://www.dsm.org/how-tos/tttt/tttt3.html After all that I still had some idle problems and every once and awhile the car would stall. The only thing I hadn't replaced was an o2 sensor. I robbed one off a junk car and slapped it in and now she runs great. No idle issues and no stalling. The old o2 sensor was telling the ecu to give it more fuel because it was reading that the mixture was too lean. Too much fuel + no air = shi**y idle and stalling

 

[khrispy]

Really? I thought the ECU didn't use the O2 sensor until it reached operating temperature- and mine stalls at cold start.

 

[maynard]

Really? I thought the ECU didn't use the O2 sensor until it reached operating temperature- and mine stalls at cold start.

You're right. It runs in open loop mode until warm. I didn't realize it stalled when it was cold

 

[GTM]

...

Of course I'll replace the exhaust manifold anyway,
...

Depending on how bad the crack it can be brazed (not welded) but it takes someone who knows what they are doing. The key is getting it hot and then cooling slowly both can be acheived by running the engine.

Cheers,
GTM